The present invention relates to a method of controlling vegetation and to an apparatus for performing the method.

Conventionally, vegetation, such as weeds, is typically controlled either by physical removal, such as hand weeding, or by the use of herbicides. Hand weeding can be very hard work. Plants are becoming increasingly resistant to herbicides. In addition, there is increasing public resistance to the use of chemicals in the environment, and legislative changes to reduce use of toxic chemicals as herbicides. Therefore there is an interest in methods of controlling vegetation which do not use synthetic chemicals and which avoid hand weeding.

Thermal methods have been suggested for weed control. A variety of methods have been used to provide heat to weeds, for example, hot water, steam, hot air, flames and hot foam. These methods can overcome some of the disadvantages of herbicides such as spray drift and soil or groundwater pollution. WO 02/07513 (Waipuna

International) describes a process for weed control by application of hot foam, in which hot liquid containing a foaming agent is supplied to an applicator head, and air is supplied from a blower through a hose, so a hot foam is created. The hot liquid and air may be supplied to a nozzle within a shroud. However, to obtain hot foam, the air will either lower the temperature of the foam, or it will be necessary to preheat the air, which adds complexity to the equipment. An improved way of making foam for this purpose would be advantageous.

The present invention aims to improve a mobile apparatus for use in controlling vegetation.

The present invention provides an apparatus suitable for controlling vegetation which apparatus comprises a container for a hot liquid, release means connected to the container, and a nozzle assembly for applying a stream of foam which contains steam, connected to the container through the release means, wherein the nozzle assembly comprises a spraying duct enclosed by a shroud, so the shroud defines an enclosure around the spraying duct, the spraying duct defining an inlet passage communicating through a flow-restricting aperture with a primary mixing chamber of larger cross- sectional area than the flow-restricting aperture, the spraying duct defining at least one inlet port communicating between the primary mixing chamber and the enclosure, the spraying duct also defining a duct of varying cross-sectional area communicating between the primary mixing chamber and a secondary mixing chamber defining an outlet spraying nozzle, wherein a turbulence-enhancing structure is enclosed within the secondary mixing chamber. In a second aspect, the present invention provides a nozzle assembly for applying a stream of foam which contains steam, wherein the nozzle assembly comprises a spraying duct enclosed by a shroud, so the shroud defines an enclosure around the spraying duct, the spraying duct defining an inlet passage communicating through a flow-restricting aperture with a primary mixing chamber of larger cross- sectional area than the flow-restricting aperture, the spraying duct defining at least one inlet port communicating between the primary mixing chamber and the enclosure, the spraying duct also defining a duct of varying cross-sectional area communicating between the primary mixing chamber and a secondary mixing chamber defining an outlet spraying nozzle, wherein a turbulence-enhancing structure is enclosed within the secondary mixing chamber.

The duct of varying cross-sectional area may be of venturi shape.

In one example the turbulence-enhancing structure is a mesh of metal wires. It may comprise knitted metal wire mesh, for example of stainless-steel.

In one example the spraying duct also comprises a baffle outside the outlet spraying nozzle, to deflect fluids emerging through the outlet spraying nozzle. For example if the spraying duct extends generally vertically, such a baffle may be arranged so that the fluids are deflected to a generally horizontal direction. The emerging fluids typically emerge as a diverging spray of foam, and may for example diverge to form a conical spray pattern.

