Foams

Abstract

A foam comprising water and a salt of a hydroxy fatty acid, where the hydroxy fatty acid salt does not only comprise an ethanolamine or hexanolamine salt of 12-hydroxystearic acid (12-HSA). A foam and

Description

Field of the invention The present invention relates to aqueous foams which can be used in agriculture, for example to protect plants from cold, heat and / or pests, and / or to supply nutrients to plants. Background of the invention There are plenty of causes of damage to plants, including frost and cold weather, heat, pests, disease and malnutrition. Damage to plants may result in plant death, inhibit their growth, prevent their ability to reproduce and / or make them more susceptible to disease. Damage to plants is clearly undesirable, particularly where the plants are being grown for the production of food or drinks such as wine. Frost and cold weather can lead to significant damage in plants. Such damage can occur directly to the plant, for example in the leaves. This may occur when water that is present in all plant cells begins to freeze. Since water expands when it freezes this process will damage the plant cells, for example by rupturing the cell wall. Dissolved salts within plant sap prevents the liquid content of plants from freezing above -2°C. Frost can therefore cause damage, and possibly death, to the whole plant, particularly if temperatures fall below -2°C Plants can also be damaged by hot weather, for example because water is lost from leaves more quickly than it can be replaced (e.g. leaf scorch). Pests can also cause damage to plants, for example because pests such as birds or insects may eat all or part of the plant. Due to climate change, damage to plants from extremes of temperatures (either cold or hot) may become more common. In addition, pests that were previously not able to survive in a certain region may now be able to survive or thrive in the same region, due to the change in climate. Climate change may therefore in some instances exacerbate the issues caused by weather and / or pests, particularly in an agricultural setting. An additional problem caused by climate change is that bud break, the process where buds start to grow after a period of colder weather (e.g. in spring, after winter), is now often happening earlier in the season than it would have historically, due to the warmer weather and / or climate. This can leave the newly exposed buds at risk from a late frost. Applying water (or a water-based solution) directly to plants can help to prevent damage caused by the cold (e.g. frost damage) because the water can freeze on the plant, thereby providing a release of latent heat and / or acting as an insulating layer. However, significant volumes of water and / or other chemicals are needed, which is both environmentally unfriendly and can cause significant issues at the site of application either at the time of application or after the water melts (e.g. due to flooding or waterlogged ground). In fact, such significant volumes of water are needed for this process that it is practically unfeasible in most cases. There is therefore a need to provide a method for protecting plants from physical damage, such as frost and cold weather, heat and pests, which uses as little water as possible. It would also be useful to provide a method of supplying nutrients to plants, for example to help prevent plant malnutrition in extreme conditions of the type that may be caused or become more common due to climate change. It would also be desirable to provide a method for protecting plants from physical damage, such as frost and cold weather, where the product or composition used actively releases and / or distributes heat to keep the temperature around the plant above freezing for as long as possible. Such a mechanism may be more effective than simply forming an insulating layer around the plant. Summary of the invention In one aspect the present invention provides a foamable composition comprising water and a salt of a hydroxy fatty acid. In one aspect the present invention provides a foam comprising water and a salt of a hydroxy fatty acid. Preferably the hydroxy fatty acid is hydroxystearic acid. In another aspect the present invention provides a method of protecting one or more plants, comprising applying to the one or more plants a foam of the invention. In one aspect the present invention provides a method of protecting one or more plants, comprising applying to the soil surrounding the one or more plants a foam of the invention. In one aspect the present invention provides a method of reducing and / or preventing the growth of weeds or other unwanted plants, comprising applying to soil a foam of the invention. In one aspect the present invention provides a method of reducing and / or preventing the growth of weeds or other unwanted plants, comprising applying to the weeds or other unwanted plants a foam of the invention. Also provided is a method of supplying nutrients to one or more plants, comprising applying to the one or more plants a foam of the invention. Also provided is the use of a foam as described herein in agriculture. Also provided is the use of a foam as described herein to protect a plant. Also provided is the use of a foam as described herein to supply nutrients to a plant. List of Figures Figures 1-12 show the temperature of probes covered in different foams compared with the temperature of probes which were left uncovered in a freezer. Figure 13 show the temperature of probes covered in different foams compared with the temperature of probes which were left exposed to air during a frost event. Figure 14 shows an image of a protective layer or film formed after a foam of the invention had been applied to a branch and then dried. Detailed description As highlighted previously, plant damage as a result of frost and cold weather, heat, pests, disease and malnutrition can be problematic. In particular, frost damage in plants is a common problem during winter months in some climates, and may slow plant growth or result in plant death. The present invention therefore provides an aqueous foam which may be used to protect plants. As used herein, the term “protect plants” generally means to protect plants from environmental conditions or factors, including but not limited to undesirable temperatures, pests (including birds and / or insects), and bacteria and fungi. An undesirable temperature is any temperature which would cause damage to an unprotected plant. Undesirable temperature may be temperatures considered to be too hot or too cold. This may therefore include unusual, unexpected or undesirable high temperatures (e.g. a heatwave), and unusual, unexpected or undesirable cold temperatures (e.g. temperature below the freezing point of water (0°C), temperatures below -2°C, or temperatures resulting in frost). The skilled person would recognise that the temperature or temperatures which are considered to be undesirable may depend on factors such as the local climate and the plants or crop in question. For example, for a plant adapted to grow in a warmer climate an undesirable minimum temperature may be higher than an undesirable minimum temperature for a plant adapted to grow in a colder climate. Preferably, the present invention provides foams which may be used to protect plants from frost, or from temperatures below about 0°C or from temperatures below about -2°C. When applied to the plant, the foam forms an insulating layer which can act to keep the plant warmer than the surrounding environment. This is particularly useful overnight when the ambient temperature drops. The foam can therefore help to insulate the plant from this drop in temperature. Furthermore, without wishing to be bound by theory, it has now been found that the latent heat released as the water in the foam freezes can help to maintain the temperature of the plant when the ambient temperature drops to or below the freezing point of water, i.e. 0°C, or when the ambient temperature drops to or below -2°C. That is, unlike conventional foams designed primarily to impede heat transfer and passively insulate plant surfaces, the foams of the present invention are believed to operate by actively promoting the controlled nucleation of ice at approximately 0°C, and rapidly conducting the resulting latent heat of fusion throughout the foam. This approach fundamentally reverses the conventional paradigm: rather than merely slowing cooling, the composition ensures that freezing initiates predictably and that the associated thermal energy is distributed to maintain surface temperatures precisely at the phase change threshold. Thus, in the present invention phase transition is leveraged as a protective measure, rather than being passively delayed or suppressed. The foams of the invention can therefore be particularly effective in insulating plants when the ambient temperature drops below freezing (i.e. when the ambient temperature drops to 0°C or below), or when the ambient temperature drops to or below -2°C. At this point the foam can freeze, and latent heat is released and absorbed by the plant. The heat can also spread throughout the foam, for example via conduction and / or convection. This effect can help to maintain the temperature of the plants above the temperature at which plant damage can occur (e.g. -2°C). It has been found that the inclusion of a salt of a hydroxy fatty acid (HFA), such as choline 12-hydroxystearate, may provide the above-mentioned effects. Surprisingly, it has also been found that foams comprising alternative surfactants, including salts of non-hydroxy fatty acids such as stearic acid, do not provide the same degree of protection as the foams of the invention, particularly from cold temperatures (e.g. at or below about 0°C such as at or below about -2°C). Without wishing to be bound by theory, it is believed that the hydroxy group on the fatty acid is important to allow the formation of structure within the foam that promotes the consistent initiation of ice formation near the freezing point of water, enabling the latent heat of fusion to be released in a narrow temperature interval. This rapid and concentrated release of thermal energy sustains surface temperatures during frost events, improving protection when ambient conditions approach 0°C. In contrast, conventional unstructured foams (e.g. as formed using non-hydroxy fatty acids) exhibit delayed, staged