Weed control apparatus and method

Abstract

Apparatus for selective control of weeds amongst crops comprises at least one autonomous flying (1) or ground (15) vehicle provided with cameras (5,5a) and image processing equipment so that it may identify a weed (7) and move to its vicinity. The vehicle (1,15) is equipped with a microwave generator and a probe (4) that is inserted into soil adjacent the roots (9) of the weed (7). The probe (4) acts as an antenna to transmit microwave radiation which is absorbed by the soil and by the roots (9) of the weed (7). Water within the roots (9) and adjacent soil is heated by the microwaves, killing the roots (9) and / or lower stem of the weed (7), and hence the weed (7) as a whole. A base station (10) allows recharging and storage for multiple vehicles (1,15) and is ideally powered by solar cells (14).

Description

WEED CONTROL APPARATUS AND METHOD

[0001] The present invention relates to apparatus for the suppression of weeds, and to a method for suppressing weeds. More particularly but not exclusively, the invention relates to apparatus for the suppression of weeds by the targeted application of microwave radiation, and to a method for applying targeted microwave radiation to suppress weeds.

[0002] It is a perennial problem for those growing crops that unwanted plants grow intermingled with the desired plants, competing for nutrients and moisture, and possibly even contaminating the crop on harvesting. Such unwanted plants are customarily referred to as "weeds".

[0003] In modem agriculture, the most common approach to controlling and eliminating weeds is the widespread application of herbicides. Herbicides have been developed that selectively attack certain types of weed while having less effect on the desired crop, but there are still significant drawbacks with this approach.

[0004] At the time of writing, around the world an estimated 1.7 million tonnes of herbicides are applied to crops each year. The financial cost of this is immense, and the environmental and health issues associated with these materials are well-documented. Some of the more effective herbicides are known carcinogens, such as glyphosate, and most are too indiscriminate in their action on non-target species. Even well-managed use can result in run-off to watercourses and fish kills. The contamination of irrigation and drainage channels in turn leads to the inhibition of phytoplankton, periphyton and macrophytes, and reduced invertebrate species richness and abundance, resulting in a reduction of sensitive species and reduced abundance of even tolerant species.

[0005] It is therefore desirable to devise alternative approaches that obviate the application of herbicides, thus reducing their unintended health risks and environmental consequences, while still having the required effect on unwanted plant life.

[0006] One approach that has been considered is the application of heat. The direct application of conventional heat sources, such as flame, is far too indiscriminate in most situations, but some progress has been made with microwave heating, using technologies related to those used for domestic cooking and for some industrial heating processes.

[0007] Systems such as those disclosed in US Patent Application No US 2019 / 0274296 and US Patent Application No US 2002 / 0090268 have been put forward for weed control. However, these systems are for blanket weed suppression, and they are as indiscriminate as many herbicides. Similar systems with only slightly greater power are used to decontaminate soil polluted with organic toxins through the pyrolysis of the majority of the organic matter present.

[0008] The mode of action of these systems is simple overheating and thermal stress killing the plants. There may be a use for such systems to control weeds around the more robust plants, such as vines or trees, but they are not viable with most crops. Existing systems of this type are expensive in terms of capital and running costs. The equipment required is large and heavy, and not particularly mobile. Since they flood the underlying soil indiscriminately with microwaves, they can kill all the plants in a treated area, except for the most robust and deep-rooted, and so are of little use around most crops. The broadcast heating also affects any microbes, insects and other invertebrateswithin the soil, including many generally beneficial organisms. There is even a risk of killing or injuring vertebrates unable to escape, in a painful and arguably inhumane manner. These systems can hence have a greater adverse effect on the environment as a whole than broad-spectrum herbicides.

[0009] It is hence an object of the present invention to provide a method for suppressing weeds using microwave radiation that obviates the drawbacks of existing methods. It is a further object of the present invention to provide equipment for suppressing weeds using microwave radiation that obviates the drawbacks of existing equipment. It is also an object of the present invention to provide an integrated system for suppressing weeds.

[0010] According to a first aspect of the present invention, there is provided a method for suppressing weeds, comprising the steps of: providing a source of microwave radiation; providing antenna means operatively connected to said source of microwave radiation; inserting the antenna means into soil adjacent a weed; and transmitting microwave radiation through the antenna means into the weed.

[0011] Preferably, the antenna means is inserted adjacent an underground portion of the weed.

[0012] Advantageously, the microwave radiation is then transmitted through the antenna means and the soil into said underground portion of the weed.

[0013] The microwave radiation thus causes heating of the weed, especially the underground portion of the weed and / or a portion of the weed adjacent a surface of the soil, sufficient to cause the weed to wilt and die.

[0014] The underground portion of the weed preferably comprises a root structure of the weed.

[0015] Preferably, the method comprises the step of providing vehicle means to which the source of microwave radiation and the antenna means are mounted.

[0016] Advantageously, said vehicle means comprises autonomous vehicle means.

[0017] The method may comprise the step of providing the vehicle means with data processing means and programming the data processing means to control a course and speed of the vehicle means.

[0018] The method may comprise the step of providing vehicle means capable of flight.

[0019] The method may then comprise the step of providing vehicle means comprising at least one rotor means.

