GB2636031A

GB2636031A - Hydroponic plant-growing tray

Info

Publication number: GB2636031A
Application number: GB2308783.6A
Authority: GB
Inventors: Cooley John
Original assignee: International Plant Propagation Technology Ltd
Current assignee: International Plant Propagation Technology Ltd
Priority date: 2023-06-13
Filing date: 2023-06-13
Publication date: 2025-06-11
Also published as: GB202308783D0

Abstract

A hydroponic plant-growing tray 100 that is configured to float in a body of water comprises a tray top (102, figure 1) and a tray skirt (104) connected to the tray top. The tray skirt extends downwards from the tray top and defines a perimeter of an air-trapping area below an underside of the tray top. A plurality of bulkheads 116 extends downwards from the underside of the tray top, the plurality of bulkheads forming a plurality of open-bottomed air compartments in the air-trapping area under the tray top. An array of cells 106, which are configured to contain a substrate for propagating plants, each comprise an open upper end, a cell wall which extends downwards from the tray top to a cell base, and a drain hole defined in the cell base. The tray top, the tray skirt, the bulkheads and the cell walls are configured to trap air under the tray top when the tray is placed in a body of water, and the open-bottomed compartments are configured to trap air to generate a tray buoyancy. A first plurality of the open-bottomed air compartments are cell-bulkhead-air-chambers into which cells extend under the tray top, and a second plurality of the open-bottomed air compartments 118 are buoyancy compartments which do not contain a cell. At least 60% of the tray’s trapped air volume may be trapped in buoyancy chambers which do not contain cells.

Description

Hydroponic Plant-Growing Tray The invention relates to a hydroponic plant-growing tray, in particular to a hydroponic plant-growing tray, or a hydroponic raft, configured to float in a body of water with a plurality of cells configured to contain substrate for propagating plants.

Background

In commercial plant-propagation systems, plants may be grown, or propagated, with their roots in any of a number of conventional growing media, or substrates, such as soil, peat or coir.

When large numbers of plants are to be propagated, they may be arranged in trays, each tray holding a plurality of plants, such as typically between 6 and 800 plants. Trays are typically rectangular. In some cases, the trays are handled by hand and in some cases by automated machinery. In use, trays are typically arranged on the ground or on benching or tables.

A tray typically comprises an array of cup-shaped cells, each cell for containing a substrate for propagation of a plant. Traditionally, cells are filled with a loose substrate such as compost and plant seeds or cuttings. During growth, the plants in a cell develop a system of roots which holds together the substrate in a "rootball" or "plug". A well-developed rootball can be removed from a cell as a single unit of substrate and plant roots, but this only works when enough roots have developed to hold the substrate together.

In some cases, it is desirable to be able to remove rootballs from cells before the roots have fully developed. For example, the grading of plants is often done when the plants are very young and the rootball has not fully developed. It is also desirable to be able to remove the contents of cells that have not successfully grown a plant. However, this is not possible with loose-filled substrates. A popular way of overcoming this problem is to use a stabilised medium, which typically comprises compost contained within some form of material which holds the compost together while the roots of the plant develop, or compost mixed with a binder which holds the compost together. A variety of types of stabilised medium are available, including some which use polymer glues to hold the compost together, and others which contain the compost in a mesh or other suitable material, such as Jiffy (RTM) plugs.

A particularly popular form of stabilised medium is a cylindrical, or tubular, stabilised medium, such as an Ellepot (RTM), in which a volume of compost is held in a membrane of a permeable material, such as paper. The membrane is designed to retain the compost so there is no need to wait for roots to develop to be able to extract a plant from a cell, if desired. Cylindrical stabilised media such as Ellepots (RTM) comprise a continuous extruded tube of soil, which is wrapped in a membrane and cut into individual cylindrical "plugs" of an appropriate length. Cylindrical stabilised media are therefore naturally parallel sided.

During hydroponic growth of various plants, plants are typically held in trays which are either suspended above a body of water, or in floating trays known as "rafts" which float on the body of water. The roots growing from the plant can grow from a drain hole in the tray or raft, downwards into the water.

In prior art hydroponic systems which use floating rafts, or EPP rafts (expanded polystyrene rafts) and EPP rafts (expanded polypropylene rafts) have typically been used as they are naturally very buoyant. Floatable expanded polystyrene trays have become the standard globally in a lot of commercial large scale float propagation systems but have significant disadvantages such as: a) being very difficult to sterilize; b) methyl bromide, which has been used for sterilizing these trays successfully in recent decades, has now been banned globally, so sterilization is now a much bigger issue than it was in the early days of EPS and EPP use; c) whilst cheaper at purchase time, EPS and EPP trays are expensive and environmentally-unfriendly in the long term as they need to be replaced frequently especially now that sterilization is more difficult; d) beads from the EPS and EPP trays can break off and get into watercourses or plant rootballs, or worse still, into the product which is sent to the supermarket and consumer.

These disadvantages are significant and floating expanded polystyrene trays continue to be used for hydroponic growth only because there has been no viable alternative.

Floating trays formed from injection-moulded plastic rather than EPS or EPP are known from international patent application W02016/147128A1. However, the trays of W02016/147128A1 were designed so that the drain holes in the bottom of the cells would always be fully-submerged in water when the tray is floating. This means that the soil in the cells of this tray is constantly saturated, while roots emerging out of the cell drain holes are underwater. For plant such as tobacco, this is highly desirable. For plants grown hydroponically, however, this is completely unsuitable, as for hydroponic propagation it is desirable that roots should be able to reach the water, but that the soil should not be saturated with water. The tray design of W02016/147128A1 is also unsuitable for use with cylindrical stabilised media, as the cells are tapered and designed to receive loose soil instead of a vertical-sided stabilised medium.

It is an aim of the present invention to provide a hydroponic plant-growing tray for hydroponic propagation of plants.

Summary of the Invention

The invention provides a hydroponic plant-growing tray, which may be called a hydroponic plant-growing raft, as defined in the appended independent claims, to which reference should now be made. Preferred or advantageous features of the invention are set out in dependent sub-claims.

In a first aspect, the invention may provide a hydroponic plant-growing tray configured to float in a body of water. The tray may comprise a tray top, and a tray skirt connected to the tray top, the tray skirt extending downwards from the tray top and defining a perimeter of an air-trapping area below an underside of the tray top. A plurality of bulkheads extend downwards from the underside of the tray top, the plurality of bulkheads forming a plurality of open-bottomed air compartments in the air-trapping area under the tray top.

The tray comprises an array of cells. Each cell comprises an open upper end, a cell wall which extends downwards from the tray top to a cell base, and a drain hole defined in the cell base.

The tray top, the tray skirt, the bulkheads and the cell walls are configured to trap air in the air-trapping area under the tray top when the tray is placed in a body of water.

The open-bottomed compartments are configured to trap air to generate a tray buoyancy.

Preferably a first plurality of the open-bottomed air compartments are cell-bulkhead-airchambers into which cells extend under the tray top, and a second plurality of the open-bottomed air compartments are buoyancy compartments which do not contain a cell. As described below, the presence of buoyancy compartments under the tray top provides the floating tray with a variety of benefits in terms of increased buoyancy to support heavier loads of plants and substrate, and increased stability while floating.

The relative heights of the tray skirt, the bulkheads and the cell walls may be configured so that, when the tray is floating in a body of water, the cell bases are suspended at least 5 mm above the surface of the body of water.

Typically within a hydroponic float raft nursery plants are grown in two separate stages -a propagation stage and a growing stage. During the propagation stage, plants are contained in a tray with, for example, 216 cells, whereas for the growth stage plants are transplanted into a floating "raft" tray containing fewer cells, for example only 18 cells. Sometimes an intermediary size is used, for example with 54 cells, where a plant from the 216-cell propagation tray is placed with four times as much space available per plant before again being transplanted into the raft with even more space available. Typically the first, or first and second, stage before the raft is done in a conventional plant tray and mechanically supported just above the water so that water roots are produced underneath the tray during propagation and before the plant is transplanted into the next container size. Roots typically might be about 4" or 5" long hanging out of the bottom of the cell in the propagation tray before the plants are transplanted into a different tray. At transplanting time the plants are lifted out of the first-stage tray and put into the next-stage tray, where the roots hanging below the propagation module or root ball pass through the drain hole so that they will be in water underneath the tray during growth.

The hydroponic plant-growing tray of the present disclosure may be termed a hydroponic raft, or a hydroponic plant-growing raft. The term raft is commonly used in the industry for floating plant trays such as the present tray. Should the term raft be used, the tray skirt may alternatively be known as a raft skirt, the tray top may be known as a raft top, and the tray buoyancy may be a raft buoyancy.

The hydroponic plant-growing tray of the present disclosure is preferably a growth-stage hydroponic plant-growing tray. The tray is preferably configured to support plants during the hydroponic growth phase which follows the initial propagation phase as described above.