The spraying duct may define a plurality of inlet ports communicating between the primary mixing chamber and the enclosure. The nozzle assembly may also include an adjustable obturator to adjust the effective area of the inlet port or ports. The effect of the fluid flowing through the flow-restricting aperture into the primary mixing chamber is that gas from the enclosure is sucked into the fluids in the primary mixing chamber. When a water and surfactant is supplied to the nozzle assembly at above the boiling point of water, the fluids emerge from the outlet spraying nozzle as a foam which contains steam, so that the gas in the enclosure is a mixture of air and steam and is well above ambient temperature. In this way the air in the foam is pre-heated by the steam. The nozzle assembly is particularly effective at producing such a foam which contains steam. This foam would provide a thermal blanket effect. This is effective at killing weeds. The shroud may be of a resilient, flexible material, arranged as a curtain to surround the spraying duct. The nozzle assembly may, in use, be moved across the ground to kill weeds over an area. Preferably the shroud is shaped such that the shroud is close to the ground at the front of the nozzle assembly, but is clear of the ground at the rear of the nozzle assembly. This ensures that deposited foam is not wiped off by the shroud. Where an apparatus comprises a plurality of nozzle assemblies, each nozzle assembly may be provided with a separate shroud, or alternatively a plurality of nozzle assemblies may be surrounded by a common shroud. For example a single shroud may enclose three nozzle assemblies. The apparatus of the invention may be mounted on a wheeled trolley or trailer which may be propelled by a vehicle, or the apparatus may be mounted on a vehicle. The vehicle may be, for example, a tractor or quad bike. As a preferred option, the apparatus is mounted on a tractor. The hot-liquid container may comprise a heating means. The reservoir may therefore heat liquid only on demand, rather than storing heated liquid. The heating means is typically a heating element. The apparatus may comprise an input for mains power and a heating element suitable for connection to mains power. Where the apparatus is used in conjunction with a motor vehicle such as a tractor, the electrical power may be generated by the vehicle. When the apparatus is connected to a source of power, the relevant heating element or elements may be arranged to heat the liquid until the pressure in the container is above atmospheric pressure. In the case of water, a suitable temperature is between 95° and Ι Οδ'Ό. The release means controls and restricts the outflow of liquid, while the pressure within the hot-liquid container remains above atmospheric pressure. For example the pressure may be between 125 kPa and 300 kPa (absolute), for example between 175 kPa and 275 kPa, for example about 200 kPa or 225 kPa.

The pressure in the hot-liquid container may additionally be raised by a pump. For example the pressure may be up to 1 .5 MPa (15 atmospheres), more preferably up to 1 .2 MPa, for example 1000 kPa or 500 kPa. In one example the pressure is at 1 .0 MPa (10 atmospheres). The apparatus may also include a storage tank for liquid which may be at ambient temperature, and means to supply liquid from the storage tank to the container, where it is heated. In this case the hot-liquid container need only contain the liquid required for immediate use, as it heats liquid only on demand, rather than storing heated liquid. The total liquid capacity for an apparatus placed in a trailer or on a vehicle will depend on the size and carrying capacity of the trailer or vehicle, but it may be more than 2000 litres, more typically up to 1000 litres, for example up to 600 litres for a vehicle such as a tractor, and for a smaller vehicle or trailer more preferably up to 350 litres, and is typically up to 150 litres, for example from 75 to 125 litres, such as about 100 litres.

The liquid supplied to the reservoir is typically water combined with a surfactant. The surfactant is typically used at a concentration of up to 2%, preferably 0.05% to 2%, more preferably from 0.1 % to 1 %. The surfactant may be organic or inorganic. The surfactant is preferably a biodegradable foaming agent such as alkyl polyglycoside and is preferably a natural product. The surfactant may be mixed with the liquid in the reservoir, before or after the water is introduced into the reservoir, or it may be mixed with the hot liquid as it is dispensed from the reservoir. The liquid may also include a sequestering agent, to sequester metal ions that would cause hardness; the provision of such a sequestering agent may provide a more consistent foam.

The release means, for example, is a valve which allows liquid to exit from the reservoir when open. In one embodiment, the valve allows a limited pressure to build up in the reservoir so as to cause the liquid to exit from the nozzle when the valve is released. The apparatus may include a pressure sensor to prevent the pressure from becoming too high in the reservoir. However, as mentioned above, the pressure in the reservoir may be raised partly by heating the liquid and partly by a pump, so as to achieve a higher pressure.