[0020] The method may comprise the step of providing vehicle means adapted to travel on a ground surface.

[0021] The method may then comprise the step of providing vehicle means having wheels.

[0022] The method may comprise the step of providing vehicle means having track means.

[0023] The method may comprise the step of providing vehicle means equipped with air cushion means.

[0024] Preferably, the method comprises the step of providing equipment having means to locate and identify weeds.

[0025] Advantageously, the method comprises the step of providing image capture equipment adapted to locate and identify weeds.

[0026] The method may comprise the step of providing the means to locate and identify weeds with image processing means.

[0027] The method may comprise the step of providing data processing means programmed or programmable to process captured images so as to compare them with reference images.

[0028] The image processing means may then comprise database means comprising a plurality of images of plants deemed to be weeds.

[0029] The means to locate and identify weeds may thus identify and locate weeds by means of their visual similarity to reference images of known weeds.

[0030] Preferably, the method comprises the step of operatively connecting the means to locate and identify weeds to control means comprising data processing means programmed or programmable to control a course and speed of the vehicle means.

[0031] The method may then comprise the step of programming the control means to cause the vehicle means to move adjacent to an identified and located weed.

[0032] The method preferably comprises the step of providing location means adapted to determine a location of the equipment.

[0033] The method advantageously comprises the step of providing the location means with data defining a region within which weeds are to be suppressed.

[0034] The method may then comprise the step of operatively connecting the location means to control means for the equipment, such that said control means restricts operation of the equipment to within said region.

[0035] Preferably, the source of microwave radiation comprises solid state microwave generator means.

[0036] Preferably, the source of microwave radiation has a power of at least 250W, ideally at least 500W.

[0037] According to a second aspect of the present invention, there is provided equipment for weed control comprising: a source of microwave radiation; and antenna means operatively connected thereto; wherein the antenna means is insertable into soil adjacent a weed; and is adapted to transmit microwave radiation from said source into said weed.

[0038] Preferably, the antenna means is insertable into soil adjacent an underground portion of the weed.

[0039] Advantageously, the antenna means is adapted to transmit microwave radiation through the soil to said underground portion of the weed.

[0040] Preferably, the equipment comprises vehicle means to which the source of microwave radiation and the antenna means are mounted.

[0041] Advantageously, the vehicle means comprises autonomous vehicle means.

[0042] The vehicle means may then comprise data processing means programmed or programmable to control a course and speed of the vehicle means.

[0043] Said vehicle means may be capable of flight.

[0044] The vehicle means may then be provided with at least one rotor means.

[0045] Said vehicle means may be adapted to travel on a ground surface.

[0046] The vehicle means may then be provided with wheels.

[0047] The vehicle means may be provided with track means.

[0048] The vehicle means may be provided with air cushion means.

[0049] Preferably, the equipment is provided with means to locate and identify weeds.

[0050] Advantageously, the means to locate and identify weeds comprises image capture equipment.

[0051] The means to locate and identify weeds may comprise image processing means.

[0052] Said image processing means may comprise data processing means programmed or programmable to process captured images so as to compare them with reference images.

[0053] The image processing means may then comprise database means comprising a plurality of images of plants deemed to be weeds.

[0054] The means to locate and identify weeds may thus identify and locate weeds by means of their visual similarity to reference images of known weeds.

[0055] Preferably, the means to locate and identify weeds is operatively connected to control means comprising data processing means programmed or programmable to control a course and speed of the vehicle means.

[0056] Said control means may be programmed or programmable to cause the vehicle means to move adjacent to an identified and located weed.

[0057] Preferably, the equipment comprises location means adapted to determine a location of the equipment.

[0058] Advantageously, the location means is provided with data defining a region within which weeds are to be suppressed.

[0059] The location means may then be operatively connected to control means for the equipment, said control means restricting operation on the equipment to within said region.

[0060] Preferably, the source of microwave radiation comprises solid state microwave generator means.

[0061] Preferably, the source of microwave radiation has a power of at least 250W, ideally at least 500W.

[0062] According to a third aspect of the present invention, there is provided a system for weed suppression comprising a base station and a plurality of pieces of equipment as described in the second aspect above.

[0063] Preferably said base station comprises a plurality of equipment charging stations, each adapted to receive at least one of said pieces of equipment.

[0064] Advantageously, each equipment charging station is operatively connectable to a piece of equipment in order to transfer electrical power thereto.

[0065] The base station may be provided with energy collection means, such as solar panel means or wind turbine means.

[0066] The base station may be transportable to a selected site for use.

[0067] The base station may be provided with wheels permanently or detachably mounted thereto, for example to allow transportation by being towed.