freezing and less predictable thermal buffering performance due to stochastic nucleation behaviour. Fatty acids (FAs) are carboxylic acids with an aliphatic chain, which is either saturated or unsaturated. Hydroxy fatty acids (HFAs) are a subset of fatty acids which have one or more hydroxyl functional groups attached to the principal chain. Salts of HFAs have been previously found to self-assemble into supramolecular multilayer micron and / or nanometresized tubes (e.g. having a diameter of about 600 nm and a length of about 10 pm). Without wishing to be bound by theory, it is believed that such structures are present in the foams of the present invention and can be used to protect plants as described herein. The present invention may make use biocompatible salts of HFAs to allow the foams to be safely applied directly to plants without causing any damage to the plant or the nearby environment. For example, biocompatible salts of HFAs may be less phytotoxic than other similar surfactants or foaming agents. It has also been found that because the largest component of the foam (by volume) is air, the total amount of water that needs to be applied (as part of the foam) to the plant(s) is less than systems where water (or solutions comprising water) are applied in non-foam form. This can avoid the issues (e.g. use of large quantities of water, continuous spraying, and / or flooding) created by such systems. Additionally, the water is effectively held in place by the foam, i.e. it may be held on or close to the plant, thereby providing the insulating and / or warming effect described herein in the best possible location. As described herein, the insulating and / or warming effect (e.g. due to the latent heat of water as it freezes) is also enhanced by the use of the hydroxy fatty acids salts described herein (e.g. as compared to alternative foaming agents or surfactants). On the other hand, it has also been found that the presence of water can help to provide the latent heat release that allows the foams of the present invention to be unexpectedly effective in keeping plants warm during cold weather (e.g. during a frost event). Increased amounts of water may in fact lead to an improved effect, as there would be a greater latent heat released as the water releases the freezing point. However, too much water can make the foams of the invention too runny (i.e. non-viscous), meaning that they can not be effectively applied to and retained on plants. Additionally, if the amount of water compared to gas (e.g. air) is too much, the improved effect from adding water can be offset by the loss of the insulating effect of the gas. The amount of water in the foam may be from about 5 to about 25 vol.%, such as from about 8 to about 20 vol.%, from about 10 to about 18 vol.% or from about 12 to about 16 vol.%. Preferably the amount of water in the foam is about 14 vol.%. The amount of water in the foam may depend on the application. For example, when applying to sugar beet or sugar beet clamp it may be desirable for the foam to contain from about 15 to about 25 vol.% water, such as from about 20 to about 25 vol.%. Such foams can freeze to form a foam with a harder shell than foams containing less water. Conversely, where the foam is primarily acting as an insulating layer, less water (e.g. from about 5 to about 15 vol.% water, such as from about 5 to about 10 vol.%) may be used. The foam is also an ultra-stable foam, meaning that it remains stable for an extended period of time, such as for at least about 12 hours. This means that foam of the present invention can adhere to a plant and remain stable throughout the night, protecting the plant from frost and cold weather. This may enable the foams described herein to be used as a scaffold or platform for other technologies that can take advantage of the long period of contact time with the plant. When desired, or when it rains, the foam can be washed off with the application of water. This can be useful, for example to allow insects to pollinate the plants. It should also be noted that there is generally no risk of rain during a late spring frost event, as one of its necessary meteorological conditions is that there is no cloud cover during a frost event. The foam of the present invention may also be used to protect plants from heat. When the foams are heated or exposed to sunlight the water can evaporate, leaving a dehydrated residue or protective film or layer behind. Without wishing to be bound by theory, it is believed that this residue is a film or layer of the supramolecular multilayer micron and / or nanometre-sized tubes present in the foam. This is believed to at reflect at least some infrared light, keeping the plant at a cooler temperature than its surroundings. Whilst the plant is protected from the heat, some or all of the UV light (e.g. from the sun) can still be transmitted through the residue or film to the plant, to allow photosynthesis to occur. The foam may also be used to protect plants from pests. For example, the foam may visually camouflage plants from pests (e.g. birds) that are attracted by colour by concealing, for example, ripening grape berries. The foam may also camouflage the scent of the plants, protecting them from insects that are attracted by smell, such as aphids (which may, for example, be attracted to the smell of oil seed rape and / or sugar beet). The foam can also trap and kill insects, such as aphids. The foam may also be used to protect plants against bacterial and fungal attacks, for example because the pH of the foam is high enough to prevent or inhibit the growth of bacteria or fungi that might otherwise cause damage to the plant. The foam may also carry additional bactericides and / or fungicides. The foam may also be used to reduce and / or prevent the growth of weeds or other unwanted plants. To achieve this, the foam can be applied to soil, e.g. bare soil, where weeds are expected or predicted to grow, or where weeds have previously grown. Alternatively and / or additionally, the foam can comprise a biocide, weedkiller or means to prevent photosynthesis and be applied directly to the weeds or other unwanted plants. The biocide, weedkiller or means to prevent photosynthesis can then act to kill any weeds or other unwanted plants, whilst the foam remains in place afterwards to prevent or reduce any regrowth. Furthermore, the foam of the present invention may also be used in a method of providing nutrients to plants. In particular the foam may contain particular nutrients which, when applied to the plant, can be absorbed through the plant leaves. The foam can therefore also be used to protect plants from malnutrition. Foam The foams described and used herein comprise water and one or more salts of one or more hydroxy fatty acids (HFAs, also called hydroxylated fatty acids). A mixture of salts of a single hydroxy fatty acid may be used, or a mixture of salts of different hydroxy fatty acids (including a mixture of one salt of multiple hydroxy fatty acids, or multiple different salts of different hydroxy fatty acids). Preferably a single hydroxy fatty acid is used. For example, one or two different salts of a single hydroxy fatty acid may be used in combination. In other embodiments a single salt of a single hydroxy fatty acid is used. Salts of a fatty acid or hydroxy fatty acid (such as 12-hydroxystearic acid) may be formed by mixing a fatty acid or hydroxy fatty acid with a suitable salt. Suitable hydroxy fatty acids that may be used in the present invention include those having from about 10 to about 40 carbon atoms, such as from about 12 to about 30, from about 14 to about 22, or from about 16 to about 18 carbon atoms. In embodiments, the fatty acids may contain up to 4 double bonds, i.e. 0, 1, 2, 3 or 4 double bonds. In embodiments, the fatty acids may contain up to 3 double bonds (i.e. 0, 1, 2 or 3), such as up to 2 double bonds, i.e. 0, 1 or 2). Preferably the fatty acids contain 0 or 1 double bonds, and most preferably the fatty acids contain 0 double bonds (i.e. they are saturated fatty acids). Specific hydroxy fatty acids which may be used in the present invention include hydroxy stearic acid (C18:0), hydroxy oleic acid (C18:1), hydroxy lauric acid (C12:0), hydroxy myristic acid (C14:0) and hydroxy palmitic acid (C16:0). Preferably the fatty acids are singly-hydroxylated (i.e. they contain a single hydroxy group). Preferably the hydroxy group is a mid-chain or secondary group, i.e. it is not present on the end of the fatty acid chain. More preferably, the hydroxy group is at least 4 carbons from the free end of the fatty acid chain (i.e. it is on carbon 5 or more when counting from the free end of the fatty acid). In embodiments, the hydroxy fatty acid has from about 14 to about 22 carbon atoms, such as from about 16 to about 18 carbon atoms, and the hydroxy group(s) (preferably the one hydroxy group) is at least 4 carbons from the free end of the fatty acid chain, such as at least 5 or at least 6. The hydroxy group(s) (preferably the one hydroxy group) may be less than 12, such as less than 10, or less than 8 carbons from the free end of the fatty acid chain. Preferably the hydroxy fatty acid is hydroxystearic acid, such as 8-, 10- or 12-hydroxystearic acid. Preferable the hydroxy fatty acid is 10-hydroxystearic acid or 12-hydroxystearic acid, more preferably 12-hydroxystearic acid. Preferably, the salt of 12-hydroxystearic acid is the only fatty acid salt present. Any stereoisomer of the salt may be used. Preferably the salts are those which will not damage the plant or cause any environmental damage. These salts may be termed “biocompatible”. The skilled person would understand which salts would be termed biocompatible. Choline is just one example of a biocompatible salt. Other examples include metal salts, such as a sodium, potassium, calcium or magnesium salts. In the foams and compositions disclosed herein, the one or more salts of one or more hydroxy fatty acids does not consist of an ethanolamine salt of 12-hydroxystearic acid or a hexanolamine salt of 12-hydroxystearic acid. In embodiments, the one or more salts of one or more hydroxy fatty acids does not consist of an ethanolamine salt of 12-hydroxystearic acid and / or a hexanolamine salt of 12-hydroxystearic acid. Thus, in embodiments the foams and compositions disclosed herein do not include ethanolamine or hexanolamine salt of 12-hydroxystearic