An embodiment suitable for use with a vehicle such as a tractor, or mounted on a vehicle, may include several nozzle assemblies, so that a wide area of ground can be treated in one pass of the vehicle.

The shroud may be of a size so as to cover a small weed, thus creating a steam/foam chamber. Where the nozzle and shroud or a plurality of nozzles and shrouds are used while moving the associated vehicle, each shroud may be arranged to enclose respective nozzles at the front and sides, leaving an opening at the rear, relative to the direction of movement. In one embodiment each nozzle assembly is supported by a support bar that includes a hinge, so that if the nozzle assembly collides with an obstruction as the apparatus is moved over the ground, the nozzle assembly can swing back to clear the obstruction, and return once the obstruction has been passed.

A preferred embodiment comprises several nozzle assemblies arranged in groups, the orientation of each group of nozzle assemblies being adjustable. For example the nozzle assemblies may be carried by a horizontal support element that carries the groups of nozzle assemblies at positions that are spaced apart along the horizontal support element. By changing the orientation of the nozzles within each group, the distribution of foam over the ground can be altered. For example the nozzle assemblies within each group may be aligned generally parallel to the direction of movement, for example to kill weeds growing in the gaps between rows of crop plants without affecting the crop plants. If each group of nozzles assemblies is turned so as to extend transverse to the direction of movement, this increases the width of ground treated by each group of nozzle assemblies. The spacing between the groups of nozzle assemblies may be such that in this orientation there are no gaps between the strips of ground treated by the nozzle assemblies, so the entire surface area is treated. Where the apparatus comprises one or more heating elements, then each heating element is typically connected to a thermostat or a thermocouple which is used to turn the heater off when the liquid reaches the required temperature. The apparatus may include a plurality of heating elements, arranged so that the liquid is heated successively by the heating elements. For example there might be three heating elements in the container, each arranged to raise the temperature of the liquid by the same amount (e.g. 25° or 30°C), the liquid flowing past the three heating elements in series, in a container in which the liquid is heated on demand.

The present invention also provides the use of an apparatus of the invention to control vegetation.

The hot foam mixture comprises steam, for example up to 10% steam, or up to 5% steam (these being proportions by weight). The water in the container may be heated to a temperature of from 96 ^° C to 105 ^° C, preferably at least 102 ^° C, and optionally up to 1 10 ^' C, such as 107 ^* C.

In one embodiment the hot foam mixture further comprises a dye, and the foam is coloured or the residue left when the foam collapses is coloured. Foliage that is sprayed with the hot foam mixture typically dies within 1 day. This does depend on the type of plant and the amount of foam that contacts the plant. Plants that have a very waxy coating may require a larger amount of foam to impart sufficient heat to the plant. Some plants wilt and undergo a colour change within an hour.

Established weeds with a tap root may require repeat treatment.

The invention will now be further and more particularly described, by way of example only, and with reference to the accompanying drawings, in which:

Figure 1 shows a side view of a weed treating apparatus, also showing a tractor;

Figure 2 shows a rear view of the weed treating apparatus of figure 1 (but not showing the tractor);

Figure 3 shows a plan view of the foam dispensing machine of the weed treating apparatus of figure 1 ;

Figure 4 shows a side view of a component of the foam dispensing machine of figure 3; Figure 5 shows a side view in the direction of arrow B of figure 3;

Figure 6 shows a perspective view, partly in section, of a nozzle assembly of the invention; and

Figure 7 shows a perspective view of group of the nozzle assemblies of figure 6. Referring to figure 1 , there is shown a weed treating apparatus 10 and a tractor 5

(represented schematically). The weed treating apparatus 10 comprises a foam generating apparatus 12, which is intended to be mounted at the rear of the tractor 5, and a foam dispensing machine 14 which is intended to be mounted at the front of the tractor 5. It will be appreciated that the apparatus 10 might instead be mounted on a different agricultural vehicle, which may be a vehicle dedicated to this use. The foam dispensing machine 14 is envisaged as being mounted at the front of such a vehicle, as this enables the driver to align it accurately to kill weeds between rows of crops, but it will be appreciated that the two components of the weed treating apparatus 10 might instead both be mounted on a trailer to be towed behind such an agricultural vehicle.