[0068] Embodiments of the present invention will now be more particularly described by way of example and with reference to the Figures of the accompanying drawings, in which:Figure 1 is a schematic side elevation of a first, aerial vehicle embodying the present invention in operation;Figure 2 is a schematic perspective view of a base station for the aerial vehicle of Figure 1 ;Figure 3 is a schematic side elevation of a second, ground vehicle embodying the present invention;Figures 4a to 4c are a schematic perspective view, a schematic side elevation, and a schematic plan view, respectively, of a microwave probe as used in the present invention;Figure 5 is a plot of irradiation intensity against angle for microwave probes having different lengths relative to a wavelength of microwave radiation emitted therefrom;Figures 6a to 6c are thermal images for soil irradiated by a microwave probe having a relatively short length;Figures 7a to 7c are thermal images for soil irradiated by a microwave probe having a relatively long length;Figures 8a to 8c are thermal images for dry soil irradiated by the microwave probe of Figures 6a to 6c;Figures 9a to 9d are plan views from above of pots of basil seedlings that have been irradiated for varying periods;Figure 10 is a plot of the duration of irradiation versus a radius at which the basil seedlings of Figures 9a to 9d are killed; andFigures 11a to 11d are plan views from above of pots of thyme seedlings that have been irradiated for varying periods.

[0069] Referring now to the Figures, and to Figure 1 in particular, a weedkilling LIAV 1 (LIAV = “unmanned aerial vehicle”, often referred to as simply a “drone”) embodying the present invention comprises a main body 2 having four propel lers / rotors 3 mounted to its upper, in use, surface (only two propellers 3 are visible in this view, the others being hidden behind the visible propellers 3). The propellers 3 are powered by electric motors within the main body 2 such that the UAV 1 is capable of flight, including vertical take-off and landing (“VTOL”) and hovering. An elongate probe 4 extends downwardly from an underside of the main body 2 of the UAV 1. The probe 4 is operatively connected to a source of microwave radiation located within the main body 2, such that the probe 4 can act as a microwave antenna.

[0070] Also located on an underside of the main body 2 of the UAV 1 is a camera 5. The camera 5 is connected to a data processing unit, also located within the main body 2, which is programmed with image processing software that enable it to identify and locate known types of weed 7.

[0071] In operation, the UAV 1 flies over a field 6 or the like under autonomous control until the camera 5 and associated data processing unit identify an example of a weed 7 that the UAV 1 is intended to eradicate. The UAV 1 then flies over to the weed 7 and reduces altitude, such that its probe 4 is inserted into the soil of the field 6 adjacent the weed 7. A distal portion 8 of the probe 4 is thus located close to a root structure 9 of the weed 7. (Alternatively, the UAV 1 may simply land on a surface of the field 6, thus inserting substantially the whole of the probe 4 into the soil).

[0072] The probe 4 is then activated by operating the source of microwave radiation, thus transmitting microwaves through the probe 4, particularly its distal portion 8 (see below for details). This causes localised healing of soil adjacent the distal portion 8 of the probe 4 and so also the root structure 9 of the weed 7. In any case, the root structure 9 of the weed 7 will in most cases have a relatively high moisture content and will therefore preferentially absorb microwave radiation of the appropriate frequency. (Additionally, if the probe 4happens not to be fully inserted into the soil, microwaves will also be transmitted through open air to adjacent portions of the weed 7 above a surface of the soil, such as a lower portion of its stem).

[0073] Thus, the root structure 9 of the weed 7 can be selectively heated to a point at which it is likely to die off, causing the weed 7 as a whole to wilt and die. (The duration and intensity of irradiation required can be established as disclosed below). The location of the probe 4 adjacent a specific identified weed 7 means that only the targeted weed 7 is likely to be affected. Targeting the root structure 9 is far more likely to kill off the weed 7 than existing methods, which primarily act on above-ground portions of a weed 7 - mainly its foliage. Many weeds 7 are capable of regrowth from a root stock 9 even if their above-ground portions are killed by microwave heating (or simply by being mowed down to ground level).

[0074] The image processing software will be selectively programmed to distinguish weeds 7 from desirable plants to be found within the field 6, even if the desirable plants are only in the form of seedlings. This contrasts with existing microwave heating systems that would indiscriminately "cook" any plant extending above the soil surface.

[0075] It is envisaged that these weed-killing UAVs 1 would be deployed in swarms from a base station 10 such as that shown in Figure 2. These base stations 10 could for example be transportable on a trailer or the like, or could be provided with wheels and towed. In the example of Figure 2, the base station 10 has been placed in a corner of a chosen field 6, adjacent a hedge 11 or fence.

[0076] Before the base station 10 is deployed, the field 6 would be surveyed and standard geofencing techniques used to define an area beyond which the UAVs 1 may not fly.

[0077] The base station 10 has a plurality of launch bays or cells 12, each containing one or more UAVs 1. Each launch bay 12 can be opened torelease its UAV or UAVs 1 - see opened launch bay 13. While within its launch bay 12, a UAV 1 will be charged up for use, here using power from an array of solar panels 14 located on an upper surface of the base station 10. The UAVs 1 can also be programmed or re-programmed while within their launch bays 12 to help to distinguish desirable plants present in the field 6 from unwanted weeds 7.

[0078] The UAVs I are thus released from their launch bays 12,13 to fly autonomously within the geofenced boundaries of the field 6, hunting for weeds 7, inserting their probes 4 into the soil adjacent a root structure of an identified weed 7, and irradiating the root structure 9 so as to kill off that weed 7.

[0079] When a UAV 1 is running low on power, it will automatically return to a launch bay 12,13 of the base station 10, where it will be recharged, then launched to continue operation.