The cells may take a variety of forms within the scope of the present disclosure, without affecting the floating characteristics of the tray. The cells may be configured to contain stabilised media, for example cylindrical stabilised media for propagating plants, in which case the cell base is the section of the cell which supports a base of a cylindrical stabilised medium in the cell. Alternatively the cells may be configured to receive other forms of plant-growing substrate, such as peat-blocks or loose-fill soil.

The tray is preferably configured to support a load weight while the tray is floating in a body of water. In use, the load weight is the weight of a plurality of plants and their associated growth substrate (cylindrical stabilised media/peat-block/soil), as when the tray is in use, the cells of the tray each contain substrate out of which a plant grows. The load weight does not include the weight of the tray itself; only the weight of the plants and substrate which are held in the tray.

As the tray supports the substrate and plants over a period of time while the plants are growing, the load weight supported by the tray will vary over time. When the tray is first loaded by filling its cells with substrate which have seedlings growing from them, the load weight will be equal to the weight of the substrate and the seedlings. As time passes and the seedlings grow progressively into larger and larger plants, the load weight will also grow towards a maximum load weight which is achieved when it is time for the plants to be harvested. This variation in load weight throughout the plant growth cycle will mean that the floating height at which the tray floats on a body of water will gradually become lower as the plants grow heavier.

In order for the hydroponic tray to encourage good hydroponic growth, it is important that the substrate is kept above the water throughout the plant-growing cycle, so that even when the load weight is at its maximum, the substrate is still suspended above the surface of the body of water.

In the present tray, the relative heights of the tray skirt, the bulkheads and the cell walls are configured to trap enough air so that, when the tray is floating in a body of water, the cell bases are always suspended at least 5 mm above the surface of the body of water. Enough air is trapped so that even when the tray is supporting a maximum load weight of fully-grown plants, the cell bases are suspended at least 5 mm above the surface of the body of water.

The dimensions of the tray top, the tray skirt, the dimensions and number of bulkheads, and the height of the cell walls are preferably selected to determine the height at which the tray floats relative to the surface of the body of water. In order to achieve good hydroponic growth, the tray must float with the cell bases close enough to the water's surface that the roots can reach the water through the drain hole in the cell base. In order to achieve this, the tray is configured to trap the right amount of air so that the cell bases are suspended at least 5 mm above the surface of the body of water throughout the growth cycle of the plants in the tray. The volume of trapped air is controlled so that, when the plants are first placed in the tray and the load weight is at its minimum, the cell bases are preferably less than 20 mm above the surface of the body of water, and when the plants are fully-grown and the load weight is at its maximum, the cell bases are still more than 5 mm above the surface of the body of water.

The number of cells, the tray dimensions and the range of supported load weights may vary depending on the requirements of any particular scenario.

As the size, weight and growth characteristics of different plants can vary significantly, the air-trapping characteristics of the tray may be selected to suit a particular application.

For any given plant, for example, a tray may be designed for use during a defined growth stage of that plant. Such a tray may be configured to support a minimum load weight which is the weight of a batch of substrate and seeds, or seedlings of that plant, and to hold the plants until they grow to a maximum load weight at which the plants are ready for transplanting or harvesting. The skilled person will appreciate that the present invention can be easily adapted to work for a range of plants in this way.

For a hydroponic tray which is configured for growing lettuce from seedling to fully-grown for example, the minimum load weight will be that of a batch of substrate rootballs containing lettuce seedlings, and the maximum load weight will be that of the same batch of rootballs containing fully-grown lettuces.

In a particularly preferred embodiment, for example, the tray contains 18 cells, each of which is configured to receive a cylindrical stabilised medium.

The minimum load weight of lettuce and cylindrical stabilised media for this tray will thus be 18 x (weight of cylindrical stabilised medium and lettuce seedling -eg. 20 g) = 360 g. The maximum load weight of lettuce and cylindrical stabilised media for this tray will thus be 18 x (weight of cylindrical stabilised medium and ready-to-harvest lettuce -eg. 670 g) = 12.06 kg.

In order to ensure good hydroponic growth as the lettuce grows from seedlings into fully-grown plants, the tray is configured to trap a volume of air which results in the tray floating at a height relative to the surface of the body of water, at which the cell bases are suspended less than 20 mm above the surface of the body of water when the load weight is the minimum load weight, and at least 5 mm above the surface of the body of water when the load weight is the maximum load weight.

Preferably the tray is configured so that, when the tray is floating in a body of water and supporting a minimum load weight, the cell bases are suspended less than 20 mm above the surface of the body of water.

Preferably the tray is configured so that, when the tray is floating in a body of water and supporting a maximum load weight, the cell bases are suspended at least 5 mm above the surface of the body of water.

Preferably the tray is configured so that, when the tray is floating in a body of water and supporting a load weight of 0.5 kg, the cell bases are suspended less than 20 mm above the surface of the body of water. This load weight of 0.5 kg may correspond to a convenient minimum load weight of rootballs and seeds, or seedlings, or juvenile plants, when the rootballs are first loaded into the tray.

In a preferred embodiment, however, the tray is configured to support a load weight of up to 12 kg, or up to 13 kg, or up to 14 kg, while the cell bases are suspended at least 5 mm above the surface of the body of water.

Preferably the tray is configured so that, when the tray is floating in a body of water and supporting a load weight of 12 kg, the cell bases are suspended at least 5 mm above the surface of the body of water. This load weight of 12 kg may correspond to a convenient maximum load weight of rootballs and plants, when the plants are ready for harvesting.

Thus even when the plants supported by the tray are fully grown to a combined weight of 12kg, the cell bases are still supported above the water, so the substrate in the cells is not saturated. This ensures healthy hydroponic growth even when the plants are fully grown and at their heaviest.

The tray skirt, the bulkheads and the cell walls are preferably configured so that, when the tray is floating in a body of water, the cell bases are suspended between 5 mm and 20 mm, or preferably between 8 mm and 12 mm above the surface of the body of water.

The tray skirt preferably extends below the tray top by a tray skirt height, and in which the tray skirt height is at least 0.05 m, or at least 0.08 m. The tray skirt height may be between 0.04 m and 0.12 m, or between 0.05 m and 0.10 m, or between 0.06 m and 0.08 m. The tray skirt height is important in determining the volume of air that is trapped under the tray when floating, and the inventor has found that tray skirt heights in this range are particularly suitable for trays configured to support a wide range of plant types, with a correspondingly wide range of plant weights.

The tray top, the tray skirt, the bulkheads and the cell walls are preferably configured to trap a trapped-air-volume when the tray is placed in a body of water. The trapped-airvolume is the volume of air which is trapped under the underside of the tray when the tray is placed on a body of water. The magnitude of the trapped-air-volume is determined by the tray skirt height, the height of the bulkheads, and the height of the drain holes relative to the tray top, as the drain holes provide a way for air to escape from under the tray.

The cell base is the section of the cell which supports the base of a substrate rootball in the cell. While a drain hole defined in the cell base, in some embodiments the drain hole may be elongated -for example a drain hole tube may be connected to the drain hole. When this is the case, it is the lowermost extent of the drain hole, or drain hole tube, that governs the ability of air to escape upwards through the drain hole. For healthy hydroponic root growth, however, it is the height of the rootball above the water which is important. As the base of the rootball is positioned at the cell base, it is the height of the cell base above the water which should remain at least 5 mm throughout the growth of the plants in the tray.

The trapped-air-volume is not simply the tray skirt height multiplied by the trapped-air area bounded by the tray skirt. As the drain holes in the cell bases provide a route through which air can escape upwards, the trapped-air volume is affected by the height of the drain hole relative to the bulkheads and tray skirt. The higher (further away from the surface of the water) the drain holes, the more air can escape upwards through the drain holes when the tray is placed on a body of water, so the lower the overall trapped-air-volume.

In preferred embodiments, the trapped-air-volume is at least 0.036 m3, or at least 0.043 m3, or at least 0.057 m3. Preferably the trapped-air-volume may be between 0.035 m3 and 0.060 m3, or between 0.040 m3 and 0.050 rn3. While the ideal trapped-air-volume will vary depending on the number of plants, and the weight of the plants, that the tray is configured to support, the inventor has found that air volumes in these ranges float in the desired height range for load weights of between 350 g and 13 kg.

As the number of cells in the tray may be varied, and as a result vary the range of load weights to be supported by the tray, the trapped-air-volume may be defined as a trappedair-volume-per-cell. In preferred embodiments tested by the inventor, the trapped air volume is at least 0.0020 m3 percell, or at least 0.0023 m3 per cell, or at least 0.0031 m3 per cell. Preferably the trapped-air-volume may be between 0.0020 m3 percell and 0.0035 m3 per cell, or between 0.0025 m3 per cell and 0.0030 m3 per cell. While the ideal trapped-air-volume per cell will vary depending on the type and therefore weight of the plants that the tray is configured to support, the inventor has found that trapped-air-volumes in these ranges float in the desired height range for load weights of between 20 g and 0.75 kg per cell.