The tractor 5 includes an engine 6, a cab 7, a three-point linkage 8 at the front, and a power takeoff 9 at the rear. Such features are conventional. The tractor 5 may also provide the facility for raising and lowering the three-point linkage 8. The foam generating apparatus 12 includes a steel frame 20 with a bracket 21 on the front side so it can be mounted onto the rear of the tractor 5.

Referring also to figure 2, within the lower part of the steel frame 20 is mounted a generator 24, which is adapted to be linked by a telescopic drive shaft with an overrun clutch to the power take off 9 from the tractor 5, so that the generator 24 is driven by the tractor's engine 6. In this embodiment the generator 24 is a three-phase 415 V generator, generating about 80 kVA. Supported by the steel frame 20 above the generator 24 is a water storage tank 26, which may for example have a capacity of 400 800 or 1200 litres. The steel frame 20, in this embodiment, also carries boxes 28, 29, 30 and 31 , which may be of a weatherproof material such as stainless steel or glass fibre reinforced composite material, and which provide enclosures for electrical or electronic components. The box 28 (which is shown partly broken away) encloses three electrical heaters

34 each provided with thermal insulation. The box 30 on the left side (as shown) of the generator 24 encloses control electronics 32 (shown schematically in broken lines), while the box 31 on the right side (as shown) of the generator 24 encloses a pump 36 (shown schematically in broken lines) and solenoid valves 38 (represented schematically). The box 29 above the heater box 28 may also contain electronic components. Each electrical heater 34 includes an electrical heating element and a cast aluminium block.

In a modification, the control electronics 32 and the solenoid valves 38 are enclosed in the box 29, and the pump 36 is adjacent to the generator 24, so the boxes 30 and 31 may be omitted.

In operation of the foam generating apparatus 12 the water which is to be heated is pumped by the pump 36 from the water tank 26 through the three electrical heaters 34 in series. Each heater 34 may have a nominal power of say 24 kW or 28 kW, and each may be powered by a different phase of the output from the generator 24. Each heater 34 is controlled by the control electronics 32 in accordance with the flow rate of the water, so that the water temperature is raised in stages, typically by between 20 °C and 30°C at each stage, to a final temperature between 95°C and 105°C or up to 1 10 ^° C, which may be above 102°C, for example 107 ^° C. The control electronics 32 monitors the water temperature at each stage, and also the temperature of the heater 34, for example using thermocouples.

The hot water emerging from the heater box 28 is at an elevated pressure for example 120 kPa (18 psi) to 135 kPa (20 psi) (absolute pressures), but more typically a higher pressure such as 400 kPa, 500 kPa, or up to 1000 kPa or 1200 kPa because of the pressure created by the pump 36. The hot water may not boil, because of the elevated pressure, until it emerges from a nozzle. By way of example the water flow rate may be up to 10 litres/minute or more, in this example, for example between 6 and 20 litres/min, depending on the electrical power available, which can supply several foam- dispensing nozzles; but if not all the foam-dispensing nozzles are in use, the flow rate of water to the electrical heaters 34 in the heater box 28 is reduced. This may utilise an electronically-controlled valve to divert some of the water from the pump 36 back to the water tank 26.

The heater box 28 contains only the water required for immediate use, as the water is heated only as it is being dispensed. That is to say, the water is heated on demand.

A surfactant such as an alkyl polyglycoside may be mixed with the water in the water tank 26 before or after the water is introduced into the water tank 26, or it may be mixed with the water as it is passed into the heater box 28, or may be mixed with the hot water as it flows out of the heater box 28, or after it has emerged from the heater box 28. The quantity of surfactant may be about 0.1 % by weight of water, typically between 0.05% and 0.5%, although it may be higher, for example 1 %, 1 .5% or 2%.

