Materials for use in plant propagation

Abstract: A low density (5-12 Kgs/M³), rigid, unicellular (i.e. closed cell) expanded polyurethane foam is perforated through at least part of its thickness by mechanical means (20a,20b), resulting in a hydrophilic, heterocellular (open cell) foam, whereby to facilitate the passage of emergent plant roots and also to provide groups of enlarged voids (in contrast to the fine cell structure of the unmodified foam) so as to facilitate water absorption and retention.

Description: The present invention is concerned with synthetic plastics materials for use in plant propagation, and in particular to such materials in modular form. The traditional method of rooting cuttings and raising seedlings has involved the use of natural materials such as soil, peat and the like. In recent years, however, such natural materials have been replaced to a large extent by the use of synthetic plastics-based materials. These can be in a wide variety of forms, for example granular, shredded or cellular. It is with the latter, cellular type of material that the present invention is particularly concerned. To provide the optimum conditions for either the successful rooting of cuttings or seed raising, a synthetic substrate must be permeable both to air and to the developing root structure. It must also be easily wettable by water and exhibit adequate levels of moisture retention. Where the substrate is a continuous phase (as opposed to granular or shredded particles) cellular plastics material, for example phenolics or polyurethanes, permeability to air and the provision of adequate moisture (and moisture retention) are, to a significant extent, mutually exclusive objectives. This arises as a result of the poor capillarity of conventional cellular materials, consisting of substantially similar pore size. It is known from GB-A-2036523 that root development of plants grown using a synthetic foam plastics substrate can be improved by forming perforations through the thickness of a foam panel. Such perforations form air-passages which occupy from 5-40% of the volume of the foam panel and preferably, extend from the top to the bottom of the panel. It is a principal object of the present invention to provide an improved plant propagation body for use in the rooting of cuttings and in the raising of seedlings. In accordance with a first aspect of the present invention there is provided a plant propagation body formed of rigid, unicellular expanded polyurethane foam which has a density in the range 5-12 Kg/M³, said body being perforated through at least part of its thickness to result in a hydrophilic, heterocellular foam structure which defines groups of enlarged voids to facilitate water absorption and retention, characterised in that said body comprises a single cuboidal block of said foam provided with two opposed, parallel, coplanar slots extending into the foam block from opposite ends thereof, the foam having a grain which is substantially parallel to the plane containing said slots, whereby the deformation of the foam along an axis perpendicular to the plane of the slots and half-way between these slots will cause formation of a cruciform recess for receiving a plant to be propagated. In accordance with a second aspect of the present invention there is provided a method of forming a plant propagation unit having a plant-receiving recess, comprising forming two opposed, parallel, coplanar slots in opposite end surfaces of a cuboidal block of unicellular, expanded polyurethane foam, having a density in the range 5-12 Kg/M³, the foam having a grain which is substantially parallel to the plane containing said slots; perforating said block of foam through at least part of its thickness whereby to result in a hydrophilic, heterocellular foam structure defining groups of enlarged voids to facilitate water absorbtion and retention; positioning a bottom surface of the cuboidal foam block over the mouth of a receptacle; applying a tool to the block to engage its top surface along a line perpendicular to the plane of said slots and half-way between the slots; displacing the tool such as to force the block into the receptacle so that the opposite end surfaces, end up substantially in a common plane; and then withdrawing the tool to leave the two slots and the junction between the two end surfaces of the block together defining a cruciform slot for receiving a plant to be propagated. The perforations can be effected by any suitable means. For example, in one arrangement, perforation can be achieved by means of a toothed or pronged tool (or tools) which is forced into a body of unmodified foam. In another arrangement, the perforations can be achieved by the application of intermittent pulses of compressed air, applied via jets arrayed in groups and placed in intimate contact with the surface of unmodified polyurethane foam. The invention is described further hereinafter, by way of example only, with reference to the accompanying drawings, in which:- Figs. 1 and 2 are diagrammatic illustrations showing two possible ways of forming a material for use in a plant propogation body in accordance with the present invention; Fig.3 shows a foam block in accordance with one embodiment of this invention; Fig.4a is a diagrammatic side section illustrating one use of the embodiment of Fig.3; Fig.4b is a