The tray comprises the tray skirt which extends downwards from the tray top and defines the perimeter of the air-trapping area under the tray top. In preferred embodiments the tray skirt extends downwards from the perimeter of the tray top, though alternative arrangements are possible without affective the floating characteristics. The plurality of bulkheads extend downwards from the underside of the tray top, and the plurality of bulkheads are arranged to form the plurality of open-bottomed air compartments in the air-trapping area under the tray top. Preferably the bulkheads extend across the underside of the tray from side to side, connecting opposite sides of the tray skirt to one another, and segmenting the trapped air area into a plurality of compartments. The compartments are closed by the tray top at their top end, but open at their bottom end in order to trap air when the tray is placed downwards onto a body of water.

As the cell walls extend downwards from the tray top, the cells project downwards underneath the tray top. The cells thus extend below the tray top, into at least some of the open-bottomed air compartments formed on the underside of the tray top.

The bottom-most surfaces of the tray skirt and optionally the bulkheads preferably provide a tray base on which the tray may rest when placed on a flat surface. The cells preferably extend downwards from the tray top by a distance of less than or equal to the height of the tray skirt, so that the cells are shorter than the cell skirt.

In a preferred embodiment, a first plurality of the open-bottomed air compartments are cellbulkhead-air-chambers into which cells extend downwards from the tray top, and a second plurality of the open-bottomed air compartments are buoyancy compartments which do not contain a cell.

The bulkheads advantageously strengthen the tray by increasing its rigidity, and compartmentalise the underside of the tray to improve the floating stability of the tray in the water. By arranging a plurality of bulkheads under the tray, the tray captures pockets of air in a series of separate open-bottomed air compartments rather than just one large trapped-air area. Compartmentalising the trapped-air area into multiple compartments improves both floating stability and reliability by ensuring that even if one or more of the compartments leaks air, this will have a minimal effect on the buoyancy of the entire tray, as the air in the other compartments will remain trapped.

In reality, individual open-bottomed air compartments may lose their air-trapping integrity due to: a hole in the tray top, which could occur either by puncture in use, or by a manufacturing defect; or a crack in the tray skirt, which could occur due to misuse or tray age.

The greater the number of open-bottomed air compartments under the tray top, the smaller the negative effect on the overall tray buoyancy when any one compartment is compromised.

The use of tray bulkheads also prevents the movement of trapped air across large lateral distances under the tray top. Thus if the tray is placed onto the body of water at an angle, or tipped to a non-level orientation while floating, the bulkheads prevent air from rushing across the underside to the highest point, which would unbalance and potentially capsize the tray.

In a preferred embodiment, a first plurality of the open-bottomed air compartments are cell-bulkhead-air-chambers into which cells extend downwards from the tray top.

A second plurality of the open-bottomed air compartments are preferably buoyancy compartments which do not contain a cell. As only some of the open-bottomed air compartments contain cells, thus only some of the open-bottomed air compartments contain cell drain holes through which air can escape. For the second plurality of open-bottomed air compartments, these compartments do not contain a cell, and thus these compartments are completely sealed apart from their open bottom ends. These second plurality of compartments are buoyancy compartment which trap air when the tray is floating, and thus increase the buoyancy of the tray.

While trays such as the floating tobacco tray of W02016/147128A1 employ bulkheads which divide the tray underside into compartments, all of the under-tray compartments in W02016/147128A1 contain cells, and none were buoyancy chambers which did not contain cells. The buoyancy compartments in the present tray enable the present tray to support load weights which are significantly greater than those supportable with the propagation tray of W02016/147128A1. The buoyancy compartments also provide additional buoyancy which allows the tray to float above the water throughout the large change in load weight during the growth phase of the plants. In W02016/147128A1, the propagation of early-stage plants such as tobacco does not create a significant difference in weight during the cycle of growth which takes place in the tray, and in W02016/147128A1 the plants are kept intentionally partly below the waterline, which is unacceptable for most hydroponic growth.

The present inventor has found that buoyancy chambers are particularly useful in ensuring that the tray floats reliably at a floating height which retains the cell bases at least 5 mm above the water at all times, as the buoyancy chambers contain a proportion of the trapped-air-volume which is unaffected by the cells and the airflow through the drain holes.

As described above, the total trapped-air-volume is affected by the height of the drain holes relative to the tray skirt and bulkheads, but the provision of buoyancy chambers means that the drain hole height only affects the volume of air trapped in the cell-bulkhead-airchambers which contain cells. The drain hole height thus impacts on a limited proportion of the total trapped-air-volume.

Preferably the tray is configured so that at least 60%, or at least 65%, or at least 70%, or at least 75% of the tray's trapped-air-volume is trapped in buoyancy chambers which do not contain cells. Trapping a large proportion of the trapped-air-volume in buoyancy chambers ensures that the tray has a high degree of buoyancy, regardless of the size, shape and style of the cells, and the height of the drain hole relative to the bulkheads. This allows more flexibility in the style of cells which are usable in the tray, as the buoyancy chambers help the tray to float at the desired height for a wide range of cell designs.

The buoyancy chambers preferably occupy 55-70% of the air-trapping area under the tray top, preferably 60-65% of the air-trapping area under the tray top. As the buoyancy chambers are filled with trapped air in use, while the cell-bulkhead-air-chambers may be only partially filled due to the escape of air through the cell drain hole, the proportion of trapped-air-volume held by the buoyancy chambers is typically greater than the proportion of the trapped-air area that is occupied by the buoyancy chambers.

In a preferred embodiment, the tray comprises a ring of buoyancy compartments extending around the underside of the tray top, the ring of buoyancy compartments being positioned between the tray skirt and the cells. The tray may comprise a plurality of rows of buoyancy compartments arranged in a grid on the underside of the tray top, the grid being arranged around the cell-bulkhead-air-chambers such that the cell-bulkhead-air-chambers are separated from one another by the rows of buoyancy compartments. Positioning buoyancy chambers around the perimeter of the trapped-air area, and/or around the cell-bulkheadair-chambers, provides the tray with advantageously stable floating characteristics, as the buoyancy provided by the buoyancy compartments is evenly spread across the tray.

The tray preferably comprises a plurality of cell-adjacent bulkheads, in which each cell is surrounded by one or more cell-adjacent bulkheads underneath the tray top. The cell-adjacent bulkheads form the cell-bulkhead-air-chambers. Each cell-bulkhead-air-chamber extends between an outside of the cell wall, an inside of the one or more cell-adjacent bulkheads surrounding said cell, and the underside of the tray top, such that air is trapped in the cell-bulkhead-air-chamber when the tray is placed in a body of water.

The cell-bulkhead-air-chambers preferably occupy 30-45% of the air-trapping area under the tray top, preferably 35-40% of the air-trapping area under the tray top. Preferably the tray is configured so that at least 25%, or at least 30%, or at least 35%, or at least 40% of the tray's trapped-air-volume is trapped in the cell-bulkhead-air-chambers when the tray is floating.

The volume of air that is trapped in the cell-bulkhead-air-chambers will depend on the height of the drain hole relative to the cell-adjacent bulkheads, as air can escape from the cell-bulkhead-air-chamber upwards through the drain hole unless the passage of air is blocked by water or a cell wall.

The drain hole is a hole formed in the base of each cell. The drain hole may optionally be connected to a drain tube which increases the effective height of the drain hole.

In one preferred embodiment, each cell comprises a drain tube which extends downwards from an upper end of the drain tube, which is connected to the drain hole in the cell base, to an open lower end of the drain tube. The lower end of the drain tube is preferably positioned at a height which is level with, or higher than, the bottom of the one or more cell-adjacent bulkheads surrounding the cell. The cell-bulkhead-air-chamber is then formed by an inside of the one or more cell-adjacent bulkheads, an outside of the cell wall, an outside of the drain tube and the underside of the tray top, such that air is trapped in the cellbulkhead-air-chamber when the tray is placed in a body of water.

In this embodiment, the height of the lower end of the drain tube limits the passage of air from the cell-bulkhead-air-chamber into the drain hole and upwards through the cell. If the lower end of the drain tube is positioned at a height which is level with the bottom of the cell-adjacent bulkheads surrounding the cell, then as soon as the tray is placed in the body of water, no air can pass from the cell-bulkhead-air-chamber into the drain hole, so no air can escape. If the lower end of the drain tube is positioned at a height which is higher (closer to the tray top) than the bottom of the cell-adjacent bulkheads surrounding the cell, then when the tray is placed in the body of water, air can pass from the cell-bulkhead-airchamber into the drain hole and escape, until the surface of the water reaches the lower end of the drain tube, or until the buoyancy of the tray reaches equilibrium.

