Weed control by heat
Thermal weeding is a physical control strategy against weeds that consists of applying a thermal shock at a high temperature (by direct flame or infrared) and for a very short exposure time (3 to 5 seconds) causing the bursting of plant cells. It is an alternative technique to the use of glyphosate during the intercrop period in particular.
Presentation
Characterization of the technique
Implementation means and conditions for better effectiveness:
For all crops burning is generally done before sowing or pre-emergence. The weeds must be germinated and developing to be affected (between cotyledon stage and four true leaves).
Thermal weeding is valid for all weeds. However, for monocotyledons, perennials and weeds with a vegetative point at ground level (rosette weeds), effectiveness is limited when the stage is advanced. In this case, two quicker passes are more effective than one slow pass.
For optimal effectiveness, it is preferable to intervene when there is no longer dew. The soil can be moist but there must be no wind.
Equipment:
There are three types of thermal weeding devices :
- Boom systems (exposure of weeds to flames): thermal weeding with naked flame works by heating plant tissues without causing their combustion. The temperature obtained must be above 95 – 100 °C and maintained for at least 0.1 seconds.
- Oven (use of infrared rays): No direct contact between flame and vegetation with this system. The effectiveness is the same, energy consumption is minimized, but the equipment cost is higher.
- Steam projector (used in non-agricultural areas).
In this system, devices operate on propane (because it has a pressure of 6 bars at 0 °C, facilitating its use). They are equipped with burners with controlled and secured ignition, and supplied in gaseous or liquid phase either by classic propane gas bottles or by a secured tank fixed to the front of the tractor.
In liquid phase: It allows to obtain directly and constantly a significant power and a faster forward speed. But there is a higher risk of clogging for the user.
In this system, a liquid phase burner with evaporator is used (more powerful and more constant, but due to its complexity, it is more costly in energy and maintenance).
In gaseous phase: If the demand is too high at the nozzles, the liquid will not have time to vaporize, which will cause frost formation making the device quickly inoperative. Generally, lower forward speeds and risks for the user. Here a double aspiration gaseous phase burner is used (allows obtaining a flame of 1400 °C; it is less expensive to purchase and maintain).
Settings:
It is important to properly adjust the device and the forward speed. The latter must be adapted according to the type of device.
Boom height : burners must be positioned about 15 cm from the ground. The goal is to adjust the height so that the blue flame reaches the weeds.
Forward speed : depends on the machine used and the development stage of the weeds. The more developed the weeds, the more heat is needed and thus the speed must be reduced.
Settings check : press the weed leaf between fingers; after release, the finger mark must be visible. If not, the effectiveness is insufficient and the forward speed must be reduced. Conversely, if the leaf is scorched or smoking, the forward speed can be increased.
Example of implementation :
On soybean: thermal weeding at emergence and at the latest at closed cotyledon stage (window of 1 to 2 days), then a harrow pass at the 2 unifoliate leaf stage and finally a combined inter-row hoeing and directed burning towards the base of the plants at the 3-4 leaf stage of soybean (in this case false seedbed is optional). Example taken from the ITAB sheet (see bibliography).
On onion: possibility to intervene in crop, at the crook stage, then from the first leaf fall to the beginning of bulb formation (See technical sheet 19 of the Vegetable Design Guide)
Details on the technique :
Ideally, intervention should be early, when the crop is at its most resistant stage and the weeds at their most sensitive stage (cotyledon stage).
The technique can be used for false seedbeds: destruction of the last false seedbed by thermal weeding is also practiced because it causes no soil disturbance, thus no germination of new unwanted plants.
Thermal weeding is less effective on creeping plants and/or deep-rooted plants, such as broadleaf dock, thistles, sowthistles, dandelion or couch grass. Plants with broad leaves and shallow roots, like ribwort plantain, are more easily controlled with this technique.
Application of the technique to...
Implementation period On established crop
Spatial scale of implementation Plot
All crops : Not generalizable, this technique is mainly used on row crops (corn, soybean, sunflower, beet, onion, garlic, carrot...)
All soil types : Easily generalizable, thermal weeding is an interesting technique given its effectiveness and especially its low soil requirements (moisture and structure). The practice is therefore relevant where the soil cannot be worked, notably if it is too wet or too steep. This technique is thus suitable for regions and crops where this situation is frequent. However, beware of stony or cloddy soils.