The solenoid valves 38 control the outflow of hot fluid through various outlet nozzles. For example hot fluid may be supplied to a hand-held spray lance with a nozzle at its end, so an operator walking alongside the tractor 5 can treat individual weeds; or hot fluid may be supplied to spray nozzles mounted behind the rear wheels of the tractor 5, so as to treat weeds growing in the wheel ruts. As the hot fluid emerges from the nozzle it forms a foam which contains steam, which blankets the surface of the weed, and kills the weed. The foam dispensing machine 14, described in more detail below, can carry several such dispensing nozzles and enables a large area to be treated easily.

Referring now to figure 3, the foam dispensing machine 14 consists of a support bar 40 supported at each end by a wheel 42. The rear face of the support bar 40 is connected by brackets 43 and hinged parallel plate linkages 44 to a crossbar 46. The crossbar 46 carries brackets 48 and an upwardly-extending link bar 41 , which enable it to be connected to the three-point linkage 8 at the front of the tractor 5.

The front face of the crossbar 46 carries two manifold boxes 50 on thermally insulating mounts 49, each with an inlet 51 which is connected by a hose 52 (only one is represented, schematically) to one of the solenoid valves 38; such hoses 52 may extend along the underside of the tractor 5. Each manifold box 50, in this example, has four outlet ports 54 to which outlet hoses 55 (only one is represented, schematically) can be connected using dry break hydraulic connectors. Each manifold box 50 also has an outlet 53 which is connected by a hose equivalent to the hose 52 to carry any excess liquid back to the water tank 26. Each hose 52 that extends along the underside of the tractor 5 is provided with a dry break hydraulic connector. A support tube 60 is fixed a short distance in front of the front face of the support bar 40, supported by a bracket 59 at the middle of the support bar 40 and by end plates 62 which are bolted onto the ends of the support bar 40. Several nozzle modules 64 can be mounted along this support tube 60, six being shown in figures 2 and 3. In figure 4 one such nozzle module 64 is shown in isolation. The module 64 includes an L-shaped clamping block 65 shaped to rest against the front face and the underside of the support bar 40. This defines a transverse hole 66 through which the support tube 60 passes, and a clamp 67 to secure the block 65 to the support tube 60. It also defines vertical hole through which a support bar 68 can slide, and a clamp 69 to secure the support bar 68 to the block 65. The lower end of the support bar 68 is linked by a hinge 70 to a curved L-shaped bracket 72 to which a nozzle support plate 73 is connected. The nozzle support plate 73 supports three nozzles 74 spaced apart along a straight line, and arranged to spray the hot liquid below the nozzle support plate 73; in plan view, as shown in figure 3, the nozzle support plate 73 is lozenge-shaped, with rounded ends, but of length about five times its width. Below the nozzle support plate 73 is a flexible rubber curtain or skirt 75 which splays out. Considering the dimensions at the base of the curtain or skirt 75, the length may be 350 mm and the breadth 100 mm; it is desirable to have a ratio of length to breadth of at least 2.5 and preferably at least three, considering the region over which foam is being sprayed at any one time.

The curtain or skirt 75 may consist of a plurality of flexible rubber curtain portions each of which, along its top edge, can slide along one of two grooves adjacent to the periphery of the nozzle support plate 73, each being of length less than that of the periphery. By adjusting the positions of these curtain portions, and the extent to which they overlap each other, a gap in the skirt 75 may be provided at the rear side of the nozzle support plate 73. Consequently when the hot liquid from the foam generating apparatus 12 is dispensed through the nozzles 74, foam is generated below the nozzle support plate 73 in a space that is enclosed on the front and the sides, but is not enclosed at the rear. Each nozzle 74 generates a conical spray of foam (indicated as 74a in figure 4).