plan view of the structure formed in accordance with Fig.4a; Fig.4c shows the foam grain in the resulting unit; Fig.5a is a diagrammatic plan view corresponding to Fig. 4b but illustrating the insertion of the embodiment of Fig. 3 into a square sectional receptable; Fig.5b corresponds to Fig.4b; and Fig.5c shows front and side views of the tool used in the operation of Figs. 4b and 5a. The present invention makes use of low density, rigid, polyurethane foams having closed (uni-cellular) elongate cells. The elongation of the cells occurs uniform in a given plane within the material and, in effect, results in the material having an identifiable 'grain'. The term 'rigid' is one used commonly in the polyurethane industry and distinguishes a foam from a 'flexible' foam. A rigid polyurethane foam can either be open celled or closed celled, usually the latter, since the major uses of closed celled foam lies in insulation and buoyancy applications. A normal typical open cell rigid polyurethane has a density typically of the order of 32 kgs/m³, its rigidity deriving from the physical properties on the polyurethane formulation itself. and distinguishes a foam from a 'flexible' foam. A rigid polyurethane foam can either be open celled or closed celled, usually the latter, since the major uses of closed celled foam lies in insulation and buoyancy applications. A normal typical open cell rigid polyurethane has a density typically of the order of 32 kgs/m³, its rigidity deriving from the physical properties on the polyurethane formulation itself. By the term "low density" as used herein, we mean a foam having a density of the order of 5-12 kgs/m³. Reduction of density to 5-12 kgs/m³ dramatically reduces the physical properties of the polyurethane cell wall membranes. The apparent rigidity (or stiffness) is provided by gas trapped in the closed walls. If these cells are ruptured then the foam loses its stiffness and is then somewhat analogous to a flexible foam, with, however, very limited resilience and a very high compression set. The stiffness of the original closed cell foam provides an aid to processing in that it facilitates cutting and perforation (as now described). In the formation of a plant propogation body in accordance with the present invention, low density rigid foam strip is perforated to at least partially break down the closed cell structure. This can be achieved by a number of means, two examples of which are illustrated in Figs. 1 and 2. The mechanical perforation means illustrated in Fig.1 comprises a pair of opposed rollers 20a, 20b, each carrying a plurality of radial pins 22. These pins are staggered between adjacent radial rows. In a typical example, the pins are staggered at 18mm pitch. A strip of the low density foam is passed through the rollers which are rotated as indicated by the arrows. By this means the closed cells are progressively ruptured by the pins. The process of perforation/texturing may include passage of the foam through rollers, ensuring that substantially all cells are ruptured. Fig.2 shows a means of obtaining perforation pneumatically by pulsing jets of air at 100 psi into both sides of a strip of the low density foam in a predetermined pattern. When the material thus formed is deformed into profiled containers having a volume less by a predetermined factor than that of the original foam to be introduced, the cells on the outer walls rupture first, so that an increase in density occurs differentially, and beneficially, on the outer surfaces of the foam in contact with the container walls. This densification is beneficial since the increased density exhibits a higher level of water absorption/retention in those areas away from the stem of the plant, as will be seen. Since nutrients are supplied in solution, nutrient salt concentrations will be at the highest in the enhanced density areas and lowest in the area around the plant stem. Also, the deformation of the foam when forced into a profiled container, given its high compression set, is permanent. Thus, the starting point is a non-hydrophilic foam, which, by perforation/texturing, cell rupturing and subsequent densification, results in a hydrophilic foam providing a beneficial air:water ratio, with good water absorption, retention and drainage. Further water has to be readily available to the plants' roots. Very small pore sizes reduce availability because of surface tension; water contained in the larger voids is more readily available. The perforation/texturing provides a series of what will become a pattern of voids on the surface of the resultant growing units, enhancing (in combination with the fine cells of the main foam structure) capillarity and water retention. Additionally, the structure of the growing units must allow for the free passage of emergent roots. Whilst the roots of plants which have a strong vigorous growth habit will readily penetrate the undensified areas of the foam, less vigorous roots would be impeded and growth stunted. Even vigorous roots however, cannot penetrate the densified areas, unless their passage is facilitated by the presence of passages provided by the perforating processes. The pattern and depth of the perforations are designed so that for a given thickness of foam the surface is uniformly textured and the length of the spikes used for perforation/texturing carried on