[0080] This approach may be used not only with aerial UAVs 1 but also with autonomous ground vehicles or UGVs 15, such as the alternative embodiment of the present invention shown in Figure 3. The UGV 15 comprises a main body 16, here mounted on four wheels 17, although tracked versions are also envisaged. This particular example has both a camera 5 located on an underside of the main body 16 to permit accurate location of the autonomous ground vehicle 15 relative to a weed 7, and a second camera 5a on an end of the main body 16 to allow location and recognition of weeds 7 at a distance, such that the vehicle 15 can then move towards the identified weed 7.

[0081] The UGV 15 operates in substantially the same manner as the corresponding UAV 1 . It moves within the boundaries of the field 6 as defined by geofencing techniques, until it locates a weed 7. It then moves to the weed 7 and parks over it.

[0082] Like the UAV 1 , the UGV 15 has an elongate probe 4 extending downwardly from the underside of its main body 16, connected to a source ofmicrowave radiation so that it can act as a microwave antenna. A major difference between the UAV 1 and the UGV 15 that the probe 4 of the latter is controllably extensible, so that its distal portion can be inserted into the soil of the field 6 adjacent the weed 7. (In alternative embodiments, this can also be achieved by providing suspension arrangements for the wheels 17 that allow the main body 16 to be lowered as a whole, towards or into contact with the surface of the field 6; this allows a probe 4 fixed to the main body 16 to be inserted into the soil, as an alternative to an extendable probe 4).

[0083] Once a distal portion of the probe 4 is disposed in the soil adjacent the root structure 9 of a weed 7, microwave radiation is transmitted through the probe 4, heating the soil locally and in particular heating the root structure 9 selectively, killing the weed 7.

[0084] A typical monopole antenna suitable for use in the present invention is shown in Figures 4a to 4c.

[0085] This comprises an elongate probe 4, here shown extending vertically upwardly - NB in the apparatus of Figures 1 and 3, this probe 4 is mounted to extend vertically downwardly. A 500W solid state microwave generator (not shown) aligned along the z-axis is mounted directly to a coaxial connector 21 of the probe 4. An elongate core of the probe 4 extends along the z-axis to act as a microwave antenna, while an outer conductor 20 of the probe extends laterally from a base of the probe 4 to form a substantially discoidal ground plane 20.

[0086] As well as the monopole antenna shown in Figures 4a to 4c, it is possible to use dipole or multipole antennae. It is also possible to use a leaky waveguide arrangement. However, for simplicity, the behaviours of systems employing the basic monopole antenna is described below.

[0087] The arrangement shown allows for the effect of the length of the probe 4 (as measured from the ground plane 20 to its tip) to be determined. Most useful is to assess the magnitude and distribution of the microwave fieldproduced by different lengths of probe 4, measured relative to the wavelength of the microwave radiation supplied by the microwave generator to the probe 4.

[0088] Figure 5 shows the different field distributions produced with probes 4 having lengths of 0.25A, 0.5A and 0.625A, where A is the wavelength in free space of the microwaves supplied through the probe 4. In each case, the probe 4 is aligned vertically. The shortest probe 4 (length = 0.25A) gives a field pattern that has the greatest intensity of the three towards the vertical (90°), but has the lowest intensity of the three towards the horizontal (0°). The middle probe 4 (length = 0.5A) produces a field pattern biased more towards the horizontal, while the longest probe 4 (0.625A) produces a field pattern even more strongly biased towards the horizontal, except for two small additional lobes aligned more towards the vertical.

[0089] It should be noted that the effective electrical length of the antenna formed by the probe 4 is not the same as its simple physical length, except when it is tested in vacuo or in air. When inserted into a material such as soil, especially moist soil, the effective electrical length of the probe 4 rises with the dielectric properties of the material. Thus, accurately predicting an ideal probe 4 length for this equipment remains challenging. Nevertheless, it has been found that the moisture content of the soil is probably the most influential variable. Hence, in practical terms, selecting a probe 4 length on the basis of the soil moisture content for the terrain being treated appears to be sufficiently reliable to ensure that the probe 4 chosen produces a useful field pattern, which ensures that roots 9 adjacent the probe 4 are within a zone receiving close to the maximum available levels of microwave irradiation.

[0090] Figures 6a to 7c show the results of experiments into the heating of soil with such probes. One aim of these experiments was to confirm that the relationship between the probe length and the spatial distribution of the microwaves also applies to the actual heating effect of the microwaves. This should allow selection of a preferred length for the probe.

[0091] The effect on weed roots is believed to be linked to the elevated temperatures experienced, rather than any direct effect of microwave irradiation on the roots. The principle behind these tests was therefore to deliver a standard amount of microwave energy into the soil via an embedded probe 4 and to observe the temperature distributions generated, for probes 4 of different lengths. Since the heating effect is as a general rule linearly dependent on the total energy provided, it was only necessary to vary the duration of irradiation using a constant power output. In these experiments, a standard 500Wwas applied for periods of 10, 20 and 40 seconds.

[0092] Three probe lengths were trialled, measuring 22mm, 55mm and 75mm from the ground plane 20 to the distal tip of the probe 4. These substantially corresponded to the 0.25A, 0.5A and 0.625A probes of Figure 5 above. In the event, the results of the 55mm probe were found to be intermediate between those of the 22mm and 75mm probes, and so they are not presented herein.