In addition the L-shaped bracket 72 also carries a liquid supply manifold 76, with an inlet 78 to communicate with a hose 55 from the manifold box 50 (see figure 3), and with three outlets 80 (only two of which are visible in figure 4) which are connected by hoses 82 (one of which is shown schematically) to the nozzles 74.

The orientation of the nozzle support plate 73 relative to the L-shaped bracket 72, and so relative to the direction of movement of the foam dispensing machine 14, can be adjusted, and can be fixed by a clamp 84.

As shown in figure 5, the wheels 42 are fixed to the ends of the support bar 40 by a clamp 86 which is clamped around the support bar 40 and by an inclined shaft 87 to ensure that the wheel 42 rides up over any ruts in the ground. The inclined shaft 87 includes a jack mechanism with a handle 88 so the position of the wheel 42 relative to the support bar 40 can be adjusted. In a modification, the wheels 42 are castered, so they come into alignment with the direction of travel. The nozzle modules 64 are hence supported by the support tube 60. By temporarily disconnecting one end plate 62 and removing the wheel support clamp 86 some nozzle modules 64 may be removed, or additional nozzle modules 64 installed. The manifold boxes 50 provide a total of eight outlet ports 54, so there can be up to eight nozzle modules 64 mounted on the support tube 60. The transverse position of each nozzle module 64 can be adjusted by sliding it along the support tube 60, and then fixed with the clamp 67. The vertical height of the nozzle support plate 73 can be adjusted by raising or lowering the support bar 68, and fixed with the clamp 69. The hinge 70 ensures that the nozzle support plate 73 can swing back to clear any obstacle in its path, such as a stone.

With the nozzle support plates 73 in the orientation as shown, there are narrow gaps between the regions of the ground that are covered with foam. If the nozzle support plates 73 are adjusted so as to be aligned more closely with the direction of movement, then the gaps between the regions that are covered with foam become larger. On the other hand, if the nozzle support plates 73 are adjusted to extend at a larger angle to the direction of movement, they can be arranged so there are no gaps between the regions treated with foam. Hence the foam dispensing machine 14 can be adjusted either to treat weeds that are growing between rows of plants, or to treat an entire area. Typically operation of the weed treating apparatus 10 is controlled by the operator using a control panel in the cab 7. In operation the pump 36 supplies a stream of water from the water tank 26 to the electrical heaters 34 in the heater box 28, this water containing surfactant. The pump 36 controls both the pressure and the flow rate of the fluid stream. The flow rate is set in accordance with the number of nozzles 74 that are in use; the operator may for example actuate switches or touch-screen buttons within the control electronics 32 to indicate the number of operating nozzles 74. The resulting high pressure liquid stream is supplied through the solenoid valves 38 and the hoses 52 to the inlets 51 on the manifold boxes 50, and so through the hoses 55 to the nozzle modules 64. Any excess liquid in the manifold boxes 50 is recirculated back to the water tank 26. In some applications, the flow rate is controlled in accordance with the number of nozzles 74 sufficiently accurately that there is no excess liquid in the manifold box 50, and in this case the outlet 53 and the return hose can be omitted.

As a desirable option the cab 7 may include a computer linked to or including a GPS aerial, and linked to the control electronics 32. Hence the computer can store data that indicates which areas have been treated, and when they were treated. Referring again to Figure 5, the foam dispensing machine 14 may include a pneumatic damper and control ram 90 which extends between the upwardly-extending link bar 41 and the support bar 40. This has two effects. When the foam dispensing machine 14 is being moved over bumpy ground, the ram 90 dampens the bouncing movement of the support bar 40 and the components fixed to it. If the driver of the tractor 5 raises the three-point linkage 8 with the foam dispensing machine 14 attached, the ram 90 limits the extent to which the support bar 40 can drop, restricting the movement of the parallel plate linkages 44, and so enabling the driver to lift the foam dispensing machine 14 clear of the ground, for example at the end of a row of crops. The foam dispensing machine 14 may also be provided with adjustable legs or a jockey wheel 92, as indicated in broken lines in figure 5, so that the crossbar 46 does not rest on the ground when disconnected from the tractor 5.