In order to allow the roots to grow from the cylindrical stabilised medium down to the water, the drain hole is preferably large relative to the size of the cell and the cylindrical stabilised medium contained in the cell. The drain hole may, for example, have a horizontal cross-sectional area of at least 80%, or at least 85%, or at least 90% of an area of the cell base.

The cell base preferably has a cell base diameter, and the drain hole has a drain hole diameter. The drain hole diameter is preferably at least 90%, or at least 95%, or equal to, the cell base diameter.

When plants are put into the floating tray, roots hang from the base of the root ball (for example from the base of a cylindrical stabilised medium) and it is desirable to have these roots flow through the drain hole. If there are no roots of this type hanging under the root ball then it will be very difficult for the plant to take up water as there are no roots touching the water. This is thus an integral part of the system.

The cell base is configured to support the cylindrical stabilised medium over the drain hole. The cell base may preferably comprise a plurality of projections which extend inwards from the cell wall across a portion of the drain hole, the plurality of projections being configured to support the lower end of the cylindrical stabilised medium on top of the projections.

In a preferred embodiment, the projections of the cell base extend inwards from the cell wall by a projection length. The projection length is preferably less than 20%, or less than 15%, or less than 10% of the drain hole diameter. This advantageously ensures that very little of the drain hole is blocked by the projections, so the roots can grow hydroponically without the base of the cylindrical stabilised medium being obstructed, and without the roots becoming tangled in the cell base. Cell bases on conventional plant trays typically comprise sets of ribs which cross the drain hole to support the soil inside the cell, but roots can become entangled in such ribs, risking damage when the plants are eventually removed from the cells.

The drain holes in the cell bases are preferably the only openings through the tray, through which air may escape from the trapped-air area when the tray is placed on a body of water.

The cell wall of the cells preferably does not comprise any openings, such that the cell drain hole is the only opening between the cell and the underside of the tray.

In some preferred embodiments, the tray is configured so that the open upper ends of the cells are elevated above the tray top. Preferably the open upper ends of the cells are elevated to a height of 17-22 mm above the tray top. This may advantageously allow improved air-flow underneath the leaves of the plant, which is beneficial for disease control.

This feature also raises the plant further above the tray top, which improves access to plant stems during the harvesting process.

Each cell may comprise a raised cell rim which projects upwards above the tray top and extends around at least a portion of the cell. The raised cell rims define the open upper ends of the cells, so that it is the raised cell rims which are preferably elevated to a height of 17-22 mm above the tray top. The raised cell rims preferably comprise a chamfered inner surface which slopes inwards towards the cell. This may advantageously make placement of cylindrical stabilised media into, and removal of the cylindrical stabilised media from, the cells easier and less likely to damage the rootballs.

The cells may each comprise a cell chamber which is inside the cell wall, and which is configured to receive the rootball. The shape of the cell chamber may vary depending on whether the cells are designed to receive rootballs grown from cylindrical stabilised media, or loose-fill substrate, or peat-blocks for example.

In some preferred embodiments configured to receive cylindrical stabilised media, the cell chamber is preferably substantially cylindrical.

Optionally the cell chamber is tapered from a wider upper end to a narrower lower end at the cell base, to reduce friction between the rootball and the cell wall when the rootball is removed from the cell. In some preferred embodiments the cell chamber at the upper end has a diameter which is less than 15% or less than 10% larger than the diameter of the cell chamber at the cell base.

In a particularly preferred embodiment, each cell comprises a plurality of slots located around the cell. The slots are open to the cell chamber and extend downwards from the tray top towards the cell base. The cell wall extends around and encompasses the plurality of slots, so that the slots are positioned inside the cell wall but outside the cell chamber. A vertical cross-section of the slots, taken radially through the cell, tapers from a wide upper end to a narrower lower end.

The lower end of the slots is preferably positioned level with the cell base, or above the cell base. The lower end of the slots is preferably positioned close to the height of the cell base, for example in the lowermost 10%, or the lowermost 20%, of the cell height.

The lower end of the slots preferably have a slot base which is configured to direct water into the cell chamber. The lower ends of the slots is preferably closed. This may advantageously ensure that the slots direct water into the sides of the rootball in the cells, and act as a reservoir to hold water which may be gradually soaked up by the substrate or drained through the drain hole.

The slots are preferably configured to allow the fingers of a mechanised plant-handling system to be inserted into the slots to place the rootball in the cell and/or to pick up and remove the rootball from the cell. The upper ends of the slots are preferably open, so that a slim mechanised finger for tray-handling can be inserted downwards into the slot from above the tray top.

Apart from their open upper ends and the side which is open to the cell chamber, the slots are preferably enclosed by the cell walls, so that the slots do not provide any passage for air to pass from the underside of the tray to the upper side of the tray.

The upper ends of the slots are positioned level with the tray top, so that water on the tray top is configured to flow into the slots.

Preferably four slots are arranged radially around each cell, particularly preferably at intervals of 90 degrees.

In a preferred embodiment, the tray top comprises a plurality of sloped catchment areas, each cell being associated with a respective catchment area. The area of tray top within each catchment area is preferably sloped to direct water towards the cell in that catchment area. These catchment areas advantageously ensure that all water on the tray top, whether rain or water from plant-watering, drains into one of the cells of the tray. The catchment area may optionally comprise one or more grooves or channels for directing water towards the cell. Particularly in a floating tray where floating height is carefully controlled, it would be undesirable to allow an additional weight of water to sit on the tray top and weigh-down the tray, so these catchment areas both conserve water and solve this problem.

The dimensions of the tray, and the size of the tray top, may be varied according to the requirements for any given version of the tray. In a preferred embodiment, however, the tray top has an area of at least 0.035 m2, or 0.04 m2 per cell. This area of tray top per cell is unusually large compared to conventional plant trays, but advantageously allows a large area for each plant to grow, such that the plants can be allowed to grow for a long period before they need to either harvested, or transplanted to a larger tray.

In a preferred embodiment the tray contains 12 to 60 cells, for example 18 to 50 cells, preferably only 12, or 18, or 50 cells.

The hydroponic plant-growing tray preferably comprises at least 5 buoyancy chambers, or at least 10 buoyancy chambers, or at least 15 buoyancy chambers, or at least 25 buoyancy chambers, or at least 50 buoyancy chambers, or at least 70 buoyancy chambers. The greater the number of buoyancy chambers, the more resilient the floating tray is against the failure of any one buoyancy chamber, and the greater the tray stability while floating.

The tray is preferably formed from injection-moulded thermoplastic. This is a heavy material relative to EPS and EPP, so is not typically used for trays which are required to float. By employing the air-trapping features described above, however, the present applicants have found that it is possible to make the present tray using injection-moulded or thermoformed plastic. This presents the benefits of being recyclable, manufacturable from recycled materials, and easy to sterilise.

Automation Fingers In a second aspect, there is provided a plant-growing tray comprising: a tray top; an array of cells configured to contain substrate for propagating plants, each cell comprising an open upper end, a cell wall which extends downwards from the tray top to a cell base, and a drain hole defined in the cell base; in which each cell comprises a plurality of slots located around the cell, in which the slots are open to the cell chamber and extend downwards from the tray top towards the cell base.

The cell wall extends around and encloses the plurality of slots, so that the slots are positioned inside the cell wall but outside the cell chamber. A vertical cross-section of the slots, taken radially through the cell, tapers from a wide upper end to a narrower lower end.

The lower end of the slots preferably have a slot base which is configured to direct water into the cell chamber. The lower ends of the slots is preferably closed. This may advantageously ensure that the slots direct water into the sides of the substrate in the cells, and act as a reservoir to hold water which may be gradually soaked up by the cylindrical stabilised media or drained through the drain hole.

The slots are preferably configured to allow the fingers of a mechanised plant-handling system to be inserted into the slots to place a substrate rootball in the cell and/or to pick up and remove the rootball from the cell. The upper ends of the slots are preferably open, so that a slim mechanised finger for tray-handling can be inserted downwards into the slot from above the tray top.

Particularly preferably, the cells may each be configured to receive a cylindrical stabilised medium.

The tray of the second aspect may have any of the features described above in relation to the first aspect. However, the tray of the second aspect need not be a floating hydroponic tray having the features of the first aspect. The tray of the second aspect may incorporate the features of the second aspect into a variety of trays other than hydroponic trays.