[0093] A sample of soil was held in a clear PMMA container, with the probe 4 inserted into the soil extending vertically downwardly. The probe 4 was inserted to its full length, with the ground plane 20 in contact with an upper surface of the soil within the container. After irradiation with microwaves at 500W for a selected period, the container was split in a vertical plane to allow thermal imaging of a vertical cross-section of the soil. NB: the vertical plane was located as close as possible to the vertical axis of the probe 4. The spatial mapping of the thermal effects produced by the microwave irradiation would thus indicate where the maximum heating effects were taking place, the spatial extent of heating and the degree of heating produced.

[0094] In Figures 6a and 7a, the thermal effects are presented as a colour map; in Figures 6b and 7b, the same data are presented as a greyscale map; and in Figures 6c and 7c, the same data are presented as a contour plot. In each instance, the results shown are from 40 seconds' irradiation at 500W, the highest net irradiation, which shows the effects most clearly.

[0095] It is notable that within each image, there is an elongate roughly vertical region having a slightly lower temperature than its surroundings. This appears to correspond to the position occupied by the probe 4. The heating effects in all instances extended as far as the surface of the soil.

[0096] Referring now to Figures 6a to 6c, the significant temperature inceases extend at least as far horizontally as they do vertically. The greatest temperature increase, indicated as point 22 on each Figure, equates to a soil temperature of around 90°C. The background shade / contour, by contrast, is as low as 5°C.

[0097] Thus, the shortest probe (22mm, approx. 0.25X) is producing a maximum temperature in the soil that is liable to cause significant thermal harm to a root structure, and this extends over significant distances both vertically and horizontally. The effect along the vertical axis appears to extend significantly beyond the distal tip of the 22mm probe.

[0098] Referring next to Figures 7a to 7c, these thermal effects are produced by the 77mm probe, again as a result of 40 seconds’ irradiation at 500W. The significant thermal effects extend proportionately further vertically than horizontally. Comparison of the scales shown on Figures 6a and 7a indicates that the heating in Figures 7a to 7c extends much less far horizontally than it does for Figures 6a to 6c.

[0099] The greatest temperature increase, at a point marked 23, equates to a temperature of maybe 70-80°C. This is a significant effect, but clearly less pronounced than that shown in Figures 6a to 6c.

[0100] It is notable that the shape of the heated zones shown in Figures 7a to 7c is not quite the same as the shape that might be predicted on the basis of Figure 5. This is believed to be due to thermal conduction within the soil, spreading out the initial heating effects.

[0101] These results suggest that a relatively short probe 4 may well be preferable in most situations. This geometry provides the widest horizontal spread of significant heating, so that the probe 4 does not have to be inserted immediately next to a root 9 of a weed 7 in order to kill it. Additionally, heating and killing an upper portion of the root 9, close to the surface of the soil, is effective because this will almost immediately prevent transfer of moisture and nutrients from lower portions of the roots 9 to above-ground portions of the weed 7, leading to rapid death of the above-ground portions of the weed 7, too.

[0102] A few weeds 7, such as dandelions and thistles, have deep tap roots 9 that might survive this treatment, even with their foliage killed off. However, probes 4 having a greater length, so as to heat deeper portions of the roots 9, could be used if such weeds 7 were to be a specific target.

[0103] A further experiment, the results of which are shown in Figures 8a to 8c, addresses the effects of moisture levels in the soil on the effectiveness of this method.

[0104] It might be expected that using microwaves tuned to frequencies known to be absorbed by water, the moisture level in the soil might be important. Although some soils, especially those containing metal sulphides, may be inherently capable of absorbing microwaves, in most cases water is expected to be the main microwave absorbing material.

[0105] Note: the experiments here have been carried out using a microwave generator set to emit microwaves at approximately 2.45GHz - i.e. the frequency used for microwave ovens in Europe. However, there are other microwave frequencies that are strongly absorbed by liquid water, such as 5.8GHz. The skilled person would appreciate that there are thus a number of frequencies within the ISM (“industrial, scientific & medical” - as opposed to telecommunications) microwave frequency range that should also be effective in place of the 2.45GHz systems used in the present examples.

[0106] To confirm the effect of moisture levels, the experiment of Figures 6a to 6c was repeated, but using soil that had been baked dry and then re-hydrated to 10% by weight water. As a comparison, the composts used as the soil for Figures 6a to 6c did not appear particularly damp, but still had a moisture level of around 55% by weight and so would generally be regarded as a relatively wet soil. A soil with just 10% by weight water present would generally be regarded as a dry or even parched soil, but there are weeds that have the reputation of surviving such conditions better than would desirable crop plants.

[0107] The results displayed in Figures 6a to 6c, and in Figures 8a to 8c, can thus be usefully compared.

[0108] Referring now to Figures 8a to 8c, there is clearly a heating effect extending a considerable distance into the dry soil, both horizontally and vertically, but the actual rise in temperature is only moderate. The greatest temperature rise, at a point marked 24 adjacent the proximal end of the probe, reaches maybe 55°C, However, as shown by comparing the scales on Figures 6a and 8a, this is relatively localised, and the majority of the area with noticeable heating is at a far lower temperature.