In the embodiment described above, each support plate 73 carries three nozzles 74 surrounded by a common shroud 75. In an alternative arrangement, each nozzle 74 is provided with its own shroud.

Referring now to Figure 6 there is shown a perspective, partly sectional view, of a nozzle assembly 100 which may be used in the foam dispensing machine 14, the nozzle assembly 100 corresponding to a single nozzle 74 with its own shroud. The nozzle assembly 100 includes a rigid top plate 101 which is of generally triangular shape in plan, with rounded corners, and which may be of a metal or an engineering plastic such as acetal. Around the periphery of the top plate 101 is attached a shroud 102 of flexible rubber for example a fluoroelastomer (such as Viton (trade mark)) which hangs down to close to the ground (when in the operating position as shown for example in figure 2). In this example the shroud 102 is cut away at a rear portion 103, so that it does not displace deposited foam as the foam dispensing machine 14 is moved.

The nozzle assembly 100 also includes a spraying duct 104 enclosed by the shroud 102, so the shroud 102 defines an enclosure around the spraying duct 104. The spraying duct 104 define an inlet tube 106 of internal diameter 4 mm, which projects above the top plate 101 , and to which a hose 82 (see fig 4) may be connected. The inlet tube 106 is integral with a circular flange 107 and with a short tubular portion 108 that projects below the flange 107. Below the top plate 101 the inlet tube 106 is screw fitted onto a throttle duct 1 10 which defines a circular flange 1 1 1 , such that top plate 101 is sandwiched between the flanges 107 and 1 1 1 . Inset into the short tubular portion 108 is a tubular block 1 12 with a conically-tapered base, which defines a flow-restricting aperture 1 13 which in this example is of diameter 0.7 mm. The inlet tube 106 and the throttle duct 1 10 may for example be of nickel plated brass, while the tubular block 1 12 may be of stainless steel.

The throttle duct 1 10 defines a primary mixing chamber 1 14, of diameter about 10 mm, which tapers down to a 2 mm tube portion 1 15 and which then broadens out again more rapidly into a secondary mixing chamber 1 16, so overall having a venturi shape. The throttle duct 1 10 also defines four 4 mm diameter holes 1 18 through the wall just below the circular flange 1 1 1 . A ring 1 19 which also defines four 4 mm diameter holes 120 surrounds the throttle duct 1 10 at the location of the holes 1 18, and is secured by a nut 122. This enables the effective diameter of the holes 1 18 to be adjusted; in a modification, the ring 1 19 and the nut 122 may be omitted. Abutting the lower end of the throttle duct 1 10 is a knitted stainless steel wire mesh 124 fixed in a brass ring 125.

A sleeve 130 fits around the lower part of the throttle duct 1 10 and the brass ring 125, being sealed by an O-ring in a circumferential groove 131 in the throttle duct 1 10, and being secured by a pin 132 which engages a second circumferential groove 133 in the throttle duct 1 10. Below the brass ring 125 the sleeve 130 tapers conically to a 4 mm diameter outlet 134, below which it defines an inclined baffle 135. The sleeve 130 may be of an engineering plastic such as nylon. The outer surface of the sleeve 130 defines opposed flats.

A bracket 140 is fixed to the underside of the top plate 101 , projects down alongside the rear side of the spraying duct 104, that is to say the throttle duct 1 10 and the sleeve 130; then projects forward, defining a circular hole with two flats to engage the outer surface of the sleeve 130; and then curves downwardly to protect the front face of the lower part of the sleeve 130. In a modification in which the sleeve 130 is supported by the bracket 140, the pin 132 may be omitted.