rollers on the upper and lower surfaces of the foam strip and their relative position is such that adequate passages for root growth are formed even after subsequent deformation. Figs. 3 to 5 illustrate another embodiment of the ruptured/textured material described above. A block 30 of foam is formed with two slots/slits 32a,32b in opposite edges and is subjected to perforation as described above. The direction of the grain of the foam is indicated in Fig. 3 by the arrow A, i.e. vertical when viewed in Fig. 3. Thus the slots/slits 32a, 32b lie in planes parallel to the grain direction A. A typical block size is 110mm x 41mm x 25mm. The slit foam block blank so formed is placed over a tapered recess of circular section of a vacuum formed tray beneath a tongue-like insertion tool 34 (see Fig.7c) which can be displaced vertically. By lowering the tool 34, the blank is forced into the tray recess so that the opposite end surfaces 36a,36b end up in a substantially horizontal plane level with the top of the recess, the two slots 32a,32b and the join between the two ends of the block together defining a cruciform slot as shown in plan in Fig.6a. It will be appreciated that when this distortion of the blank has taken place, the foam grain in the region marked 3 will still be substantially vertical (when viewed in Fig. 3), but the grain in the region marked c will be substantially horizontal. [Fig.7 shows the same situation in relation to a tapered recess of rectangular section]. Seedlings or plant cuttings can be introduced into such an arrangement to provide an ideal growing environment. Root development is influenced and facilitated by the cruciform interstices provided in the growing units formed in the plastic trays. This construction has a number of other advantages. Firstly, the insertion of plant cuttings of widely differing stem diameters can be accommodated. Secondly, when a plant rooted in such a module is transplanted into compost, as the stem diameter increases the four upright segments will move outwards to accommodate. This latter is even more advantageous where a plantlet which subsequently forms a bulb is rooted in the growing units; initially the increase in volume at the base of the plant is taken up by foam densification, subsequently in earth or compost, further increase in volume displaces the vertical components outwards. Plantlets raised by tissue culture methods, having been rooted in agar are already rooted, it can be seen that by partially removing the growing units from their container, the four vertical elements can be pushed apart to allow the roots to be accommodated, then, when the unit is pushed back into the tapered container, the plant stem and roots are held in place as the unit takes up its original shape. It can be seen that the same method is equally of utility in facilitating the planting of large seeds and small bulbs or bubils. Insertion is easy, and as the foam is pushed back into its container, the foam surrounding the seed/bulb densifies to form an intimate interface with the plant material.

Claims: A plant propogation body formed of rigid, unicellular expanded polyurethane foam which has a density in the range 5-12 Kg/M³, said body being perforated through at least part of its thickness to result in a hydrophilic, heterocellular foam structure which defines groups of enlarged voids to facilitate water absorption and retention, characterised in that said body comprises a single cuboidal block (30) of said foam provided with two opposed, parallel, coplanar slots (32a,32b) extending into the foam block from opposite ends (36a,36b) thereof, the foam having a grain which is substantially parallel to the plane containing said slots (32a,32b), whereby the deformation of the foam along an axis perpendicular to the plane of the slots (32a,32b) and half-way between these slots will cause formation of a cruciform recess for receiving a plant to be propagated. A method of forming a plant propagation unit having a plant-receiving recess, comprising forming two opposed, parallel, coplanar slots (32a,32b) in opposite end surfaces (36a,36b) of a cuboidal block (30) of unicellular, expanded polyurethane foam, having a density in the range 5-12 Kg/M³, the foam having a grain which is substantially parallel to the plane containing said slots (36a,36b); perforating said block of foam (30) through at least part of its thickness whereby to result in a hydrophilic, heterocellular foam structure defining groups of enlarged voids to facilitate water absorbtion and retention; positioning a bottom surface of the cuboidal foam block (30) over the mouth of a receptacle; applying a tool (34) to the block to engage its top surface along a line perpendicular to the plane of said slots and half-way between the slots; displacing the tool (34) such as to force the block (30) into the receptacle so that the opposite end surfaces (36a,36b) end up substantially in a common plane; and then withdrawing the tool to leave the two slots (32a,32b) and the junction between the two end surfaces (36a,36b) of the block together defining a cruciform slot for receiving a plant to be propagated.

Inventors: PERRIN ALAN PHILIP, PERRIN, ALAN PHILIP
Applicants: SYNTHETIC SUB STRATES HOLDINGS, SYNTHETIC SUB-STRATES (HOLDINGS) LIMITED
Application year: 1989
Main classification: A01G31
IPC codes: A01G31, A01G9