[0109] This shows that dry soils are not particularly good absorbers of microwaves, but it does also indicate that the dry soil is relatively transparent to microwaves, which appears to have counterbalancing benefits as set out below.

[0110] Further experiments were therefore carried out with vegetable matter present in the soil sample to represent model weed roots. The same preferred conditions were used: 22mm probe, 500W irradiation for 40m seconds. In one experiment, a small carrot was sectioned to represent a weed with a tap root. The probe was inserted approximately 5-10mm from the carrot. When after irradiation the soil was split and thermal images were generated, heating was found to be very localised, most of the heating having taken place selectively within the carrot.

[0111] In another experiment, a dandelion root was embedded in the soil and the experimental procedure repeated. In this case, not only was the heating largely localised in the root, but surprisingly the thermal imaging results almost perfectly mapped the shape of the dandelion root.

[0112] In retrospect, since a carrot contains up to 90% water and a dandelion root is also mainly water, they might be expected to be significantly more susceptible to heating than even the wettest soil. Nevertheless, it remains highly encouraging that the target weed roots appear selectively vulnerable to microwave heating. In a dry soil, this selective heating effect on the root itself would be even more pronounced, and as noted above, dry soils can be almost transparent to microwaves, which should permit microwave heating to selectively affect weed roots at even greater ranges from the probe.

[0113] In the next series of tests, the effect of microwave heating was tested on actual plants. Four model plants were used: basil, thyme, dandelion and a small conifer. These were chosen for ready availability and to represent a range of different types of weed.

[0114] Basil is a good model for soft-stemmed weeds, and is readily available with multiple basil seedlings in a single pot.

[0115] Thyme is a more woody plant than basil, acting as a model for hardier types of weed, having a lower moisture content. It is hence an increased challenge for the present invention.

[0116] Dandelion is a notoriously difficult weed to eradicate, and so represents an even greater challenge for the method of the present invention.

[0117] The conifer was chosen to explore the vulnerability of such a strong plant to the method of the invention.

[0118] For each plant, a standard microwave irradiation set up was used - a 22mm probe connected to a 500W microwave source. A range of irradiation periods was examined, in order to produce as much useful data as possible.

[0119] With the basil samples, there were enough seedlings in a pot that the effect of the distance between the probe and the "weed" could also usefully be explored. Referring now to Figures 9a to 9d, four initially very similar pots of basil seedlings are shown, four days after irradiation with microwaves for 10 seconds, 20 seconds, 40 seconds and 60 seconds respectively. Wilting of seedlings was evident very soon after irradiation for the 40 second and 60 second dosages. Damage was obvious within a day for all dosages. The stems of the affected seedlings had begun to wilt from their bases upwards, confirming damage at or just below the surface of the soil. By four days from irradiation, all affected plants were effectively dead. A few leaves were still alive, but given the conditions of their stems, revival would not occur.

[0120] In each pot, it was evident that, beyond a certain radius, seedlings were unaffected, but closer to the probe position, seedlings did not survive. To quantify this effect, the seedlings were trimmed to leave only a lower part of each stem. The dead stems were marked with white paint to differentiate them more clearly, and a clear plastics sheet was superimposed on the pot. This sheet was marked up with concentric circles at 5mm intervals at radii between 10mm and 35mm. The centre of these circles was located over the point marked 25 in these figures, which was where the probe had been inserted.

[0121] For each pot, this allowed the determination of an effective radius around the probe position, within which all seedlings were killed. The radius of this "kill zone" could then be plotted against the duration of microwave irradiation, producing a graph as shown in Figure 10.

[0122] This demonstrated a very definite correlation between the duration of exposure to microwaves and the radius of the resulting kill zone. Indeed, the relationship beyond 20 seconds’ exposure is almost linear. (Note: logically,there must be a point as shown on the graph at (0,0) where no microwaves produce no kill zone, so extrapolation of the linear portion of the graph should be carried out with caution).

[0123] The results of the graph of Figure 10 thus appear entirely consistent with the effects hoped-for from the present invention.

[0124] Figures 11a to 11d show the results of similar experiments carried out on thyme plants. In this set of experiments, there was unfortunately a problem with the pot that was to be irradiated for 60 seconds, so this was omitted. Figure 11 b shows a 10 second dosage, Figure 11c shows a 20 second dosage and Figure 11d shows a 40 second dosage. Figure 11a shows a control pot that was not irradiated.

[0125] In this set of experiments, the results were less clear-cut. While the control pot may look like multiple seedlings, it is actually a single thyme plant, as is the case for the irradiated pots. This was expected to lead to a simple "kill" or "no effect" result for each pot, depending on whether the root of the plant had been killed off. Nevertheless, there still appeared to be a gradient in effect, depending on a distance from the probe position, as indicated by the presence of dead or green leaves.

[0126] While this confirms that the method is generally effective against woody weeds, it is not immediately clear what mechanism is operating. At present, it is believed that the varying exposure times may be destroying the root / stem structure to different degrees, the longer exposure times causing more complete destruction and so producing a more rapid die-off of the plant as a whole. This theory may need confirmation, but the effectiveness of the method appears clear regardless.