acid, unless used in combination with another salt of a hydroxy fatty acid (which may be a different salt of 12-hydrostearic acid). Put another way, if the salt of the hydroxy fatty acid is an ethanolamine or hexanolamine salt of 12-hydroxystearic acid, at least one further salt of a hydroxy fatty acid is preferably present. The foam preferably does not comprise only water and an ethanolamine or hexanolamine salt of 12-hydroxystearic acid. In embodiments, the foam does not include an ethanolamine or hexanolamine salt of 12-hydroxystearic acid at all. In embodiments the foam does not include any ethanolamine or hexanolamine salts of a hydroxy fatty acid, i.e. the one or more salts is not an ethanolamine or hexanolamine salt. In embodiments the foam does not include any ethanolamine or hexanolamine salts. The salt may be any suitable salt known to the skilled person, including both organic and inorganic salts. For example, the salt may be a metal salt, such as a sodium, potassium, lithium, calcium or magnesium salt. The salt may also be an organic salt, such as an ammonium, diethanolamine, triethanolamine or choline salt. Preferably the salt is a sodium, potassium, ammonium or choline salt. More preferably the salt is a sodium, potassium or choline salt. Even more preferably the salt is a sodium or choline salt. Most preferably the salt is a choline salt. Thus, the salt of a hydroxy fatty acid used herein may be choline hydroxystearate, such as choline 8-hydroxystearate, choline 10-hydroxystearate or choline 12-hydroxystearate. The preferred salt for use herein is choline 12-hydroxystearate. Salts of a fatty acid (such as choline hydroxystearate) may be formed by combining the fatty acid or hydroxy fatty acid (such as hydroxystearic acid) with a salt (e.g. a choline salt such as choline hydroxide or choline bicarbonate). This may be done before, during and / or after formation of the foam. Thus, the foam may comprise a hydroxy fatty acid such as hydroxy stearic acid (e.g. 12-hydroxystearic acid) and a salt such as a choline salt (e.g. choline bicarbonate), which together form a salt of a HFA (e.g. choline hydroxy stearate) in situ. The skilled person would be able to select suitable salts depending on the desired final product. For example, hydroxide salts such as sodium hydroxide, potassium hydroxide, lithium hydroxide, calcium hydroxide, magnesium hydroxide, ammonium hydroxide or choline hydroxide may be used to form the desired salt of a hydroxy fatty acid. Bicarbonate salts may also be used. Salts of a fatty acid may alternatively be formed by the saponification of lipids (usually triglycerides) with a strong base (e.g. the salt), leading to the formation of glycerol and fatty acid salts. For example, stearin can be saponified using a salt, such as those mentioned above (e.g. NaOH etc.), to form glycerol and a salt of stearic acid. Preferably any glycerol formed in the process is removed, as this can otherwise act as an antifreeze. The foam preferably comprises a total of from about 1 to about 40 wt.% of the one or more hydroxy fatty acid salts, such as from about 3 to about 30 wt.%, from about 5 to about 25 wt.%, from about 8 to about 22 wt.% or from about 10 to about 20 wt.%. The foam preferably comprises a total of from about 1 to about 40 wt.% of 12-hydroxystearic acid, such as from about 3 to about 30 wt.%, from about 5 to about 25 wt.%, from about 8 to about 22 wt.% or from about 10 to about 20 wt.%. The foams may also comprise cellulosic nanoparticles. The cellulosic nanoparticles may be cellulose nanocrystals or cellulose nanofibers. The inclusion of cellulosic nanoparticles can help to provide structure or “body” to the foam. Cellulosic nanoparticles may make the foam longer-lasting and / or more stable. Cellulosic nanoparticles can also provide an insulating effect, both within the foam and after evaporation or loss of the water. The latter effect may occur as the cellulosic nanoparticles are left behind and form a fleece-like coating which can provide an insulating effect (from both the heat and the cold). This may also help to reflect IR radiation, thereby helping to maintain plants below a given temperature (e.g. ambient temperature) when desired (e.g. during hot weather). The nanoparticles may have a number average particle size of less than 1000 nm, such as from about 1 to about 500 nm, from about 2 to about 250 nm, from about 5 to about 100 nm, from about 5 to about 50 nm, or from about 6 to about 20 nm. As used herein, the term “particle size” refers to the longest dimension of a particle, e.g. length. For spherical particles this is the diameter. Preferably, any nanoparticles make up from about 0.1 to about 20 wt.% of the foam. More preferably, any nanoparticles make up from about 0.5 to about 10 wt.% of the foam. Even more preferably, any nanoparticles make up from about 0.5 to about 5 wt.% or from about 0.5 to about 2 wt.% of the foam. The cellulose used in the present invention may be fibrillated cellulose, microcrystalline cellulose, regenerated cellulose, bacterial cellulose, nanocellulose, or combinations thereof. The term nanocellulose can include cellulose nanocrystal (CNC) or nanofibrillated cellulose (NFC). Preferably, the cellulose is fibrillated cellulose, cellulose nanocrystals (CNC) or nanofibrillated cellulose (NFC). Any fibrillated cellulose may be microfibrillated cellulose. In another embodiment, the fibrillated cellulose may be nanofibrillated cellulose. Preferably the foam is free from or essentially free from organic solvents. Thus, the foam preferably contains less than about 5 wt.%, such as less than about 1 wt.% or less than about 0.1 wt.% organic solvents. The foam may also comprise one or more additional surfactants, such as sugar esters and / or lecithin, to improve the consistency of the foam and / or the ability of the foam to adhere to plants once applied. The foam may also comprise one or more nucleating agents. Suitable nucleating agents include inorganic particles, such as talc, calcium carbonate, silica, barium sulfate, titanium dioxide, sodium benzoate, felspar, and inorganic clays such as kaolin, montmorillonite, sepiolite, illite and bentonite. Preferably the nucleating agent is felspar or an inorganic clay, more preferably kaolin. If one or more nucleating agents is present, the foam may comprise a total of up to about 20 wt.% nucleating agent, such as up to about 15 wt.% or up to about 10 wt.%. For example, the foam may comprise a total of from about 0.1 to about 20 wt.% nucleating agent, from about 0.5 to about 15 wt.% or from about 1 to about 10 wt.%. The nucleating agent can help to create the desired foam structure when the temperature drops below 0°C, thereby assisting with the release of latent heat as described herein. The foam may also comprise one or more gelling agents. Suitable gelling agents would be known to the skilled person, and may include polysaccharide gelling agents, such as nonionic polysaccharide gelling agents. Suitable gelling agents include hyaluronic acid, chitosan, agar-agar, gelatin, pectin, carrageenan, pullulan, glucomannan, xanthan gum, guar gum, locust bean gum, konjac powder, psyllium husk, tara gum, gellan gum, and alginate (e.g. sodium alginate or calcium alginate). Preferably the gelling agent is selected from pullulan, xanthan gum and alginate (e.g. sodium alginate). If one or more gelling agents is present, the foam may comprise a total of from about 0.01 to about 10 wt.% gelling agent, such as from about 0.1 to about 10 wt.%, from about 0.5 to about 8 wt.% or from about 1 to about 6 wt.%. The foam may alternatively comprise from about 0.01 to about 2 wt.% gelling agent, such as from about 0.05 to about 1 wt.%. In embodiments both xanthan gum and alginate (e.g. sodium alginate) are present in the foam, in the amounts of from about 0.1 to about 5 wt.% and from about 0.1 to about 3 wt.% respectively. In other embodiments both xanthan gum and alginate (e.g. sodium alginate) are present in the foam, in the amounts of from about 0.4 to about 4 wt.% and from about 0.2 to about 2 wt.% respectively. It has been found that including a gelling agent may increase the stability of the foams. This may be measured by determining the foam drainage (i.e. the volume loss of the foam) over time. A gelling agent (such as xanthan gum) can reduce the foam drainage. In particular, the gelling agent may allow for the formation of a water resistant membrane on the surface of the foam on exposure to air. The resulting foam 'cocoon' has been found to be very stable and can remain in situ for many days without change. This can enable the user to apply the foam well in advance of when it is needed, for example well in advance of anticipated or predicted cold weather such as a frost event. It has also been found that the water resistant membrane can act as a barrier against further dehydration while still allowing water vapor exchange. Thus, during dry conditions the foam can partially dehydrate at the surface, forming the protective membrane. Conversely, during wet conditions (e.g. rain, mist, dew), the foam can reabsorb water and rehydrate, restoring porosity and flexibility. This hydration-dehydration cycle is reversible and repeatable, enabling the foam to remain outdoors over extended periods (e.g. over several months, such as an entire winter season), without loss of integrity. The water resistant membrane can also assist in insulating the plants within the foam. In some embodiments it is preferable for the foam to be hydrophobic, as this can lead to increased stability over time, particularly when the foam is exposed to environmental conditions (e.g. rain etc.). This can be achieved by the addition of resin and / or oil, for example an amphiphilic tree resin like mastic or pine rosin, and / or tung oil, optionally in combination with a polysaccharide such as glucomannan. If present, these components may be present in the total amount of from about 0.1 to about 5 wt% of the foam or foamable composition, such as from about 0.5 to about 3 wt% or from about 1 to about 2 wt%. The foam may also comprise hexagonal boron nitride platelets which are edge hydroxylated (e.g. using with heat, shear and choline hydroxide), optionally in combination with sodium silicate. The inclusion of edge hydroxylated hexagonal boron nitride platelets are expected to be able to absorb heat and / or smoke particles. For example, without wishing to be bound by theory, smoke particles may interact with the interface of each bubble in the foam, resulting in the smoke particles being bound to the hydroxyls exposed on the edges of the boron nitride. This can therefore protect plants (e.g. vines) from damage from heat and / or smoke, e.g. due to a nearby forest fire. For example, when vineyards and grapes are exposed to smoke this can ultimately result in wines with undesirable sensory characters, such as smoky, burnt, ashy or medicinal. The foams of the present invention can help to prevent smoke taint, particularly when edge hydroxylated hexagonal boron nitride platelets are present. The foam may comprise, consist essentially of, or consist of, water, gelling agent, and a salt of a hydroxy fatty acid, such as choline 12-hydroxysteate or a combination of choline 12-hydroxysteate and triethanolamine 12-hydroxystearate. The foam may comprise, consist essentially of, or consist of, water, cellulosic nanoparticles, and a salt of a hydroxy fatty acid, such as choline 12-hydroxysteate or a combination of choline 12-hydroxysteate and triethanolamine 12-hydroxystearate. The foam may comprise, consist essentially of, or consist of, water, gelling agent, cellulosic nanoparticles, and a salt of a hydroxy fatty acid, such as choline 12-hydroxysteate or a combination of choline 12-hydroxysteate and triethanolamine 12-hydroxystearate. The foam may comprise, consist essentially of, or consist of water; from about 1 to about 20 wt.