It is known that plants can be grown in mineral wool growth substrates. Such growth substrates are typically provided as a coherent plug, block, slab or mat/blanket and generally include a binder, usually an organic binder, in order to provide structural integrity to the product.
Typically, the growth process of the plant is managed in two stages: a first stage managed by a “propagator” in which the plant is grown from seed; and a second stage managed by a “grower” during which the plant is sustained and any harvest taken. For example, in the case of the tomato plant, the propagator may plant individual tomato seeds in cylindrical plugs having a thickness in the order of 25-30 mm and a radius of around 20-30 mm. After germination of the seed, the propagator places the plug within a cuboid block to allow further growth of the root system and the plant. The individual plant within the block is then nursed until a stage when it can be transferred from the propagator to the grower.
Although typically only a single plant is provided in each block, it is possible for multiple plants to be provided in a single block. In some examples, a single plant in a block is split into two by splitting a stem during an early phase of growth, resulting in two plants sharing a single root system. In another alternative, multiple plants may be grafted together and grown within a single block.
The use of a separate plug and block by the propagator is not essential for all plants, but has been described, for example, in European patent application EP2111746, as providing a number of advantages. In particular, the small size of the plug allows more regular watering of the plant in the initial stage without saturating its substrate.
After they are received from the propagator, the grower places a number of blocks on a single slab of mineral wool to form a plant growth system. The slab of mineral wool is typically encased in a foil or other liquid impermeable layer except for openings on an upper surface for receiving the blocks with the plants and a drain hole provided on the bottom surface.
During subsequent growth of the plant, water and nutrients are provided using drippers which deliver a liquid containing water and nutrients to the system either directly to the blocks or to the slabs. The water and nutrients in the blocks and slabs is taken up by the roots of the plants and the plants grow accordingly. Water and nutrients which are not taken up by the plant either remain in the substrate system or are drained through the drain hole.
There is a desire to use water and nutrients as efficiently as possible during the growing process. This is both for cost and environmental reasons. In particular, the nutrients are expensive to obtain, while waste water containing such nutrients is difficult to dispose of due to environmental legislation. The desire to avoid such waste is matched by a desire to improve plant growth conditions, and thereby to increase the yield and quality of fruit obtained from plants in this manner.
The use of mineral wool itself provides significant benefits in this regard as compared to traditional soil-based growing methods, but there is an ongoing requirement to further improve these characteristics. In particular, there is a conflicting desire to both produce more and consume less in plant growth processes. That is, a greater yield from the plants is desired while at the same time reducing the amount of water and/or nutrients that are used. In practice, existing growing methods and/or substrates provide limitations on both these aspects.
In nature, plant growth can be understood and divided into two distinct phases: vegetative growth and generative growth. During vegetative growth, the plant predominantly grows leaves and other green elements which enable it maximise its photosynthetic potential. During the generative growth stage the plant predominantly starts to set and grow fruits. A higher proportion of growth during a generative phase is directed towards flowers and fruit, while the absolute overall biomass grown during a vegetative phase is higher but is predominantly directed to leaves.
It is known that whether a plant enters a generative growth phase or a vegetative phase depends in large part on the water and nutrients the plant receives or has access to within the substrate. In particular, when the plant is stressed (i.e. when it receives relatively little water and/or nutrients) generative growth begins whereas when the plant is provided with plentiful water and/or nutrition vegetative growth becomes predominant.
In view of this, it is known to “steer” plants towards vegetative or generative growth by adopting an irrigation strategy in which the amount of water provided to the plant varies. For example, in practice, the mineral wool slab described above may typically be saturated prior to the placement of the plant-containing block upon the surface of the slab. This is in order to ensure that the roots from the plant in the block are encouraged to grow into the slab.
Once the block has been placed on the slab, it is known for the grower to attempt to apply a “generative impulse” to the plant by reducing the water content of the slab. The grower is keen for the plant to flower (and thus provide fruit) as soon as possible in order to obtain products/fruits which can be sold. Once this process is initiated, however, the grower wishes to encourage the overall growth of the plant. To do so, the grower then increases the water content in the slab to a level appropriate for vegetative growth. Thus, the grower steers the plant between vegetative growth and generative growth.
However, in practice, the steering of plant growth in this manner is crude and inefficient. In particular, the grower does not have accurate control over the overall water content in each slab, and moreover there are typically wide variations in the water content within the slab (for example, the water content is typically higher at the bottom of the slab than at the top). It is found in this context that to achieve a generative impulse having the desired effect, the water content level during the generative impulse must be significantly lower than that required during a subsequent continuous growth phase.
One attempt to improve the distribution of water and nutrients in a substrate is described in European patent application EP0300536. In this document, a system is described in which multiple blocks are provided on a single slab and a capillary system is used to apply a constant suction pressure across the slab. The aim of this suction pressure is to draw more water in to areas that become relatively dry, thereby increasing the uniformity of the water distribution across the slab.
In practice, there are a number of drawbacks with the system described in EP0300536. In particular, while there may be some benefit in terms of the uniformity of the water distribution across the slab in the horizontal plane, this does not provide significant benefits in terms of the vertical variation in water content that has been found. Moreover, the system of EP0300536 is relatively expensive to implement.
Using conventional techniques, there remain significant difficulties in steering plant growth between generative and vegetative states. The lack of uniformity within the slab means that the true plant growth conditions cannot be closely controlled. Moreover, the control that is possible is cumbersome, requiring a significant time period to transfer between a generative impulse and a subsequent sustainable growth state.