[0127] For dandelions, representing weeds with tap roots, individual plants were irradiated using a 22m probe connected to a 500W source, using a 20 second exposure time. The probe was inserted from 5 to 10mm from the stalk of the dandelion, a reasonable range in practice. In each case, heating wasobvious, and within 2 to 3 days, the plant was clearly dying. They were thoroughly wilted, and the bases of the leaves had begun to wither away.

[0128] A single test on the conifer was less clear. Irradiation at 500W for 40 seconds with a 22mm probe led to an apparent increase in dead leaves, starting from the base of the plant. However, conifers are very robust, and it is not yet evident whether this will lead to death of the plant or to damage from which it will recover.Checks on Effects of Microwaves

[0129] There will inevitably be a degree of concern about the possible effects of microwave heating on other organisms in the soil close to the target weed roots. The inventors have studied this issue and there should be no or very few adverse effects except for those on the weeds themselves.

[0130] As a first point, any effects from the microwave healing as described above will result solely from thermal effects - i.e. the temperature rise in the soil and its associated biota. This is just the same as if the imposed temperature rise had been caused by conventional heating. Although some unconventional effects can be produced by microwave irradiation in very specific laboratory conditions, none of these conditions apply to the present method for suppressing or killing weeds.

[0131] It is thus possible to assess the effect of microwave heating on soil biota and chemistry on the basis of how elevated a temperature is produced and where. In the relevant trials, the maximum temperature detected was 90°C (194°F). This was only for a very short period, and the volume of soil reaching such temperatures was very small, typically around 5cm3.Chemistry

[0132] No change in soil chemistry would be expected at temperatures of 90°C or lower. The chemical decomposition of organic soil componentsgenerally requires temperatures above 170°C, for prolonged periods. Even simple sugars require temperatures of over 120°C for an hour or so to show measurable decomposition, in laboratory conditions. There is a rule of thumb for such reactions that each 10°C difference in temperature has an effect of *2 on the rate of the reaction, so the rate of decomposition of simple sugars at 90°C is likely to be roughly 10x slower than at 120°C.

[0133] The inorganic components of the soil will also be little affected. Aluminosilicate minerals will be entirely unaffected at 90°C. Simple ions such as sodium, potassium or chloride will also be unaffected. Complex anions, such as the agriculturally important nitrate and phosphate ions, can show signs of decomposition at as low as 70°C in laboratory thermogravimetric analysis, but the rate of reaction is very slow. Significant decomposition on a practical timescale requires temperatures of above 150°C, which are never produced in the microwave heating methods of the present invention.

[0134] In fertilised agricultural soils, the concentrations of nitrate anions should not exceed 50mg / kg (as NOs'). Inorganic phosphates will typically be around 300mg / kg (as PC3')- Even if all the nitrate and phosphate within the heated volume of 5cm3were to be decomposed, which will not occur in practice, this would represent less than 1 mg of nitrate and 4mg of phosphate. This would be insignificant on the scale of a field or orchard. The only plant likely to be inconvenienced, even in a worst case scenario, would be the weed that is being suppressed.Bacteria

[0135] Soil bacteria can be differentiated into several different groups, depending on their ability to multiply and survive at different temperatures.

[0136] Bacteria exist that are classified as psychrophiles (extreme low temperature bacteria), thermophiles and hyperthermophiles (extreme high temperature bacteria) but these are found only in extreme environments and are not relevant to the biota of realistic agricultural soils.

[0137] The groups of bacteria classified as mesophilic and thermotolerant are the only significant types in temperate agricultural soils. Mesophilic bacteria can tolerate a range of 18° to 45°C, with an optimal temperature around 39°C. Thermotolerant bacteria can tolerate a range of 22° to 60°C, but their optimal growth rates are at below 50°C.

[0138] A temperature above 60°C can kill mesophilic bacteria in 30 minutes. However, on a shorter timescale, any bacteria except for thermophilic / hyperthermophilic bacteria would require temperatures of 88°- 95°C to be killed within 15 to 30 seconds. As noted above, these are the peak conditions likely to be produced within a volume of less than 5cm3. The effect of the methods of the present invention on soil bacteria is thus going to be negligible.Insects

[0139] Some insects living in the soil are undesirable, but many have beneficial effects on the soil ecology. It is hence desirable for the present invention to minimise effects on any organism but the need to be killed.

[0140] These types of insect can typically travel at 12m / s although they tend to be slower when within the soil itself. Typically, a body temperature of 71 °C is required to kill most insects. Given the size of the zone that is heated to an appreciable degree, insects within the soil would need to move around 5- 10cm to reach a position at which they would not be significantly harmed by the heating effects.

[0141] It is hence possible that some insects might not be able to flee far enough in time, and so might suffer harm, but the impact on insect populations with soil will be very localised and insignificant for the field as a whole.Worms

[0142] Worms are more sensitive to elevated temperatures, and prolonged exposure to temperatures as low as 35°C can kill. The effect of the methods of the present invention on worms in the soil will depend largely on their response to the disturbance caused by the probe / antenna entering the soil and the size of the worm.

[0143] Worms living in topsoil tend to flee from disturbance of the soil, so would be expected to leave the vicinity of the probe / antenna as soon as it is inserted into the soil, before the microwaves are generated and localised heating occurs. Whether this will be fast enough depends on the size of the worm. A medium-sized earthworm can travel through the soil at around 1- 5cm / s, but smaller worms may only be capable of 0.2cm / s, and may not escape in time.