All climatic contexts : Easily generalizable. Thermal weeding is an interesting technique given its effectiveness and especially its low requirement in terms of weather conditions (no need for dry weather after the pass). Thus, unlike mechanical weeding, the weather window for intervention is much more favorable.
Regulations
Thermal weeding equipment is eligible for the PVE (Plant Environment Plan), so investment aid can be received. Likewise, several MAE (Agri-Environmental Measures) territorially targeted can financially support the practice of thermal weeding.
Effects on the sustainability of the cropping system
"Environmental" criteria
Effect on air quality : Decrease
Acidification : DECREASE
Phytosanitary emissions : DECREASE in pollutant transfers to water and air thanks to reduced herbicides.
GHG emissions : INCREASE
Particle emissions : VARIABLE
Effect on water quality : Variable
N.P. : VARIABLE
Pesticides : DECREASE
Turbidity : NEUTRAL
Effect on fossil resource consumption : Increase
Fossil energy consumption : INCREASE
Phosphorus consumption : INCREASE
Thermal weeding reduces herbicide use and thus limits the risk of these products entering air or water. However, there is emission of carbon monoxide, carbon dioxide, sulfur oxides and nitrogen oxides, especially if burners are poorly adjusted.
"Agronomic" criteria
Some organisms (soil fauna and flora in particular) may be eliminated during passes of thermal weeding tools.
Heat can harm beneficial animals crawling on the soil, such as ground beetles, earwigs, spiders and woodlice.
This technique nevertheless allows, on some crops like sunflower, to weed the row without herbicides (for example in organic farming).
"Economic" criteria
Operational costs : Increase
The cost for two passes is about 95 euros/ha for equipment (machine, gas and two tractor passes) and 32.4 euros/ha for labor (10.5 euros/hour and 1.5 hours for one pass on 1 hectare).
Mechanization costs : Increase
Need for specific equipment. According to the Chamber of Agriculture Rhône Alpes: a 3.2m boom 6 rows, 12 Primatech burners (312 kg gas tank) costs 4750 euros; a 3.2m boom + chassis, 6 rows, 12 Antargaz burners (275 kg gas tank) costs 6245 euros.
Margin : Variable
Thermal weeding is relatively expensive (120 to 130 euros/ha), which is why it is mainly developed on intensive crops (market gardening) or if the product is valued accordingly (organic farming).
Other economic criteria : Increase
Fuel consumption: Increase
"Social" criteria
Working time : Variable
Variable depending on the technique used, crop and weed stage, and forward speed, generally between 3 and 6 km/h.
Increased working time due to the narrow tool width and increased number of passes.
Peak period : Increase
The number of passes required can be high. One can count 1.5 hours/ha for one pass. However, intervention windows are wider than for chemical weeding.
However, technical knowledge is necessary, especially regarding temperature sensitivity thresholds of main crops and weeds.
Risk of auxiliary destruction
All auxiliaries that walk on the soil surface can be killed by the technique, especially spiders, predatory and granivorous ground beetles, earwigs, rove beetles...
For further information
- Practical guide for the design of vegetable cropping systems saving phytopharmaceutical products. Technical sheet T19. - Launais M., Bzdrenga L., Estorgues V., Faloya V., Jeannequin B., Lheureux S., Nivet L., Scherrer B., Sinoir N., Szilvasi S., Taussig C., Terrentroy A., Trottin-Caudal Y., Villeneuve F., Ministry of Agriculture, French Agency for Biodiversity, GIS PIClég., Publication, 2014.
- Thermal weeding, Technical-economic benchmarks, Organic open-field vegetables - Chapuis S. Technical brochure, 2010.
- Thermal weeding, Vetabio - Grebert D., Vercaigne J.-P., Delanote L., Legrand M. Technical brochure, 2009.
- Alternative weeding in market gardening - Curative measures - Ferrier J-D. Chamber of Agriculture of Ain, Technical brochure, 2016 See page 8.
- Thermal: an alternative to chemical weeding? - Crenn J. Aujourd'hui & demain, n°95, Press article, 2008
Thermal weeding tool
Appendices
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