Thermal Weed Control
By: Tim Marshall
Peter Edwards demonstrates the EnviroPure steam weeder
Peter Edwards demonstrates the EnviroPure steam weeder

Several organic farmers have imported machines from Germany & other farmers, weed control companies and gas companies are developing prototype heat weeding machines of the flame or infra red type. Also in development are microwave machines, & others using electrical current.

Thermal weeding usually works by applying enough heat, for a very short period, to expand & rupture plant cells or coagulate proteins, without totally destroying the organic matter. This is probably a minimum temperature of 90-100 degrees C for one tenth of a second. Monocotyledonous plants may have a slightly higher temperature tolerance than dicots, permitting some careful selective weeding with this technique, such as with onions, a weed sensitive crop. Maize, another monocot, is particularly resistant to heat . Shielded burners conserve energy & provide control for both direct flame contact & radiated heat. Shielded burners can be used for inter-row control of weeds in the growing crop. Although more work needs to be done in Australian conditions, these techniques appear to use no more energy than cultivation with tyned implements. Further research needs to done on design of the burners, height above the crop, angle of the flame, gas pressure & speed of travel of the tractor.

As naked flame burners have obvious limitations in Australian conditions, hot water techniques are generally considered more promising. These still require a large water tanker and an exciting recent innovation is using super heated air. Air systems are being developed by at least two Australian companies at the present time. This should result in a safer, more water and fuel efficient and more mobile plant.

There are numerous different systems, using slightly different techniques. Typically, they would use either naked flame or a radiant heater (similar to the sort of thing which would be used to heat a workshop) at about 25 cm above the soil. The forward speed has to be sufficient to denature the plant proteins but should not actually shrivel the plant.

Ric and Annie Dunn are NASAA certified vegetable growers at Innaloo, in the Northern suburbs of Perth. They use thermal techniques for pre-emergence control of weeds in carrots. This crop was always labour intensive and therefore expensive to grow organically, because they do not compete well with weeds in the row and require a high level of weeding. The inter-row is generally accessible to mechanical weeding, but weeds within the row seriously reduce yield and quality.

ACA Engineering thermal machine can be powered by gas or diesel
ACA Engineering thermal machine can be powered by gas or diesel

The technique which Ric and Annie have developed is to plant two 'indicator' rows two days before the main crop. When the indicator crop emerges, they know that the main planting is about to germinate. They then drive over the patch with the flame burner (see photo) to remove all competition. The carrots then have a good head start.

The prototype burner was developed with the assistance of Kleenheat Gas Pty Ltd.

The technique is effective on small and germinating weeds only, and may also remove seed at the soil surface. European research indicates that weeds up to 75mm high may be controlled, but most Australian use is for weeds smaller than this.

Most operators agree that speeds of 4 to 6 km/hour are possible.

The factors which appear to influence susceptibility of plants to thermal weeding most are the presence of numerous leaf hairs (which provide insulation), the thickness of the wax covering on the leaf and wether the plant can regenerate from roots. (All plants, even apparently smooth leaved plants like silverbeet etc. have microscopic leaf hairs which are not visible to the naked eye).

Late growth stage or pre-harvest thermal applications are also used to defoliate onions and potatoes.

A significant advantage of thermal weeding is that it may be used when the soil is too wet for mechanical cultivation, although most users say that the weed leaves should be dry.

Another form of thermal weed control was common on Australian farms in the past, especially small market gardens, and can still occasionally be found today. That is the use of a hand held flamer, powered by kerosine or other fuels and more recently LPG.

This tool has been used for tussock burning, weed control under fences etc. where machinery does not easily reach, or for pest control in chook houses, dog kennels etc.

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