[0144] However, earthworm densities in agricultural soils are typically less than 100 individuals per square meter, and they may live at depths of up to 12cm. There will hence be an average of around 1 earthworm per litre of soil. Given the volumes of soil heated by the methods of the present invention, only about one in every hundred treatments of a weed is likely to be sufficiently close to an earthworm for it to be harmed, if it takes insufficient evasive action. This is a negligible effect in the context of natural predation by birds, for example.Vertebrates

[0145] The vertebrates living on agricultural land generally display intelligent behaviour, fleeing the slightest signs of threat, including noise or other vibrations. As the equipment of the present invention approaches the area to be treated, vertebrates such as voles and field mice would be likely to evacuate the immediate area, or failing that, they will be triggered to flee by the probe / antenna being inserted into the soil. Even if they were not startled into flight, the sensation of heat produced as the microwave irradiationcommences would be enough to trigger a panic response leading to rapid departure from the dangerous heated zone.

[0146] The number of vertebrates in a typical field will not be particularly high compared to smaller organisms, in any case, so overall the risks to vertebrates caused by the methods of the present invention are believed to be virtually non-existent.Overall

[0147] Therefore, the use of microwaves for selective thermal weed suppression would be free from harmful impact on the majority of soil- associated organisms. There may be trivial impacts on some soil bacteria and some invertebrate life. Nevertheless, given the focussed and localised nature of the heating produced, any such effects would be immeasurably small and negligible in the context of other impacts on soil biota, whether natural or resulting from existing agricultural processes.

Claims

CLAIMS1 . A method for suppressing weeds, comprising the steps of: providing a source of microwave radiation; providing antenna means operatively connected to said source of microwave radiation; inserting the antenna means into soil adjacent a weed; and transmitting microwave radiation through the antenna means into said weed.

2. A method as claimed in claim 1 , wherein the antenna means is inserted into soil adjacent an underground portion of the weed.

3. A method as claimed in either claim 1 or claim 2, wherein the underground portion of the weed comprises a root structure of the weed.

4. A method as claimed in any one of claims 1 to 3, comprising the step of providing vehicle means to which the source of microwave radiation and the antenna means are mounted.

5. A method as claimed in claim 4, wherein said vehicle means comprises autonomous vehicle means.

6. A method as claimed in either claim 4 or claim 5, wherein the vehicle means is capable of flight.

7. A method as claimed in any one of claims 4 to 6, wherein the vehicle means is adapted to travel on a ground surface.

8. A method as claimed in any one of the preceding claims, comprising the step of providing equipment having means to locate and identify weeds.

9. A method as claimed in claim 8, wherein the method comprises the step of providing image capture equipment adapted to locate and identify weeds.

10. A method as claimed in either claim 8 or claim 9, wherein the method comprises the step of providing data processing means programmed or programmable to process captured images so as to compare them with reference images.

11. A method as claimed in any one of claims 8 to 10, wherein the method comprises the step of operatively connecting the means to locate and identify weeds to control means comprising data processing means programmed or programmable to control a course and speed of the vehicle means.

12. A method as claimed in claim 11 , wherein the method comprises the step of programming the control means to cause the vehicle means to move adjacent to an identified and located weed.

13. Equipment for weed control comprising: a source of microwave radiation; and antenna means operatively connected thereto; wherein the antenna means is insertable into soil adjacent a weed; and the antenna means is adapted to transmit microwave radiation from said source into said weed.

14. Equipment as claimed in claim 13, wherein the antenna means is insertable into soil adjacent an underground portion of the weed.

15. Equipment as claimed in either claim 13 or claim 14, comprising vehicle means to which the source of microwave radiation and the antenna means are mounted.

16. Equipment as claimed in claim 15, wherein the vehicle means comprises autonomous vehicle means.

17. Equipment as claimed in either claim 15 or claim 16, wherein said vehicle means is capable of flight.

18. Equipment as claimed in any one of claims 15 to 17, wherein said vehicle means is adapted to travel on a ground surface.

19. Equipment as claimed in any one of claims 13 to 18, wherein the equipment is provided with means to locate and identify weeds.

20. Equipment as claimed in claim 19, comprising data processing means programmed or programmable to process captured images so as to compare them with reference images.

21. Equipment as claimed in claim 20, wherein the means to locate and identify weeds is operatively connected to control means comprising data processing means programmed or programmable to control a course and speed of the vehicle means.

22. Equipment as claimed in claim 21 , wherein the control means is programmed or programmable to cause the vehicle means to move adjacent to an identified and located weed.

23. A system for weed suppression comprising a base station and a plurality of pieces of equipment as claimed in any one of claims 13 to 22.

24. A system for weed suppression as claimed in claim 23, wherein said base station comprises a plurality of equipment charging stations, each adapted to receive at least one of said pieces of equipment.

25. A system for weed suppression as claimed in claim 24, wherein each equipment charging station is operatively connectable to a piece of equipment in order to transfer electrical power thereto.

26. A system for weed suppression as claimed in any one of claims 23 to 25, wherein the base station is provided with energy collection means.

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