Description of Specific Embodiments

Specific embodiments of the invention will be now be described by way of example, with reference to the accompanying drawings in which: Figure 1 is a perspective view, from above, of a hydroponic plant-growing tray according to an embodiment of the present invention; Figure 2 is a perspective view, from below, of the hydroponic plant-growing tray of Figure 1; Figure 3 is a plan view of the top of the hydroponic plant-growing tray of Figure 1; Figure 4 is a plan view of the underside of the hydroponic plant-growing tray of Figure 1; Figure 5 is a side view of the long side of the hydroponic plant-growing tray of Figure 1; Figure 6 is a side view of the short side of the hydroponic plant-growing tray of Figure 1; Figure 7 is an enlarged perspective view of a portion of the top of the tray of Figures 1 to 6; Figure 8 is an enlarged perspective view of a portion of the underside of the tray of Figures 1 to 6; Figure 9 is a partial side-on cross-section of a hydroponic plant-growing tray according to a first embodiment of the present invention; Figure 10A is a partial side-on cross-section of a hydroponic plant-growing tray according to a second embodiment of the present invention; Figure 10B is a partial side-on cross-section of a hydroponic plant-growing tray according to a third embodiment of the present invention; Figure 11A is a partial side-on cross-section of a hydroponic plant-growing tray according to a fourth embodiment of the present invention; Figure 11 B is a partial side-on cross-section of a hydroponic plant-growing tray according to a fifth embodiment of the present invention; Figure 12A is a perspective view of an alternative cell usable with the tray of the present invention; and Figure 12B is a side-on cross-section of the cell of Figure 12B.

As described above, the hydroponic plant-growing tray of the present disclosure may be made in a variety of sizes, with a variety of numbers and types of cells, and a range of different dimensions, all while employing the same inventive concepts. For the sake of illustration only, the Figures and the following description relate to an embodiment of the tray which contains 18 cells for cylindrical stabilised media, and which has dimensions selected for growing lettuce plants from those cylindrical stabilised media. The skilled person will appreciate that trays intended for hydroponically propagating different crops could incorporate more or fewer than 18 cells, and have cell spacings and dimensions different from those illustrated. Alternative shapes of cell could also be used for accommodating rootballs grown from types of substrate other than cylindrical stabilised media.

Figures 1 to 8 show different views of the same embodiment of hydroponic plant-growing tray 100, which may also be called a hydroponic raft.

Hydroponic plant-growing tray 100 is a generally rectangular tray which has a flat, rectangular tray top 102 and a tray skirt 104 which follows the outermost perimeter of the tray top 102 and extends vertically downwards from the tray top. The tray skirt 104 forms an outer wall enclosing an air-trapping area under the tray top.

The tray 100 contains an array of 18 cells 106 arranged in three rows of 6 cells. Each cell 106 is shaped to receive a cylindrical stabilised medium of a particular diameter. Although a variety of different cell-styles can be used in the hydroponic tray, all suitable cells have an open upper end, through which a cylindrical stabilised medium can be placed into, and removed from, the cell. The cells 106 are each surrounded by a cell wall 108 which extends downwards from the tray top to a cell base 110. The cell base 110 is configured to support a lower end of the cylindrical stabilised medium, and a drain hole 112 is defined in the cell base, so that roots from the cylindrical stabilised medium can extend through the drain hole to reach a body of water below.

The underside of the tray 100 is divided into a series of square and rectangular open-bottomed air compartments, or chambers, by a plurality of bulkheads 114. The bulkheads 114 are walls which extend downwards from the underside of the tray top 102, and are arranged to span the air-trapping area so that the bulkheads are connected to the tray skirt on opposing sides of the tray.

The open-bottomed air compartments are bounded by the tray top 102, the bulkheads 114 and in some cases the tray skirt 104, and the open-bottoms of these compartments means that they trap air under the tray top when the tray is placed in a body of water with the tray top 102 facing upwards.

In the illustrated embodiment, the dimensions of the tray top are 1.2 m by 0.6 m, and the tray skirt has a height of 0.06 m from the tray top to the lowermost end of the skirt.

As the cells 106 extend downwards from the tray top, the cells protrude into some of the open-bottomed air compartments. The open-bottomed air-compartments which contain cells are cell-bulkhead-air-chambers 116 which are bounded by cell-adjacent bulkheads 114 on all four sides. The volume of air trappable in a cell-bulkhead-air-chamber 116 is defined by the volume enclosed by the outside surface of the cell wall, the inside surfaces of the cell-adjacent bulkheads surrounding the chamber, and the tray top.

The open-bottomed air compartments which do not contain cells are buoyancy compartments 118 which function to trap air which increases the buoyancy of the tray 100.

The volume of air trappable in a buoyancy compartment 118 is defined by the volume enclosed by the inside surfaces of the bulkheads surrounding the buoyancy compartments, and the tray top.

In the illustrated preferred embodiment, the tray comprises 73 buoyancy compartments and 18 cell-bulkhead-air-chambers.

In the illustrated preferred embodiment, 64% of the trapped-air area under the tray top is occupied by buoyancy compartments 118, while the remaining 36% is occupied by cell-bulkhead-air-chambers 116. The volume of air trappable in the buoyancy compartments 118 can be calculated based on the height of the bulkheads and the area of the buoyancy compartments. The volume of air trappable in the cell-bulkhead-air-chambers 116, however, is dependent on the height of the drain hole relative to the cell-adjacent bulkheads, as it is possible for trapped air to escape upwards through the drain hole as long as the lowest extent of the drain hole is located above the water level.

The relative heights of the cell wall, the bulkheads and the tray skirt, as well as the area of the tray top, are therefore important for determining the volume of air that will be trapped under the tray, and therefore the buoyancy of the tray when it is floating in a body of water.

In order to provide good hydroponic growth properties, and avoid the cylindrical stabilised media touching the water at any stage of the plants' growth, in the present tray the relative heights of the tray skirt, the bulkheads and the cell walls are selected so that, when the tray is floating in a body of water, the cell bases are suspended at least 5 mm above the surface of the body of water.

As described above, the size of the tray top and the relative heights of the cell wall, the bulkheads and the tray skirt may be selected to suit a particular crop. Based on a load weight of the expected maximum weight of the crop (for example the weight of the crop when it is ready to harvest, or ready to be transplanted out of the tray) plus the weight of the cylindrical stabilised media, the tray dimensions are selected to trap an air volume which will ensure that the cell bases are suspended at least 5 mm above the surface of the body of water even when the load weight on the tray is at its maximum.

As shown in the Figures, a ring of buoyancy compartments extends around the perimeter of the tray, with the tray skirt 104 forming an outer wall of the buoyancy compartments in this ring. Further rows of buoyancy compartments are arranged in a grid pattern with rows of buoyancy chambers dividing up the cell-bulkhead-air-chambers. This provides the advantage of spreading out the buoyancy evenly around the tray, to ensure that the tray floats evenly even if some plants grow more quickly than others.

Figures 9 to 12B illustrate cells 106 which are usable in the present tray. Figures 11A and 11 B in particular illustrate the effect of the drain hole position on the volume of air trappable in the cell-bulkhead-air-chambers 116.

In the illustrated embodiments of Figures 1 to 11 B, each cell comprises a raised cell rim 120 which defines the open upper ends of the cells, so that the open upper ends of the cells are elevated to a height of 17-22 mm above the tray top 102. This allows improved air-flow underneath the leaves of the plant, which is beneficial for disease control, and improves access to plant stems during the harvesting process.

The raised cell rim 120 projects upwards above the tray top 102 and is split into four sections, each of which extends around approximately a quarter of the cell. The raised cell rims 120 have a chamfered inner surface 122 which slopes inwards towards the cell. This may advantageously make placement of the cylindrical stabilised media into, and removal of the cylindrical stabilised media from, the cells easier and less likely to damage the rootballs.

The cell wall 106 is shaped to define a cell chamber which is substantially cylindrical, but tapered slightly from a wider upper end to a narrower lower end at the cell base 110.

The raised cell rim 120 is divided into four by four slots 124 which are arranged 90 degrees apart around the cell. The slots extend radially outwards from the cell chamber so that the inner side of the slots are open to the cell chamber. The slots 124 extend from an open upper-end at the tray top, down towards the cell base. The cell wall extends around and encompasses the plurality of slots, so that the slots are positioned inside the cell wall but outside the cell chamber.

Figures 9 to 11 B show a vertical cross-section of the slots, taken radially through the cell. As can be seen in these Figures, the slots 124 taper from a wide upper end at the tray top, to a narrower lower end at or near the cell base 110.

In the illustrated embodiments, the lower end of the slots 124 is positioned level with the cell base 110.

The lower end of each slot 124 has a slot base which is shaped to direct water into the cell chamber. The slots are only open at their top end and at the side facing the cell chamber, so the slots direct water into the sides of the cylindrical stabilised media in the cells, and act as a reservoir to hold water which may be gradually soaked up by the cylindrical stabilised media or drained through the drain hole.

The slots 124 advantageously allow the fingers of a mechanised plant-handling system to be inserted into the slots to place the cylindrical stabilised medium in the cell and/or to pick up and remove the cylindrical stabilised medium from the cell. As the upper ends of the slots are open, a mechanised finger for tray-handling can be inserted downwards into each slot from above the tray top. The four slots 124 allow up to four mechanised fingers to be inserted to pinch the cylindrical stabilised medium before lifting the cylindrical stabilised medium out of the cell 106.

