Herbicides have been the mainstay of weed control for over half a century. They turned what was often one of the most complex and time consuming activities in crop production into a relatively straight forward task. However, fifty years on, across the globe, farmers, growers and scientists are realising that herbicides are increasingly in trouble, with growing resistance (especially in herbicide resistant crops), lack of new chemistry, mounting intolerance from consumers, and legislative prohibition. The race is therefore on to find alternatives. However, few non-chemical weeding techniques get close to the silver bullet effect of herbicides, meaning that farmers and growers need to use the ‘many little hammers’ approach  to get sufficient overall weed control. However, among this complexity of tools, one stands out as being something of a sledgehammer.
Imagine a weed control technology that could achieve all of the following:
- broad spectrum, i.e. kills practically all weeds;
- had a residual period as long as the crop’s production cycle;
- could be used in any crop;
- had a nil withholding period;
- had no risk of releasing xenobiotic materials into the environment;
- had exceptional reliability levels (i.e., always works); and
- had no risk of evolved resistance.
This may seem like an impossible list for any weed control technology, including herbicides, but it is in fact possible using a technique called intrarow soil thermal weeding (ISTW).
The basic idea behind the technique is really simple. Nearly all weeds in cropping systems (arable and vegetable) originate from the ‘weed seed bank’ i.e., weed seeds in the soil. So, with the exception of perennial weeds, if there is no weed seed bank, then there will be no weeds. The elimination of the weed seed bank is a well known side-effect of the use of steam for soil sterilisation / pasteurisation in intensive cropping systems, such as glasshouses, to control soil borne pests and diseases. Back in the early 2000s, Danish scientists Bo Melander, Torben Heisel, and Martin Jørgensen wondered if this side-effect could be turned into the main event and be used for weed control [2, 3] .
However, the standard approach to soil steaming involved heating the soil for hours, often as deep as the plough layer, which was not going to be practical for weed control across a broad spectrum of cropping systems. Bo, Torben and Martin reasoned that the most problematic weeds are those in the crop-row, especially those close to the crop plants as these have the biggest competitive effect on yield, and they are hardest to control, compared with the interrow where broad-spectrum herbicides or interrow hoeing can be used. So focusing weed control on the crop row would significantly reduce the amount of steaming required. Next, unlike pests and diseases that can affect crop plants from deep in the soil by attacking roots, most crop weeds can only emerge from the top five centimetres of soil. This again meant that the amount of soil needing heating is dramatically reduced, to about 10% of that heated by standard steaming techniques. The final part of the jigsaw was to work out just how short the heating time could be made, with the answer being as low as a minute instead of hours.
With the puzzle complete, the researchers built a tractor mounted machine that just steamed the crop row (intrarow) to about 5-7 cm deep, and for long enough to kill the weed seeds. The result was weed-free crop rows! In addition, as the source of weeds had been eliminated, i.e., the weed seed bank, no further weeds could emerge from the row until new seed was introduced, either from soil mixing, or weed seed rain. The result is a technique that is broad spectrum for all weeds originating from seeds, has a residual period as long as the crop production cycle, can be used on any crop (because the soil is steamed before planting), it uses no agri-chemicals so there are no issues with withholding periods or chemical environmental pollution, it is highly reliable, and has no risk of evolved resistance as everything has a thermal death point. ISTW is therefore the only non-chemical weed control technique that can directly out-compete herbicides.
The only problem, was that for field scale machines, the size of steam boiler required made the technique unattractive to most producers, and, even with the significant reduction in the amount of soil heated, considerable amounts of fuel were still required.
To address these issues the Future Farming Centre undertook a theoretical analysis and laboratory testing to see if (1) hot air could be used instead of steam, to simplify the engineering, and (2) recycle the heat to further reduce fuel consumption.
The guts of the results are that hot air is capable of killing weed seeds in the soil so it can be used instead of steam, and that heat can be recycled / reused reducing the amount of fuel / energy required, potentially substantially.
However, there were a number of caveats discovered. While hot air can kill weed seeds, there are important interactions between temperature and the moisture content of the heat source, soil and seeds, that needs further unpicking. While recycling heat is possible, doing so on mobile field equipment could be a significant engineering design challenge and that it may be a better option for energy efficiency to remove soil from the field for treatment.
The results of this work have been published on the FFC website at www.bhu.org.nz/future-farming-centre/information/weed-management/istw
This means that at present, while ISTW is a sledgehammer, or perhaps more appropriately a steam-hammer, in terms of the level of weed control it can achieve, it still requires further research and engineering to turn it into a practical technology for use across the spread of real-world farming and growing systems. However, considering the inevitable downward trajectory of herbicides, any technique that can pull off the level of weed control that ISTW can, surely deserves some significant support, to give farmers and growers a serious weed control backstop.
The research the FFC undertook into ISTW was funded from the Sustainable Farming Fund (number L12-104 ) with backing from HorticultureNZ.
1. Liebman, M. and Gallandt, E.R., Many little hammers: ecological management of crop-weed interactions, in Ecology in Agriculture, Jackson, L.E., Editor. 1997, Academic Press: San Diego, CA. ISBN 978-0123782601 https://www.doi.org/10.1016/B978-012378260-1/50010-5
2. Melander, B., Heisel, T., and Jørgensen, M.H. Aspects of steaming the soil to reduce weed seedling emergence. in 12th EWRS Symposium. 2002: European Weed Research Society. http://orgprints.org/1547/1/Abstract_NL1.pdf
3. Melander, B., Heisel, T., and Jørgensen, M.H. Band-steaming for intra-row weed control. in 5th EWRS Workshop on Physical Weed Control. 2002. Pisa, Italy: European Weed Research Society https://orgprints.org/id/eprint/1546/1/Pisa_BME_DK_1.pdf