% salt of a hydroxy fatty acid (e.g. choline hydroxy stea rate); optionally from about 0.1 to about 10 wt.% gelling agent; and optionally from about 0.1 to about 20 wt.% cellulosic nanoparticles. The foam may comprise, consist essentially of, or consist of water; from about 3 to about 15 wt.% salt of a hydroxy fatty acid (e.g. choline hydroxy stea rate); optionally from about 0.5 to about 8 wt.% gelling agent; and optionally from about 0.5 to about 10 wt.% cellulosic nanoparticles. The foam may comprise, consist essentially of, or consist of water; from about 5 to about 12 wt.% salt of a hydroxy fatty acid (e.g. choline hydroxy stea rate); optionally from about 1 to about 6 wt.% gelling agent; and optionally from about 0.5 to about 5 wt.% cellulosic nanoparticles. The foam may comprise, consist essentially of, or consist of from about 78 to about 93.5 wt.% water; from about 5 to about 12 wt.% choline 12-hydroxystearate; and from about 0.1 to about 10 wt.% gelling agent. The foam may comprise, consist essentially of, or consist of from about 83 to about 90.5 wt.% water; from about 8 to about 12 wt.% choline 12-hydroxystearate; and from about 0.5 to about 8 wt.% gelling agent. The foam may comprise, consist essentially of, or consist of from about 78 to about 93.5 wt.% water; from about 5 to about 12 wt.% choline 12-hydroxystearate; from about 1 to about 5 wt.% triethanolamine 12-hydroxystearate; and from about 0.5 to about 5 wt.% cellulosic nanoparticles. The foam may comprise, consist essentially of, or consist of from about 83 to about 90.5 wt.% water; from about 8 to about 12 wt.% choline 12-hydroxystearate; from about 1 to about 3 wt.% triethanolamine 12-hydroxystearate; and from about 0.5 to about 2 wt.% cellulosic nanoparticles. In each case the water may be present in the amount to make up 100 wt.% (e.g. q.s water). The water may comprise from about 50, 60, 70, 75, 78, 80, 83 or 85 wt.% of the foam, to about 99, 98, 97, 95, 93, 92, 91, 90.5 or 90 wt.% of the foam. In embodiments, the water may comprise from about 50 to about 99 wt.% of the foam, such as from about 60 to about 99 wt.%, from about 70 to about 97 wt.%, from about 75 to about 95 wt.%, from about 78 to about 92 wt.%, or from about 80 to about 90 wt.%. In other embodiments, the water may comprise from about 78 to about 93.5 wt.% of the foam, such as from about 83 to about 90.5 wt.%. The foam may also comprise an active agent as used in agriculture, for example a fertilizer, herbicide, pesticide, fungicide and / or insecticide. The foam may be an ultra-stable foam. That is, the foam may remain stable for at least about 12 hours, such as at least about 24 hours, at least about 3 days or at least about 7 days. Foam stability may be measured by measuring the volume of a given foam over a period of time. Over the duration of the test the foam may be kept at a consistent temperature (e.g. 25°C) in a sealed environment (e.g. in a sealed container). The foams of the present invention preferably retain at least about 90 % of their volume over a period of at least about 12 hours, such as at least about 24 hours, at least about 3 days or at least about 7 days. Preferably the foam has a pH of above about 7, such as from about 7 to about 12, from about 8 to about 12, from about 8.5 to about 11, or from about 9 to about 11. The pH may also range from about 7 to about 10, such as from about 8 to about 10, or from about 8.5 to about 9.5. The pH may also range from about 7 to about 9. The foam may comprise at least about 70 vol.% gas (e.g. air), such as at least about 75 vol.% or at least about 80 vol.%. For example, the foam may comprise from about 70 to about 95 vol.% gas, such as from about 80 to about 92 vol.%, from about 82 to about 90 vol.%, or from about 84 to about 88 vol.%. The remainder of the foam will be water and any other components (e.g. the salt of the hydroxy fatty acid), i.e. the foamable composition described below. The volume of foamable composition within the foam may be from about 5 to about 25 vol.%, such as from about 8 to about 20 vol.%, from about 10 to about 18 vol.% or from about 12 to about 16 vol.%. Within this, the ratio of the volume of water to the volume of any other components may be from about 20:1 to about 6:1, such as from about 15:1 to about 8:1 or from about 12:1 to about 10:1. The expansion factor of the foam is the ratio of the volume of foam compared to the volume of the foamable composition used to form the foam. This may range from about 5:1 to about 20:1, such as from about 6:1 to about 15:1, from about 8:1 to about 12:1, or about 10:1. The volume ratio of gas (e.g. air) to water in the foam may range from about 80:20 to about 92:8, such as from about 82:18 to about 90:10, or from about 84:16 to about 88:12. In other embodiments the volume ratio of gas (e.g. air) to water in the foam may range from about 4:1 to about 12:1, such as from about 5:1 to about 9:1, or from about 6:1 to about 8:1. As discussed above, if the ratio of gas to water is too high (i.e. there is too little water) the foam will show a reduced latent heat effect and therefore provide less effective protection for the plant. Conversely, if the ratio of gas to water is too low (i.e. there is too much water), the foam can become too runny or non-viscous to be effectively applied and retained on a plant (e.g. it could run off too easily after application). Foamable composition The invention also provides a foamable composition, i.e. a composition which may be formed into a foam. The foamable composition may comprise all the same components of the foams described herein except the gas (e.g. air) which is present in a foam. For example, in one aspect the foamable composition comprises water and a salt of a hydroxy fatty acid (e.g. choline hydroxystearate). Due to the negligible weight of the gas within the foam, the components of the formable composition may be present in the same weights as given herein in relation to the foam. Preferably the foamable composition has a pH of above about 7, such as from about 7 to about 12, from about 8 to about 12, from about 8.5 to about 11, or from about 9 to about 11. The pH may also range from about 7 to about 10, such as from about 8 to about 10, or from about 8.5 to about 9.5. The pH may also range from about 7 to about 9. The invention also provide a foam concentrate, which is a concentrated version of the foamable composition. This can be diluted (e.g. by the end user) to form the foamable composition, which is then formed into the final foam. The foam concentrate will contain significantly less water than the foamable composition, meaning that the proportion of the other components are relatively higher. In embodiments, the foam concentrate is diluted with water to form the foamable composition, at a ratio of from about 1:100 to about 1:1 foam concentrate to water, such as from about 1:75 to about 1:5 or from about 1:50 to about 1:10. Method of making the foam The foam of the present invention may be formed using any suitable technique, including aerating a foamable composition described herein. For example, the foams may be formed by creating an aqueous solution comprising the components of the foam (e.g. a foamable composition), and then subjecting the solution or composition to high shear conditions. The foam may also be formed by aerating the aqueous solution, for example by bubbling gas (e.g. air) through the solution. The foam may be formed by blowing a gas (e.g. air) though a sponge of other porous material which holds the foamable composition. An aspirating foam nozzle may also be used. The foam may therefore be formed before and / or during application to a plant or plants. For example, a foam may be first formed and then applied, or the foam may be formed as part of the application process. A method of making the foam may comprise forming a mixture of water and a salt of a hydroxy fatty acid (e.g. choline hydroxy stearate), or water, a hydroxy fatty acid (e.g. hydroxy stearic acid) and a salt (e.g. a choline salt), and forming a foam from the mixture. Forming the foam may involve, for example, exposing the mixture to high shear conditions and / or bubbling air through the mixture. An aspirating foam nozzle may also be used. Method of protecting plants The present invention provides a method of protecting one or more plants, comprising applying to the one or more plants the aqueous foam described herein. In the context of the present invention, protecting the plant(s) includes, but is not limited to, protecting the plant(s) from cold (including frosts and / or temperatures at or below about 0°C, such as at or below about -2°C), heat (including leaf scotch and / or temperatures above about 30°C), pests (including birds and / or insects) and disease (e.g. bacterial and / or fungal diseases). The foam may be applied to the whole plant or any part or parts of the plant, including the roots, leaves, stems, flowers, petals, berries, fruit, etc. In