As the outer edges of the slots 124 are enclosed by the cell walls, the slots do not provide any passage for air to pass from the underside of the tray to the upper side of the tray.

The cells 106 illustrated in the Figures are particularly suitable for the present hydroponic plant-growing tray 100. However, the slots 124 and cells 106 shown in Figures 9 to 11 B are usable in plant-growing trays other than the hydroponic plant-growing tray 100, as the benefits provided by the slots 124 are applicable to all automated plant-handling systems, not just those intended for hydroponic propagation.

The cell base 110 is made up of four projections 126 which extend inwards from the cell wall across a portion of the drain hole 112, so that the plurality of projections support the lower end of a cylindrical stabilised medium 128 on top of the projections.

In order to support the cylindrical stabilised medium 128 without becoming entangled in the roots growing from the medium, the length of the projections is approximately 15% of the diameter of the drain hole.

The drain holes occupy the entire area of the cell base 110, apart from the area occupied by the projections 126 for supporting the cylindrical stabilised medium. This maximises the available area for roots growing hydroponically out of the cylindrical stabilised medium.

The drain holes in the cell bases are the only openings through the tray, through which air may escape from the trapped-air area when the tray is placed on a body of water. The cell wall of the cells and the slots do not comprise any other openings, so the cell drain holes are the only openings between the cell and the underside of the tray.

As discussed above, the tray 100 is configured to provide enough buoyancy so that even when the plants are at their heavies, the tray floats high enough on a body of water that the cell bases 110 are positioned at least 5 mm above the surface of the water.

However, the volume of air that is trapped in the cell-bulkhead-air-chambers depends on the height of the drain hole relative to the cell-adjacent bulkheads, as air can escape from the cell-bulkhead-air-chamber upwards through the drain hole unless the passage of air is blocked by water or a cell wall.

In order to control both of these factors independently, the drain hole formed in the cell bases 110 may optionally be connected to a drain tube 130 which extends downwards from the cell base 110 and increases (lowers towards the water) the effective height of the drain hole 112.

Figures 10A, 10B, 11A and 11 B illustrate the tray 100 floating on the surface of a body of water 140, with a cylindrical stabilised medium 128 in the cell.

As shown in Figures 10A and 10B, one way of increasing the height of the cell base 110 above the water surface 140 is by elevating the entire cell relative to the tray top to increase the clearance of the cell base above the water level.

In other preferred embodiments, each cell comprises a drain tube 130 which extends downwards from an upper end of the drain tube, which is connected to the drain hole in the cell base, to an open lower end of the drain tube. The drain tube 130 forms a continuation of the cell wall 108 and effectively extends the drain hole 112 downwards.

In the embodiment shown in Figure 11A, the lower end of the drain tube 130 is positioned at a height which is level with the bottom of the cell-adjacent bulkheads 114 surrounding the cell. The cell-bulkhead-air-chamber 116 surrounding the cell is then formed by the cell-adjacent bulkheads, an outside of the cell wall, an outside of the drain tube and the underside of the tray top, such that air is trapped in the cell-bulkhead-air-chamber when the tray is placed in a body of water. The trapped-air volume contained in the cell-bulkhead-air-chamber 116 is indicated by dashed lines in Figure 11A.

In this embodiment, as the lower end of the drain tube 130 is touching the water level 140, air that is trapped in the cell-bulkhead-air-chamber 116 cannot escape into the drain hole and upwards through the cell. This maximises the volume of air that is trapped in the cellbulkhead-air-chamber 116.

Figure 11 B shows an alternative embodiment having a shorter drain tube 130. In the embodiment shown in Figure 11 B, the lower end of the drain tube 130 is positioned at a height which is above the bottom of the cell-adjacent bulkheads 114 surrounding the cell. The cell-bulkhead-air-chamber 116 surrounding the cell is still formed by the cell-adjacent bulkheads 114, an outside of the cell wall 108, an outside of the drain tube 130 and the underside of the tray top 102, such that air is trapped in the cell-bulkhead-air-chamber when the tray is placed in a body of water. However, in this embodiment the shorter drain tube 130 means that some air can escape from the cell-bulkhead-air-chamber 116 when the tray is first placed into the water. This means that less air is trapped in the cellbulkhead-air-chamber 116 -the trapped-air volume contained in the cell-bulkhead-airchamber 116 is indicated by dashed lines in Figure 11 B, and can be seen to be less than that trapped in Figure 11A.

The difference in the volume of air trapped in the cell-bulkhead-air-chambers 116 in the embodiments of Figures 11A and 11 B affects the height at which the tray 100 floats in the body of water. In the embodiment of Figure 11B, the cell base 110 (and therefore the lower end of the cylindrical stabilised medium) is positioned closer to the water surface 140 than is the case in Figure 11A. The length of the drain tube 130 may thus be controlled to alter the buoyancy of the tray, in order to ensure that the cell bases always float at the desired height of at least 5 mm above the surface of the water.

Figures 12A and 12B illustrate an alternative embodiment of the cells, which are equally usable with the hydroponic plant-growing tray of the present disclosure.

In Figures 12A and 12B, a cell 206 comprises a generally-cylindrical cell chamber 207 surrounded by a cell wall 208. The cell 206 includes a cell base and drain hole as described above for other embodiments of cell.

The cell wall 208 extends downwards from a tray top 204 which comprises a plurality of sloped surfaces surrounding the cell. The sloped surfaces surrounding the cell form a catchment area 210 on the tray top, so that in a multi-cell tray each cell 206 is surrounded by its own respective catchment area. The area of tray top 204 within each catchment area 210 is sloped to direct water towards the cell 206 in that catchment area. These catchment areas advantageously ensure that all water on the tray top, whether rain or water from plant-watering, drains into one of the cells of the tray. Particularly in a floating tray where floating height is carefully controlled, it is undesirable to allow an additional weight of water to sit on the tray top and weigh-down the tray, so these catchment areas both conserve water and solve this problem.

The cells 206 of Figures 12A and 12B may be swapped for the cells 106, and used in hydroponic trays which incorporate all of the tray features described ion relation to Figures 1 to 11 B. In the resulting tray, instead of the tray top being flat, the tray top would be divided into an array of separate catchment areas 210, so that wherever on the tray top 204 water lands, it is directed towards one of the cells.

The hydroponic trays of the present disclosure are preferably formed from injection-moulded plastic, which is heavier than conventional EPS trays, but provides benefits in terms of lifespan, hygiene and environmental-friendliness.

Preferred Aspects Preferred aspects of the invention are defined in the following numbered clauses: 1. A hydroponic plant-growing tray configured to float in a body of water, the tray comprising: a tray top; a tray skirt connected to the tray top, the tray skirt extending downwards from the tray top and defining a perimeter of an air-trapping area below an underside of the tray top; a plurality of bulkheads extending downwards from the underside of the tray top, the plurality of bulkheads forming a plurality of open-bottomed air compartments in the air-trapping area under the tray top; and an array of cells configured to contain substrate for propagating plants, each cell comprising an open upper end, a cell wall which extends downwards from the tray top to a cell base, and a drain hole defined in the cell base; in which the tray top, the tray skirt, the bulkheads and the cell walls are configured to trap air under the tray top when the tray is placed in a body of water; in which the open-bottomed compartments are configured to trap air to generate a tray buoyancy, and in which the tray buoyancy has a magnitude such that, when the tray is floating in a body of water and supporting a load weight of 5 kg, the cell bases are suspended at least 5 mm above the surface of the body of water.

2. The hydroponic plant-growing tray of clause 1, in which the tray buoyancy has a magnitude such that when the tray is floating in a body of water and supporting a minimum load weight, the cell bases are suspended less than 20 mm above the surface of the body of water.

3. The hydroponic plant-growing tray of clause 1 or 2, in which tray buoyancy has a magnitude such that when the tray is floating in a body of water and supporting a load weight of 0.5 kg, the cell bases are suspended less than 20 mm above the surface of the body of water. 4. 5. 6. 7. 8. 9.

The hydroponic plant-growing tray of clause 1, 2 or 3, in which the tray buoyancy has a magnitude such that, when the tray is floating in a body of water and supporting a maximum load weight, the cell bases are suspended at least 5 mm above the surface of the body of water.

The hydroponic plant-growing tray of any preceding clause, in which the tray is configured to support a load weight while the tray is floating in a body of water, in which the tray buoyancy has a magnitude such that, when the tray is floating in a body of water and supporting a load weight of 8 kg, or 10 kg, or 12 kg, the cell bases are suspended at least 5 mm above the surface of the body of water.