Thus the spraying duct 104 consists of the inlet tube 106, the throttle duct 1 10 and the sleeve 130. Together these define a flow path for the hot water supplied from the heater box 28. The flow path includes the flow-restricting aperture 1 13 which imposes the major pressure drop; this opens out into the primary mixing chamber 1 14, to which air and steam are sucked from the enclosure within the shroud 102. This mixture passes through the venturi-shaped duct to the secondary mixing chamber 1 16, and then through the mesh 124. These successive steps have been found very effective at creating foam, as the mixture emerges through the outlet 134; this is because they create significant turbulence. The degree to which foam is formed depends on the flow rate through the system, which can be controlled by the operator. By way of example the pressure upstream of the restricting aperture 1 13 may be 10 atmospheres (gauge), and the flow rate through a single nozzle assembly 100 may be between 0.5 and 1 .0 litres per minute. Referring to Figure 7 there is shown an arrangement in which three separate nozzle assemblies 100, each with its own shroud 102, are supported by a single L- shaped bracket 72. A support bar 142 is rotatably attached by a swivel joint 144 to the underside of the L-shaped bracket 72, the joint 144 including the clamp 84. The support bar 142 is attached to the circular flange 107 of each of the three nozzle assemblies 100, and in each case the inlet tube 106 projects, so a hose 82 (see figure 4) can be connected to it. Consequently the orientation of the support bar 142 relative to the L- shaped bracket 72, and so relative to the direction of movement of the foam dispensing machine 14, can be adjusted, and can be fixed by the clamp 84. The orientation of the top plate 101 can be freely adjusted, as the top plate 101 is not fixed to the circular flange 107. If the orientation of the top plate 101 is changed, this turns the bracket 140 and so alters the orientation of the sleeve 130. Consequently the foam is always directed in the same direction relative to the top plate 101 . It will be appreciated that a weed killing apparatus may differ from that described in relation to the figures. Within the spraying duct 104, instead of the knitted mesh 124 there might instead be successive layers of fine stainless steel mesh arranged in a random cross-lamination arrangement. For example spraying ducts 104 may be used in place of the nozzles 74 in the apparatus shown in figures 1 -5, with for example three spraying ducts 104 supported by the support plate 73 and enclosed within a common shroud 75. As another modification the weed killing apparatus 10 may be installed permanently as part of a dedicated vehicle, rather than being mounted on a general- purpose vehicle such as the tractor 5. The width of the foam dispensing machine 14 may differ from that described, and it may carry a smaller number or a larger number of nozzle modules 64. Where there are a larger number of nozzle modules 64 it may be appropriate to use a water tank 26 of greater capacity, to prolong the period for which the apparatus 10 can operate without refilling.

The control electronics 32 preferably includes indicators, visible by the operator, indicating the values of parameters such as temperatures, flow rates, and pressures. In one embodiment a touch-screen controller is provided in a portable box connected via a flexible cable or umbilical cord to the control electronics 32. Such a touch-screen controller may include controls such as an on/off switch for the screen itself, an indicator to show if the power takeoff 9 is at the correct speed, and an emergency stop button to disconnect the power to the heaters 34 and the pump 36. This touch-screen controller would normally be mounted in the cab 7, for example on a sucker pad, so the operator can use it when in the cab 7. Alternatively the operator can remove the touch-screen controller and attach it elsewhere, or hold it in his hand, for example when the operator is controlling weeds using a hand-held spray lance, so he can continue to control foam production without having to climb back into the cab 7.

The nozzle assembly 100 has been described above in the context of a large- scale weed treating apparatus 10 carried by a tractor 5. It will be appreciated that the nozzle assembly 100 may form part of a hand-held spray lance, connected by a hose to the pump 36. Furthermore the nozzle assembly 100 may be used as part of a small- scale portable weed treating apparatus which is entirely self-contained, including a hot water reservoir, and means to dispense hot water at pressure through the nozzle assembly 100.