one aspect the foam is applied to the leaves. The plant to which the foam is applied may be any plant, but in particular are agricultural plants which may be useful in the production of food or drink. For example, the plants may be crops such as wheat, corn, sugar beet, etc. Alternatively, the plants may be vines, such as grapevines. Besides agricultural plants or crops, the foam may also be applied to horticultural, arboricultural, floricultural, aquatic crops and ornamental plants. It may also be applied in indoor protected cropping environments. Rather than being applied to the plants themselves, the foam may also be applied to an existing surface or structure designed to protect plants from the weather (e.g. a greenhouse or polytunnel). In this case, the foam may act as another layer of protection above and beyond that provided by the existing surface or structure. In this case, the plant protection device (such as a greenhouse or polytunnel) contains the plants which are to be protected. The foam may be applied to the plant when the plant is still growing, which is particularly the case for vines. Alternatively, the foam may be applied to the plant after harvest, which may be useful for crops such as wheat, corn, sugar beet, etc. For example, the foam can be applied to sugar beet clamp. The aqueous foam may be applied to a plant (or to a plant protection device such as a greenhouse or polytunnel) to form a layer of foam having an average thickness of from about 0.1 cm to about 5 cm, such as from about 0.3 cm to about 3 cm, or from about 0.5 cm to about 1.5 cm. The thickness is preferably relatively uniform, such that at each point the layer has a thickness of from about 0.1 cm to about 5 cm, such as from about 0.3 cm to about 3 cm, or from about 0.5 cm to about 1.5 cm. The thickness of the foam is generally proportional to the time in which the plant will remain protected, meaning that thicker foams may be used when it is desired to protect the plant for longer and / or from harsher conditions. The foam may be applied in any suitable way, including but not limited to spraying the foam onto the one or more plants or plant protection device. The foam may be formed before being applied, or alternatively the foam may be formed during the application process. For example, an aqueous mixture (i.e. the foamable composition described herein) may be sprayed and form a foam during and / or after spraying. Protection from the cold The aqueous foam may be applied to one or more plants (or to a plant protection device such as a greenhouse or polytunnel) to protect the plants from frost or cold weather. As discussed above, the water in the foam freezes or crystallises at temperatures of 0°C or less. Thus, when the temperature drops to 0°C or less, the aqueous foam applied to the plants freezes or crystallises. The foam may maintain the temperature of the plants above -2°C, preferably at or above -1°C, and more preferably at or above 0°C. The aqueous foam may maintain the temperature of the one or more plants above ambient temperature, such as at least about 1°C above ambient temperature, at least about 2°C above ambient temperature, at least about 3°C above ambient temperature, at least about 4°C above ambient temperature or at least about 5°C above ambient temperature. In embodiments the ambient temperature is less than about 10°C, less than about 5°C, less than about 2°C, less than about 0°C, less than about -2°C, less than about -5°C or less than about-10°C. The foam may maintain the temperature of the one or more plants at or above 0°C when the ambient temperature is less than about 0°C, such when the ambient temperature is less than about -2°C, less than about -5°C or less than about -10°C. The foam may maintain the temperature of the one or more plants above -2°C when the ambient temperature is less than about -2°C, such as when the ambient temperature is less than about -5°C or less than about -10°C. The foam preferably maintains the temperature of the one or more plants at or above the desired temperature for a period of at least about 30 minutes, such as at least about 1 hour, at least about 2 hours, at least about 5 hours, at least about 8 hours or at least about 12 hours. For example, the foam may maintain the temperature of the one or more plants at or above a temperature of 0°C when the ambient temperature is less than 0°C, for a period of at least about 2 hours, such as at least about 5 hours, at least about 8 hours or at least about 12 hours. The foam may maintain the temperature of the one or more plants at or above a temperature of 0°C when the ambient temperature is less than -2°C, for a period of at least about 2 hours, such as at least about 5 hours, at least about 8 hours or at least about 12 hours. The foam may maintain the temperature of the one or more plants above a temperature of -2°C when the ambient temperature is less than -2°C, for a period of at least about 2 hours, such as at least about 5 hours, at least about 8 hours or at least about 12 hours. The foam may maintain the temperature of the one or more plants above a temperature of -2°C when the ambient temperature is less than -5°C, for a period of at least about 2 hours, such as at least about 5 hours, at least about 8 hours or at least about 12 hours. Once the temperature increases again (e.g. during the daytime) the foam may return to a liquid state, at which point the foam may be removed from the plant, for example by rain or the application of water. The foam may also remain in a (non-solid) foam state, especially if temperatures do not fall below the freezing point of water. Any discussion above relating to the temperature of the plant (including the temperature over time and / or in comparison to the ambient temperature) may also apply to the internal temperature of the foam. This is because these two temperatures will be closely related, e.g. because the temperature of the plant will generally remain at or above the internal temperature of the foam when the ambient temperature is below the internal temperature of the foam. By way of example only, the foam may maintain an internal temperature at or above 0°C when the ambient temperature is less than 0°C, for a period of at least about 2 hours, such as at least about 5 hours, at least about 8 hours or at least about 12 hours. In embodiments the foams of the present invention can maintain an internal temperature of 0°C or more for a period of at least about 1 hour, such as at least about 1.5 hours, such as at least 2 hours, such as at least 4 hours, such as at least 8 hours, such as at least 12 hours, even when the ambient or external temperature drops below 0°C, such as below about -5°C or below about -10°C. In embodiments, the internal temperature of the foam may be maintained at least about 5°C, at least about 6°C, at least about 7°C, at least about 8°C, at least about 9°C, or at least about 10°C, higher than the external or ambient temperature, preferably for a period of at least about 1 hour, such as at least about 1.5 hours, such as at least 2 hours, such as at least 4 hours, such as at least 8 hours, such as at least 12 hours. By “internal temperature” it is meant the temperature within the foam. This temperature is generally measured at the point in the foam which is furthest from the ambient air. For example, this may be the middle of the foam, or may be the bottom surface of the foam (i.e. the surface opposite the ambient air) if the foam is applied to a surface (e.g. a plant). The “internal temperature” of the foam would be expected to be the highest temperature within the foam. When the foam is applied to a plant, the internal temperature of the foam will correspond to the temperature to which the plant is exposed. By “external” temperature it is meant the air temperature outside of the foam (e.g. the ambient air temperature). The ambient temperature may be measured as the temperature outside of the foam but less than about 10 m, such as less than about 1 m, less than about 50 cm or less than about 10 cm from the surface of the foam. Protection from the heat As discussed above the foam may also protect the one or more plants from hot weather and / or overheating. For example, the plants may be protected from a temperature at which the plant will scorch (this temperature will depend on the type of plant). For example, the foam may maintain the temperature of the plants below about 26°C, such as below about 28°C or below about 30°C. The foam may maintain the temperature of the one or more plants below about 26°C when the ambient temperature is above than about 26°C, such as when the ambient temperature is above about 30°C or above about 35°C. The foam preferably maintains the temperature of the one or more plants at or below the desired temperature for a period of at least about 30 minutes, such as at least about 1 hour, at least about 2 hours, at least about 5 hours, at least about 8 hours or at least about 12 hours. Again, any discussion above relating to the temperature of the plant (including the temperature over time and / or in comparison to the ambient temperature) may also apply to the internal temperature of the foam. This is because these two temperatures will be closely related, e.g. because the temperature of the plant will generally remain at or below the internal temperature of the foam when the ambient temperature is above the internal temperature of the foam. By way of example only, the foam may maintain an internal temperature at or below 26°C when the ambient temperature is more than about 26°C, for a period of at least about 2 hours, such as at least about 5 hours, at least about 8 hours or at least about 12 hours. The foam may act to protect the plant from hot weather via a different mechanism than that by which is protects against the cold. In particular, without wishing to be bound by theory, it is believed that when the water is removed or allowed to evaporate from the foam, a layer of the supramolecular multilayer micron and / or nanometre-sized tubes present in the foam is left behind. This forms a protective layer or film on the plant which can reflect heat (particularly from IR radiation) whilst still allowing photosynthesis to continue (e.g. because UV light can still pass through the protective layer). An image of the dried protective layer or film is shown in Figure 9. The foam may also be used to protect the plant from fire. For