The hydroponic plant-growing tray of any preceding clause, in which the load weight is, in use, substrate and a plurality of plants growing from the substrate in the cells of the tray.

The hydroponic plant-growing tray of any preceding clause, in which the tray buoyancy has a magnitude such that, when the tray is floating in a body of water, the cell bases are suspended between 5 mm and 20 mm, or preferably between 8 mm and 12 mm above the surface of the body of water.

The hydroponic plant-growing tray of any preceding clause, in which the tray buoyancy has a magnitude such that, when the tray is floating in a body of water and supporting a load weight of 5 kg, the cell bases are suspended between 5 mm and 20 mm, or preferably between 8 mm and 12 mm above the surface of the body of water.

The hydroponic plant-growing tray of any preceding clause, in which the tray skirt extends below the tray top by a tray skirt height, and in which the tray skirt height is at least 0.04 m, or at least 0.05 m, or at least 0.08 m.

10. The hydroponic plant-growing tray of any preceding clause, in which the tray top, the tray skirt, the bulkheads and the cell walls are configured to trap a trapped air volume when the tray is placed in a body of water.

11. The hydroponic plant-growing tray of any preceding clause, in which the trapped air volume is at least 0.036 m3, or at least 0.043 m3, or at least 0.057 m3.

12. The hydroponic plant-growing tray of any preceding clause, in which the trapped air volume is at least 0.0020 m3per cell, or at least 0.0023 m3 per cell, or at least 0.0031 m3per cell.

13. The hydroponic plant-growing tray of any preceding clause, in which a first plurality of the open-bottomed air compartments are cell-bulkhead-air-chambers into which cells extend downwards from the tray top, and a second plurality of the open-bottomed air compartments are buoyancy compartments which do not contain a cell.

14. The hydroponic plant-growing tray of clause 13, in which the tray is configured so that at least 60%, or at least 70%, or at least 75% of the tray's trapped-air-volume is trapped in buoyancy chambers which do not contain cells 15. The hydroponic plant-growing tray of clause 13 or 14, in which the tray comprises a ring of buoyancy compartments extending around the underside of the tray top, the ring of buoyancy compartments being positioned between the tray skirt and the cells.

16. The hydroponic plant-growing tray of clause 13, 14 or 15, in which the tray comprises a plurality of rows of buoyancy compartments arranged in a grid on the underside of the tray top, the grid being arranged around the cell-bulkhead-air-chambers such that the cell-bulkhead-air-chambers are separated from one another by the rows of buoyancy compartments.

17. The hydroponic plant-growing tray of any preceding clause, comprising a plurality of cell-adjacent bulkheads, in which each cell is surrounded by one or more cell-adjacent bulkheads underneath the tray top, forming a cell-bulkhead-air-chamber between an outside of the cell wall, an inside of the one or more cell-adjacent bulkheads surrounding said cell, and the underside of the tray top, such that air is trapped in the cell-bulkhead-air-chamber when the tray is placed in a body of water.

18. The hydroponic plant-growing tray of clause 17, in which the cell-bulkhead-airchambers occupy 30-45% of the air-trapping area under the tray top, preferably 3540% of the air-trapping area under the tray top.

19. The hydroponic plant-growing tray of any preceding clause, in which each cell comprises a drain tube which extends downwards from an upper end of the drain tube, which is connected to the drain hole in the cell base, to an open lower end of the drain tube.

20. The hydroponic plant-growing tray of clause 19, in which the lower end of the drain tube is positioned at a height which is level with, or higher than, the bottom of the one or more cell-adjacent bulkheads surrounding the cell, such that the cellbulkhead-air-chamber is formed by an inside of the one or more cell-adjacent bulkheads, an outside of the cell wall, an outside of the drain tube and the underside of the tray top, such that air is trapped in the cell-bulkhead-air-chamber when the tray is placed in a body of water.

21. The hydroponic plant-growing tray of any preceding clause, in which the drain hole has a horizontal cross-sectional area of at least 80%, or at least 85%, or at least 90% of an area of the cell base.

22. The hydroponic plant-growing tray of any preceding clause, in which the cell base has a cell base diameter and the drain hole has a drain hole diameter, in which the drain hole diameter is at least 90%, or at least 95%, or equal to, the cell base diameter.

23. The hydroponic plant-growing tray of any preceding clause, in which the cell base comprises a plurality of projections which extend inwards from the cell wall across a portion of the drain hole.

24. The hydroponic plant-growing tray of clause 23, in which the projections of the cell base extend inwards from the cell wall by a projection length, in which the projection length is less than 20%, or less than 15%, or less than 10% of the drain hole diameter.

25. The hydroponic plant-growing tray of any preceding clause, in which the cell wall does not comprise any openings, such that the cell drain hole is the only opening between the cell and the underside of the tray.

26. The hydroponic plant-growing tray of any preceding clause, in which the tray is configured so that the open upper ends of the cells are elevated above the tray top, preferably in which the open upper ends of the cells are a height of 17-22 mm above the tray top.

27. The hydroponic plant-growing tray of any preceding clause, in which each cell comprises a raised cell rim which projects upwards above the tray top and extends around at least a portion of the cell, the raised cell rims defining the open upper ends of the cells.

28. The hydroponic plant-growing tray of clause 27, in which the raised cell rims comprise a chamfered inner surface which slopes inwards towards the cell.

29. The hydroponic plant-growing tray of any preceding clause, in which the cell comprises a cell chamber, optionally in which the cell chamber is tapered from a wider upper end to a narrower cell base, preferably in which the cell chamber at the upper end has a diameter which is less than 15% or less than 10% larger than the diameter of the cell chamber at the cell base.

30. The hydroponic plant-growing tray of any preceding clause, in which each cell comprises a plurality of slots located around the cell, in which the slots are open to the cell chamber and extend downwards from the tray top towards the cell base.

31. The hydroponic plant-growing tray of clause 30, in which the cell wall extends around and encompasses the plurality of slots, the slots being positioned inside the cell wall but outside the diameter of the cell chamber, and in which a vertical cross-section of the slots tapers from a wide upper end to a narrower lower end.

32. The hydroponic plant-growing tray of clause 30 or 31, in which the lower end of the slots is positioned above the cell base, and in which the lower end of the slots have a slot base which is configured to direct water into the cell chamber.

33. The hydroponic plant-growing tray of clause 30, 31 or 32, in which the slots are configured to allow fingers of a mechanised plant-handling system to be inserted into the slots to place a plant rootball in the cell and/or to pick up and remove the rootball from the cell.

34. The hydroponic plant-growing tray of any of clauses 30 to 34, in which the lower ends of the slots is positioned level with the cell base 35. The hydroponic plant-growing tray of clauses 30 to 35, in which the upper ends of the slots are positioned level with the tray top, so that water on the tray top is configured to flow into the slots.

36. The hydroponic plant-growing tray of any preceding clause, in which the tray top comprises a plurality of sloped catchment areas, each cell being associated with a respective catchment area, in which the tray top within each catchment area is sloped to direct water towards the associated cell.

37. The hydroponic plant-growing tray of any preceding clause, in which the tray top has an area of at least 0.035 m2, or 0.04 m2 per cell.

38. The hydroponic plant-growing tray of any preceding clause, in which the tray contains 12 to 60 cells, for example 18 to 50 cells, preferably only 12, or 18, or 50 cells.

39. A hydroponic plant-growing tray configured to float in a body of water, the tray comprising: a tray top; a tray skirt connected to the tray top, the tray skirt extending downwards from the tray top and defining a perimeter of an air-trapping area below an underside of the tray top; a plurality of bulkheads extending downwards from the underside of the tray top, the plurality of bulkheads forming a plurality of open-bottomed air compartments in the air-trapping area under the tray top; and an array of cells configured to contain a substrate for propagating plants, each cell comprising an open upper end, a cell wall which extends downwards from the tray top to a cell base, and a drain hole defined in the cell base; in which the tray top, the tray skirt, the bulkheads and the cell walls are configured to trap air under the tray top when the tray is placed in a body of water; and in which the relative heights of the tray skirt, the bulkheads and the cell walls are configured so that, when the tray is floating in a body of water, the cell bases are suspended at least 5 mm above the surface of the body of water.

40. A hydroponic plant-growing tray configured to float in a body of water, the tray comprising: a tray top; a tray skirt connected to the tray top, the tray skirt extending downwards from the tray top and defining a perimeter of an air-trapping area below an underside of the tray top; a plurality of bulkheads extending downwards from the underside of the tray top, the plurality of bulkheads forming a plurality of open-bottomed air compartments in the air-trapping area under the tray top; and an array of cells configured to contain a substrate for propagating plants, each cell comprising an open upper end, a cell wall which extends downwards from the tray top to a cell base, and a drain hole defined in the cell base; in which the tray top, the tray skirt, the bulkheads and the cell walls are configured to trap air under the tray top when the tray is placed in a body of water; and in which the open-bottomed compartments are configured so that, when the tray is floating in a body of water, the cell bases are suspended at least 5 mm above the surface of the body of water.

41. A hydroponic plant-growing raft configured to float in a body of water, the raft comprising: a raft top; a raft skirt connected to the raft top, the raft skirt extending downwards from the raft top and defining a perimeter of an air-trapping area below an underside of the raft top; a plurality of bulkheads extending downwards from the underside of the raft top, the plurality of bulkheads forming a plurality of open-bottomed air compartments in the air-trapping area under the raft top; and an array of cells configured to contain a substrate for propagating plants, each cell comprising an open upper end, a cell wall which extends downwards from the raft top to a cell base, and a drain hole defined in the cell base; in which the raft top, the raft skirt, the bulkheads and the cell walls are configured to trap air under the raft top when the raft is placed in a body of water; in which the open-bottomed compartments are configured to trap air to generate a raft buoyancy, in which a first plurality of the open-bottomed air compartments are cell-bulkhead-air-chambers into which cells extend under the raft top, and a second plurality of the open-bottomed air compartments are buoyancy compartments which do not contain a cell.

Claims

Claims 1. A hydroponic plant-growing tray configured to float in a body of water, the tray comprising: a tray top; a tray skirt connected to the tray top, the tray skirt extending downwards from the tray top and defining a perimeter of an air-trapping area below an underside of the tray top; a plurality of bulkheads extending downwards from the underside of the tray top, the plurality of bulkheads forming a plurality of open-bottomed air compartments in the air-trapping area under the tray top; and an array of cells configured to contain a substrate for propagating plants, each cell comprising an open upper end, a cell wall which extends downwards from the tray top to a cell base, and a drain hole defined in the cell base; in which the tray top, the tray skirt, the bulkheads and the cell walls are configured to trap air under the tray top when the tray is placed in a body of water; in which the open-bottomed compartments are configured to trap air to generate a tray buoyancy, in which a first plurality of the open-bottomed air compartments are cell-bulkhead-air-chambers into which cells extend under the tray top, and a second plurality of the open-bottomed air compartments are buoyancy compartments which do not contain a cell.
2. The hydroponic plant-growing tray of claim 1, in which the tray buoyancy has a magnitude such that, when the tray is floating in a body of water and supporting a load weight of 5 kg, the cell bases are suspended at least 5 mm above the surface of the body of water.
3. The hydroponic plant-growing tray of claim 1 or 2, in which tray buoyancy has a magnitude such that when the tray is floating in a body of water and supporting a load weight of 0.5 kg, the cell bases are suspended less than 20 mm above the surface of the body of water.
4. The hydroponic plant-growing tray of claim 1, 2 or 3, in which the tray buoyancy has a magnitude such that, when the tray is floating in a body of water and supporting a load weight of 8 kg, or 10 kg, or 12 kg, the cell bases are suspended at least 5 mm above the surface of the body of water.
5. The hydroponic plant-growing tray of any preceding claim, in which the load weight is, in use, a plurality of plant rootballs and plants growing from the rootballs in the cells of the tray.
6. The hydroponic plant-growing tray of any preceding claim, in which the tray buoyancy has a magnitude such that, when the tray is floating in a body of water and supporting a load weight of between 0.5 kg and 12 kg, the cell bases are suspended between 5 mm and 20 mm above the surface of the body of water.
7. The hydroponic plant-growing tray of any preceding claim, in which the tray buoyancy has a magnitude such that, when the tray is floating in a body of water and supporting a load weight of 5 kg, the cell bases are suspended between 5 mm and 20 mm, or preferably between 8 mm and 12 mm above the surface of the body of water.
8. The hydroponic plant-growing tray of any preceding claim, in which the tray skirt extends below the tray top by a tray skirt height, and in which the tray skirt height is at least 0.04 m, or at least 0.05 m, or at least 0.08 m, or at least 0.10 m.
9. The hydroponic plant-growing tray of any preceding claim, in which the tray top, the tray skirt, the bulkheads and the cell walls are configured to trap a trapped air volume when the tray is placed in a body of water, in which the trapped air volume is at least 0.036 m3, or at least 0.043 m3, or at least 0.057 m3.
10. The hydroponic plant-growing tray of any preceding claim, in which the tray is configured to trap a trapped air volume of at least 0.0020 ma per cell, or at least 0.0023 m3 per cell, or at least 0.0031 m3 per cell.
11. The hydroponic plant-growing tray of any preceding claim, in which the tray is configured so that at least 60%, or at least 70%, or at least 75% of the tray's trapped-air-volume is trapped in buoyancy chambers which do not contain cells.
12. The hydroponic plant-growing tray of any preceding claim, in which the tray comprises a ring of buoyancy compartments extending around the underside of the tray top, the ring of buoyancy compartments being positioned between the tray skirt and the cells, and/or in which the tray comprises a plurality of rows of buoyancy compartments arranged in a grid on the underside of the tray top, the grid being arranged around the cell-bulkhead-air-chambers such that the cell-bulkhead-air-chambers are separated from one another by the rows of buoyancy compartments.
13. The hydroponic plant-growing tray of any preceding claim, comprising a plurality of cell-adjacent bulkheads, in which each cell is surrounded by one or more cell-adjacent bulkheads underneath the tray top, forming a cell-bulkhead-air-chamber between an outside of the cell wall, an inside of the one or more cell-adjacent bulkheads surrounding said cell, and the underside of the tray top, such that air is trapped in the cell-bulkhead-air-chamber when the tray is placed in a body of water.
14. The hydroponic plant-growing tray of claim 13, in which the cell-bulkhead-air- chambers occupy 30-45% of the air-trapping area under the tray top, preferably 35- 40% of the air-trapping area under the tray top.
15. The hydroponic plant-growing tray of any preceding claim, in which each cell comprises a drain tube which extends downwards from an upper end of the drain tube, which is connected to the drain hole in the cell base, to an open lower end of the drain tube.
16. The hydroponic plant-growing tray of claim 15, in which the lower end of the drain tube is positioned at a height which is level with, or higher than, the bottom of the one or more cell-adjacent bulkheads surrounding the cell, such that the cell-bulkhead-air-chamber is formed by an inside of the one or more cell-adjacent bulkheads, an outside of the cell wall, an outside of the drain tube and the underside of the tray top, such that air is trapped in the cell-bulkhead-air-chamber when the tray is placed in a body of water.
17. The hydroponic plant-growing tray of any preceding claim, in which the cell base has a cell base diameter and the drain hole has a drain hole diameter, in which the drain hole diameter is at least 90%, or at least 95%, or equal to, the cell base diameter.
18. The hydroponic plant-growing tray of any preceding claim, in which the cell base comprises a plurality of projections which extend inwards from the cell wall across a portion of the drain hole, preferably in which the projections of the cell base extend inwards from the cell wall by a projection length, in which the projection length is less than 20%, or less than 15%, or less than 10% of the drain hole diameter.
19. The hydroponic plant-growing tray of any preceding claim, in which each cell comprises a plurality of slots located around the cell, in which the slots are open to the cell and extend downwards from the tray top towards the cell base.
20. The hydroponic plant-growing tray of claim 19, in which the cell wall extends around and encompasses the plurality of slots, the slots being positioned inside the cell wall but outside the diameter of the cell chamber, and in which a vertical cross-section of the slots tapers from a wide upper end to a narrower lower end.
21. The hydroponic plant-growing tray of claim 19 or 20, in which the lower end of the slots is positioned level with the cell base, or above the cell base, and in which the lower end of the slots have a slot base which is configured to direct water into the cell chamber.
22. The hydroponic plant-growing tray of claim 19, 20 or 21, in which the slots are configured to allow fingers of a mechanised plant-handling system to be inserted into the slots to place the plant rootball in the cell and/or to pick up and remove the plant rootball from the cell.
23. The hydroponic plant-growing tray of claims 19 to 22, in which the upper ends of the slots are positioned level with the tray top, so that water on the tray top is configured to flow into the slots.
24. The hydroponic plant-growing tray of any preceding claim, in which the tray top comprises a plurality of sloped catchment areas, each cell being associated with a respective catchment area, in which the tray top within each catchment area is sloped to direct water towards the associated cell.
25. The hydroponic plant-growing tray of any preceding claim, in which the cells are configured to contain cylindrical stabilised media for propagating plants, each cell comprising an open upper end, a cell wall which extends downwards from the tray top to a cell base which is configured to support a lower end of the cylindrical stabilised medium, and a drain hole defined in the cell base.
26. The hydroponic plant-growing tray of any preceding claim, in which the tray comprises at least 5 buoyancy chambers, or at least 10 buoyancy chambers, or at least 15 buoyancy chambers, or at least 25 buoyancy chambers, or at least 50 buoyancy chambers, or at least 70 buoyancy chambers.