example, the foam may be applied to a plant to prevent or reduce the risk of the plant overheating and / or burning. The foam may be applied to the plant to minimise the risk of a fire starting or to protect the plant from an active fire. In both cases the plant may be a tree, and the foam may be used to help prevent or reduce the spread of forest fires. The foam may be applied to the leaves and / or trunk of one or more tree to prevent or reduce the spread of a forest fire. Protection from pests The foam may also be used to protect plants from pests. For example, the foam may visually camouflage plants from pests (e.g. birds) that are attracted by colour by concealing, for example, ripening grape berries. The foam may also camouflage the scent of the plants, protecting them from insects that are attracted by smell, such as aphids to oil seed rape and sugar beet. The foam may also trap and kill any insects (e.g. aphids) which are already present on the plant at the time of application. Thus existing insects may be killed, whilst the foam may also protect the plant against further insect attack, e.g. by masking the smell of the plants. The foam may also protect against pests in an active way. For example, the foam may comprise an active agent as used in agriculture, for example a fertilizer, herbicide, pesticide, fungicide and / or insecticide. Protection from bacterial and fungal attacks The aqueous foam may also protect plants against bacterial and fungal attacks, for example because the pH of the foam is high enough to prevent or inhibit the growth of bacteria or fungi that might otherwise cause damage to the plant. Protection against such bacteria and / or fungi is also enhanced because the stability of the foam means that it may remain on the plant for an extended period of time. As described above, the foam preferably has a pH of above about 7, such as from about 8 to about 12, from about 8.5 to about 11, or from about 9 to about 11. The pH may also range from about 8 to about 10, such as from about 8.5 to about 9.5. The foam may also protect against pests in an active way. For example, the foam may comprise an active agent as used in agriculture, for example a fungicide. An example additive may include, for example, silver, optionally in the form of nanoparticles. Another example additive is a quillaia extract. Other aspects The present invention also provides a method of supplying nutrients to one or more plants, comprising applying to the one or more plants an aqueous foam as described herein, wherein the foam also comprises one or more plant nutrients. The plant nutrients are not particularly limited, and may be any nutrients that would be normally supplied to a plant. The nutrients may be absorbed by the plant after application, for example through their leaves (and in particular through the stomata on their leaves). This method is also enhanced because the stability of the foam means that it may remain on the plant for an extended period of time. This long contact time allows the plant to draw nutrition from the foam or from the dried foam as and when it is required. Supplying the plants with nutrients in this way may be preferable to the traditional methods of spraying the plant with liquids containing nutrients, since the nutrients will not be lost e.g. through not reaching the plant and / or run off. Holding the nutrients in place on the plants will maximise the nutrient uptake for the plant whilst minimising the amount of nutrients need, thereby making the whole process of supplying nutrients to the plants more efficient. The present invention also provides a method of protecting one or more plants, comprising applying to the soil surrounding the one or more plants an aqueous foam containing cellulose. In this case the foam can prevent the loss of water from the soil (e.g. by evaporation), thereby retaining more water in the soil for uptake by the one or more plants. Thus, the present invention also provides a method of preventing water loss from soil, the method comprising applying to the soil aqueous foam containing cellulose. Additionally, the foam may also be applied to soil in order to prevent and / or reduce the growth of weeds or other unwanted plants. To achieve this, the foam can be applied to soil, e.g. bare soil, where weeds are expected or predicted to grow, or where weeds have previously grown. The foam may also or alternatively be applied directly to weeds in order to reduce and / or prevent their growth, and this may be particularly effective if the foam comprises a biocide, weedkiller or means to prevent photosynthesis. Essential oils such as carvacol (e.g. from oregano), thymol, garlic oil or other monoterpenes can be added as a biocide, and may also act as seed germination inhibitors. As used here, the term “means to prevent photosynthesis” is a component or additive of the foam or foamable composition that can act to reduce or prevent the amount of light passing through the foam, such that photosynthesis of any plants (e.g. weeds) covered by the foam is prevented or reduced (e.g. by at least about 80 % or at least about 90 %). For example, the means to prevent or reduce photosynthesis can be a light blocking material, for example a black or dark-coloured particulate material such as carbon black or biochar. However, other colours could also be used; for example, white particulate material could reflect light, such that light transmission through the foam was reduced or prevented. Using a light blocking or black particulate material is the preferred means for preventing and / or reducing the growth of weeds or other unwanted plants. Examples Examples 1-8 5 The following foam formulations were made by mixing the components together and subjecting them to shear forces to form a foam. In each case the expansion factor of the foam was 10.0 ± 0.5 : 1. Example Formulation of the foamable composition 1 10 wt.% choline 12-hydroxystearate 90 wt.% distilled water 2 10 wt.% choline 12-hydroxystearate 1 wt.% cellulose nanocrystals 89 wt.% distilled water 3 10 wt.% choline 12-hydroxystearate 4 wt.% cellulose nanocrystals 86 wt.% distilled water 4 10 wt.% choline 12-hydroxystearate 1 wt.% cellulose nanofibers 89 wt.% distilled water 5 10 wt.% triethanolamine 12-hydroxystearate 90 wt.% distilled water 6 9 wt.% choline 12-hydroxystearate 1 wt.% triethanolamine 12-hydroxystearate 90 wt.% distilled water 7 (comparative) 10 wt.% decyl glucoside 5 wt.% cocamidopropyl betaine 85 wt.% distilled water 8 (comparative) 6 wt.% sodium alpha olelin sulphonate 94 wt.% distilled water 9 (comparative) 10 wt.% choline stearate 90 wt.% distilled water 10 (comparative) 5 wt.% cocamidopropyl betaine 5 wt.% decyl glucoside 90 wt.% distilled water 11 (comparative) 10 wt.% sodium alpha olelin sulphonate 90 wt.% distilled water 12 9 wt.% choline 12-hydroxystearate 10 wt.% kaolin particles 81 wt.% distilled water Each foam was then placed in a 100ml borosilicate beaker and placed inside a freezer set to -18°C. The temperature was monitored using tinytag dataloggers equipped with two probes: one placed inside the foam and the other outside. The results are shown in Figures 1-12. As shown by these results, the foams of the present invention (Examples 1-6 and 12) were able to provide a significant insulating effect for an extended time period (e.g. at least 1 hour). In contrast, other similar foams made in the same way but using a different surfactant (Comparative Examples 7-11), were not able to provide an insulating effect for any significant time. It is particularly noteworthy that Comparative Example 9, comprising choline stearate, was not effective at maintaining the temperature at the desired level for a long period of time. This show the importance of the use of a hydroxy fatty acid in the foams of the present invention. In the UK, a severe late spring frost may be classified in vineyards for example as being caused by the ambient air temperature around plants falling to -6°C for an hour. These tests therefore indicate that the compositions of the invention are suitable to protect plants from such temperatures. Example 13 Separately, a foam was formed in the same way as Examples 1-12 but using a formulation comprising 10 wt.% choline 12-hydroxystearate, 84 wt.% distilled water, 4 wt.% xanthan gum and 2 wt.% sodium alginate. The foam of Example 13 was formed and then applied to a plant at a thickness of about 2.5 cm in advance of expected frosts (i.e. the ambient temperature falling below 0°C). The temperature was monitored using tinytag dataloggers equipped with two probes: one placed inside the foam and the other outside. The results are shown in Figure 13. A water resistant membrane was found to form on the surface of the foam on exposure to air. The resulting foam 'cocoon' was found to be incredibly stable. In particular, as shown in Figure 13, one application was sufficient to carry on protecting the plant over several frost and thaw events over a 50 hour time period. This result demonstrates the absorption and release of latent heat on freezing and thawing. As the temperature rises above 0°C the water molecules melt out of their solid ice structure and absorb thermal energy in their molecular liquid structure. Conversely, as temperatures drop below 0°C the water molecules release thermal energy as they restructure to a crystalline ice form. In the results shown in Figure 13, the temperature of the foam decreases below 0°C at about 2 hours and the water molecules in the foam start to freeze. By 3 hours the latent heat from the water freezing is being released which raises the temperature in the foam slightly, closer to 0°C. As can clearly be seen from Figure 13, the temperature of the foam is consistent (and generally above ambient temperature) until the ambient temperature rises above 0°C. At this point the temperature inside the foam also rises, but slower, and undergoes a phase change from solid to liquid. When the ambient temperature later drops below 0°C the temperature inside the foam falls, but again more slowly. Latent heat is released as the water in the foam freezes. This cycle can be repeated multiple times. Put another way, when the ambient temperature rises above 0°C the water in the foam melts and forms a liquid phase of matter, absorbing energy from its surroundings and thereby slowing down the rate of temperature rise within the foam relative to that of the ambient temperature. The reverse occurs when the ambient temperature falls below 0°C. In this way the temperature of the foam is held stable through fluctuations in ambient temperature. Example 14 A foam was formed in the same way as Example 1 but using a formulation comprising 10 wt.% choline 12-hydroxy stearate, 89.9 wt.% distilled water, and 0.1 wt.% xanthan gum (technical grade). This foam was tested for stability over time, and compared with the foam of Example 1. The test procedure involved forming the two foams, and then immediately transferring 300 mL of the foam without compression into a transparent cylindrical column vessel (inner diameter 5-10 cm, height >30 cm). Foam height at t = 0 was recorded. The column was maintained undisturbed at ambient laboratory conditions (23 ± 2 °C, 40-60% relative humidity) for 24 hours. Drainage was defined as the cumulative volume of liquid collected at the base of the column over time. After 24 hours drainage of the foam of Example 1 was observed, corresponding to about 5% of the total foam volume. For example, from 300 mL foam, about 15 mL liquid had accumulated at the base. Under identical conditions, after 24 hours the foam comprising 0.1 wt% xanthan gum exhibited no detectable drainage. The detection limit was 1 mL (<0.3% of foam volume). This demonstrates that the addition of just a small amount of gelling agent can provide significantly enhanced stability of the foam. Example 15 A viscoelastic organogel was prepared by combining tung oil and pine rosin in a 1:1 weight ratio, with 12-hydroxystearic acid (12-HSA) incorporated at 8 wt%. This organogel was blended into a 10* concentrate of choline 12-hydroxystearate aqueous solution. Glucomannan was added at 3 wt% relative to the total formulation to strengthen the foam network. 5 wt% of sepiolite clay, pre-hydrated at 20 wt% in water, was also included. The sepiolite imparts mechanical body to the foam structure and provides resistance to rain impact and splash erosion. A foam was then formed. The resulting foam was tested outdoors, and found to have rainproof stability, i.e. the foam resisted wash-off by rainfall. Additionally, in the presence of wind the foam’s outer layer formed a semi-permeable membrane, which acted as a barrier against further dehydration while still allowing water vapor exchange. The membrane was found to form during dry conditions, when partial dehydration of the foam occurred only at the surface. During wet conditions the foam reabsorbed water and rehydrated, restoring porosity and flexibility. Embodiments: 1. A foam comprising water and one or more fatty acid salts, wherein the one or more hydroxy fatty acid salts does not comprise only an ethanolamine or hexanolamine salt of 12-hydroxystearic acid. 2. A foam comprising water, a salt of a hydroxy fatty acid, and cellulosic nanoparticles. 3. A foam comprising water, a salt of a hydroxy fatty acid, and a gelling agent. 4. A foam comprising water, a salt of a hydroxy fatty acid, and a nucleating agent. 5. A foamable composition comprising water, a salt of a hydroxy fatty acid, and cellulosic nanoparticles. 6. A foamable composition comprising water, a salt of a hydroxy fatty acid, and a gelling agent. 7. A foamable composition comprising water, a salt of a hydroxy fatty acid, and a nucleating agent. 8. The foamable composition or foam of any preceding embodiment, wherein the hydroxy fatty acid comprises from about 10 to about 40 carbon atoms. 9. The foamable composition or foam of any preceding embodiment, wherein the hydroxy fatty acid comprises from about 12 to about 30 carbon atoms. 10. The foamable composition or foam of any preceding embodiment, wherein the hydroxy fatty acid comprises from about 14 to about 22 carbon atoms. 11. The foamable composition or foam of any preceding embodiment, wherein the hydroxy fatty acid comprises from about 16 to about 18 carbon atoms. 12. The foamable composition or foam of any preceding embodiment, wherein the hydroxy fatty acid comprises 0, 1,2, 3 or 4 double bonds. 13. The foamable composition or foam of any preceding embodiment, wherein the hydroxy fatty acid comprises 0, 1,2 or 3 double bonds. 14. The foamable composition or foam of any preceding embodiment, wherein the hydroxy fatty acid comprises 0, 1 or 2 double bonds. 15. The foamable composition or foam of any preceding embodiment, wherein the hydroxy fatty acid comprises 0 or 1 double bonds. 16. The foamable composition or foam of any preceding embodiment, wherein the hydroxy fatty acid comprises 0 double bonds. 17. The foamable composition or foam of any preceding embodiment, wherein the hydroxy fatty acid is selected from the group consisting of hydroxy stearic acid (C18:0), hydroxy oleic acid (C18:1), hydroxy lauric acid (C12:0), hydroxy myristic acid (C14:0) and hydroxy palmitic acid (C16:0). 18. The foamable composition or foam of any preceding embodiment, wherein the hydroxy fatty acid is selected from the group consisting of hydroxy stearic acid (C18:0), hydroxy oleic acid (C18:1), hydroxy myristic acid (C14:0) and hydroxy palmitic acid (C16:0). 19. The foamable composition or foam of any preceding embodiment, wherein the hydroxy fatty acid is selected from the group consisting of hydroxy stearic acid (C18:0), hydroxy myristic acid (C14:0) and hydroxy palmitic acid (C16:0). 20. The foamable composition or foam of any preceding embodiment, wherein the fatty acid is a singly hydroxy fatty acid. 21. The foamable composition or foam of any preceding embodiment, wherein the fatty acid is hydroxystearic acid. 22. The foamable composition or foam of any preceding embodiment, wherein the fatty acid is 8-, 10- or 12-hydroxy stearic acid. 23. The foamable composition or foam of any preceding embodiment, wherein the fatty acid is 10-hydroxystearic acid or 12-hydroxystearic acid. 24. The foamable composition or foam of any preceding embodiment, wherein the fatty acid is 12-hydroxystearic acid. 25. The foamable composition or foam of any preceding embodiment, wherein the salt is a sodium, potassium, lithium, calcium, magnesium, diethanolamine, triethanolamine, ammonium or choline salt. 26. The foamable composition or foam of any preceding embodiment, wherein the salt is a sodium, potassium, calcium, magnesium, triethanolamine, ammonium or choline salt. 27. The foamable composition or foam of any preceding embodiment, wherein the salt is a sodium, potassium, ammonium or choline salt. 28. The foamable composition or foam of any preceding embodiment, wherein the salt is a sodium, potassium, or choline salt. 29. The foamable composition or foam of any preceding embodiment, wherein the salt is a choline salt. 30. The foamable composition or foam of any preceding embodiment, wherein the salt is choline hydroxy stearate. 31. The foamable composition or foam of any preceding embodiment, wherein the salt is choline 12-hydroxystearate. 32. The foamable composition or foam of any preceding embodiment, which comprises from about 1 to about 40 wt.% of the salt of a hydroxy fatty acid. 33. The foamable composition or foam of any preceding embodiment, which comprises from about 3 to about 30 wt.% of the salt of a hydroxy fatty acid. 34. The foamable composition or foam of any preceding embodiment, which comprises from about 5 to about 25 wt.% of the salt of a hydroxy fatty acid. 35. The foamable composition or foam of any preceding embodiment, which comprises from about 8 to about 22 wt.% of the salt of a hydroxy fatty acid, such as from about 10 to about 20 wt.%. 36. The foamable composition or foam of any preceding embodiment, which comprises from about 50 to about 99 wt.% water. 37. The foamable composition or foam of any preceding embodiment, which comprises from about 70 to about 97 wt.% water. 38. The foamable composition or foam of any preceding embodiment, which comprises from about 80 to about 92 wt.% water. 39. The foam of any preceding embodiment, which comprises from about 5 to about 25 vol.% water. 40. The foam of any preceding embodiment, which comprises from about 8 to about 20 vol.% water. 41. The foam of any preceding embodiment, which comprises from about 10 to about 18 vol.% water. 42. The foam of any preceding embodiment, which comprises from about 12 to about 16 vol.% water. 43. The foam of any preceding embodiment, which at least about 70 vol.% gas (e.g. air). 44. The foam of any preceding embodiment, which comprises at least about 75 vol.% gas (e.g. air). 45. The foam of any preceding embodiment, which comprises at least about 80 vol.% gas (e.g. air). 46. The foam of any preceding embodiment, which comprises from about 70 to about 95 vol.% gas (e.g. air). 47. The foam of any preceding embodiment, which comprises from about 80 to about 92 vol.% gas (e.g. air). 48. The foam of any preceding embodiment, which comprises from about 82 to about 90 vol.% gas (e.g. air). 49. The foam of any preceding embodiment, which comprises from about 84 to about 8 vol.% gas (e.g. air). 50. The foam of any preceding embodiment, wherein the volume ratio of gas (e.g. air) to water in the foam ranges from about 80:20 to about 92:8. 51. The foam of any preceding embodiment, wherein the volume ratio of gas (e.g. air) to water in the foam ranges from about 82:18 to about 90:10. 52. The foam of any preceding embodiment, wherein the volume ratio of gas (e.g. air) to water in the foam ranges from about 84:16 to about 88:12. 53. The foam of any preceding embodiment, wherein the volume ratio of gas (e.g. air) to water in the foam ranges from about 4:1 to about 12:1. 54. The foam of any preceding embodiment, wherein the volume ratio of gas (e.g. air) to water in the foam ranges from about 5:1 to about 9:1. 55. The foam of any preceding embodiment, wherein the volume ratio of gas (e.g. air) to water in the foam ranges from about 6:1 to about 8:1. 56. The foamable composition or foam of any preceding embodiment, which further comprises one or more gelling agents and / or nucleating agents. 57. The foamable composition or foam of any preceding embodiment, wherein the one or more gelling agents are selected from the group consisting of agar-agar, gelatin, pectin, carrageenan, pullulan, glucomannan, xanthan gum, guar gum, locust bean gum, konjac powder, psyllium husk, tara gum, gellan gum, and alginate (e.g. sodium alginate or calcium alginate); and / or wherein the one or more nucleating agents are selected from the group consisting of talc, calcium carbonate, silica, barium sulfate, titanium dioxide, sodium benzoate, felspar, and inorganic clays such as kaolin, montmorillonite, sepiolite, illite and bentonite. 58. The foamable composition or foam of embodiment 57, wherein the one or more gelling agents are selected from the group consisting of pullulan, glucomannan, xanthan gum and alginate (e.g. sodium alginate); and / or wherein the one or more nucleating agents are selected from the group consisting of inorganic clays such as kaolin, montmorillonite, sepiolite, illite and bentonite, preferably kaolin. 59. The foamable composition or foam of any of embodiments 57-58, wherein the foam comprises a total of from about 0.01 to about 10 wt.% gelling agent, such as from about 0.1 to about 10 wt.%, from about 0.5 to about 8 wt.% or from about 1 to about 6 wt.%; or wherein the foam comprises from about 0.01 to about 2 wt.% gelling agent, such as from about 0.05 to about 1 wt.%; and / or wherein the foam comprises a total of up to about 20 wt.% nucleating agent, such as up to about 15 wt.% or up to about 10 wt.%. 60. The foamable composition or foam of any of embodiments 57-59, wherein the foam comprises xanthan gum and alginate (e.g. sodium alginate) in the amounts of from about 0.1 to about 5 wt.% and from about 0.1 to about 3 wt.% respectively. 61. The foamable composition or foam of any of embodiments 57-60, wherein the foam comprises xanthan gum and alginate (e.g. sodium alginate) in the amounts of from about 0.4 to about 4 wt.% and from about 0.2 to about 2 wt.% respectively. 62. The foamable composition or foam of any preceding embodiment, which further comprises cellulosic nanoparticles. 63. The foam of any preceding embodiment, wherein the foam is an ultra stable foam, and / or wherein the foam is hydrophobic. 64. The foamable composition or foam of any preceding embodiment, which further comprises an active agent as used in agriculture, for example a fertilizer, herbicide, pesticide, fungicide and / or insecticide. 65. The foam of any preceding embodiment, wherein the foam freezes or crystallises at temperatures of less than 0°C. 66. The foamable composition or foam of any preceding embodiment, which has a pH of above about 7. 67. The foamable composition or foam of any preceding embodiment, which has a pH of from about 7 to about 12, such as from about 8 to about 12. 68. The foamable composition or foam of any preceding embodiment, which has a pH of from about 7 to about 9 or from about 8 to about 10. 69. A method of protecting one or more plants, comprising applying a foam as described in any preceding embodiment to the one or more plants, or forming a foam from the foamable composition as described in any preceding embodiment and then applying the foam to the one or more plants. 70. A method of protecting one or more plants, comprising applying a foam as described in any preceding embodiment to a plant protection device such as a greenhouse or polytunnel, or forming a foam from the foamable composition as described in any preceding embodiment and then applying the foam to a plant protection device such as a greenhouse or polytunnel. 71. A method of supplying nutrients to one or more plants, comprising applying a foam as described in any preceding embodiment to the one or more plants, or forming a foam from the foamable composition as described in any preceding embodiment and then applying the foam to the one or more plants. 72. A method of reducing and / or preventing the growth of weeds or other unwanted plants, comprising applying a foam as described in any preceding embodiment to soil, or forming a foam from the foamable composition as described in any preceding embodiment and then applying the foam to soil, where the soil may be soil where weeds are expected or predicted to grow and / or where weeds have previously grown. 73. A method of reducing and / or preventing the growth of weeds or other unwanted plants, comprising applying a foam as described in any preceding embodiment to the weeds or other unwanted plants, or forming a foam from the foamable composition as described in any preceding embodiment and then applying the foam to the weeds or other unwanted plants, wherein the foam comprises a biocide, weedkiller or means to prevent photosynthesis. 74. The method of any of embodiments 69-73, wherein the plants are protected from undesirable temperatures, pests (including birds and / or insects), bacteria and / or fungi. 75. The method of any of embodiments 69-74, wherein the plants are crops (which may be either pre- or post-harvest) or grapevines (preferably pre-harvest). 76. The method of any of embodiments 69-75, wherein the method comprises protecting the plants from cold or frost damage. 77. The method of any of embodiments 69-76, wherein the foam maintains the temperature of the plants and / or the international temperature of the foam above ambient temperature, optionally for a period of at least about 30 minutes, such as at least about 1 hour, at least about 2 hours, at least about 5 hours, at least about 8 hours or at least about 12 hours; preferably wherein the ambient temperature is less than 0°C, such as less than -2°C, less than -5°C or less than -10°C. 78. The method of any of embodiments 69-77, wherein the foam maintains the one or more plants and / or the international temperature of the foam at least about 1°C above ambient temperature, optionally for a period of at least about 30 minutes, such as at least about 1 hour, at least about 2 hours, at least about 5 hours, at least about 8 hours or at least about 12 hours; preferably wherein the ambient temperature is less than 0°C, such as less than -2°C, less than -5°C or less than -10°C. 79. The method of any of embodiments 69-78, wherein the foam maintains the one or more plants and / or the international temperature of the foam at least about 2°C above ambient temperature, optionally for a period of at least about 30 minutes, such as at least about 1 hour, at least about 2 hours, at least about 5 hours, at least about 8 hours or at least about 12 hours; preferably wherein the ambient temperature is less than 0°C, such as less than -2°C, less than -5°C or less than -10°C. 80. The method of any of embodiments 69-79, wherein the foam maintains the one or more plants and / or the international temperature of the foam at least about 3°C above ambient temperature, optionally for a period of at least about 30 minutes, such as at least about 1 hour, at least about 2 hours, at least about 5 hours, at least about 8 hours or at least about 12 hours; preferably wherein the ambient temperature is less than 0°C, such as less than -2°C, less than -5°C or less than -10°C. 81. The method of any of embodiments 69-80, wherein the foam maintains the one or more plants and / or the international temperature of the foam at least about 4°C above ambient temperature, optionally for a period of at least about 30 minutes, such as at least about 1 hour, at least about 2 hours, at least about 5 hours, at least about 8 hours or at least about 12 hours; preferably wherein the ambient temperature is less than 0°C, such as less than -2°C, less than -5°C or less than -10°C. 82. The method of any of embodiments 69-81, wherein the foam maintains the one or more plants and / or the international temperature of the foam at least about 5°C above ambient temperature, optionally for a period of at least about 30 minutes, such as at least about 1 hour, at least about 2 hours, at least about 5 hours, at least about 8 hours or at least about 12 hours; preferably wherein the ambient temperature is less than 0°C, such as less than -2°C, less than -5°C or less than -10°C. 83. The method of any of embodiments 69-82, wherein the foam maintains the temperature of the plants and / or the international temperature of the foam at or above 0°C. 84. The method of any of embodiments 69-83, wherein the foam maintains the temperature of the plants and / or the international temperature of the foam at or above 0°C when the ambient temperature is less than 0°C. 85. The method of any of embodiments 69-84, wherein the foam maintains the temperature of the plants and / or the international temperature of the foam at or above -2°C when the ambient temperature is less than -2°C. 86. The method of any of embodiments 69-85, wherein the foam maintains the temperature of the plants and / or the international temperature of the foam above -2°C for a period of at least 2 hours when the ambient temperature is less than -2°C, such as less than -5°C or less than -10°C. 87. The method of any of embodiments 69-86, wherein the plants are protected from pests. 88. The method of any of embodiments 69-87, wherein the plants are protected from overheating. 89. The method of any of embodiments 69-88, wherein the foam is applied in a layer having an average thickness of from about 0.1 cm to about 5 cm. 90. The method of any of embodiments 69-89, wherein the foam is applied in a layer having an average thickness of from about 0.3 cm to about 3 cm. 91. The method of any of embodiments 69-90, wherein the foam is applied in a layer having an average thickness of from about 0.5 cm to about 1.5 cm. 92. The method of any of embodiments 69-91, wherein the foam maintains the temperature of the plants and / or the international temperature of the foam below ambient temperature when the ambient temperature is more than about 30°C. 93. The method of any of embodiments 69-92, wherein the foam is applied by spraying. 94. The method of any of embodiments 69-93, wherein the method further comprises, after application to the one or more plants, removing the water from the foam or allowing the water to evaporate. 95. The method of any of embodiments 69-94, wherein the foam forms a residue after any water is removed or lost through evaporation. 96. The method of any of embodiments 69-95, wherein the residue reflects IR radiation and does not reflect UV radiation. 97. The method of any of embodiments 69-96, wherein the residue comprises a water soluble plant based matrix of cellulose and voids or air pockets. 98. The use of an aqueous foam as described in any proceeding embodiment in agriculture. 99. The use of an aqueous foam as described in any proceeding embodiment to protect a plant. 100. The use of an aqueous foam as described in any proceeding embodiment to supply nutrients to a plant, wherein the foam further comprises one or more plant nutrients.

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