Thermal Weed Control

Non-chemical methods of weed control in crops, orchards and vineyards are becoming more important as weeds develop herbicide resistance and agricultural production moves towards environmentally friendly management. Fewer new herbicides are being developed, and the use of herbicides is being prohibited or limited on some crops.

Organic producers in New Zealand use inter-row cultivation to reduce weed growth between crop rows, but weeds can still be a problem between plants within rows.

One control method that is regaining traction is foliar flaming. In other words, treatment of emergent weeds using directed gas-powered flames. This method was common prior to the 1940’s but was abandoned when herbicides were developed and became popular. Dr Charles (Merf) Merfield, head of the Future Farming Centre at the Biological Husbandry Unit at Lincoln University, says the technique is very useful in organic vegetable growing.

“Typically a producer will till the field ready for planting, wait a week or so, drill in the seed and just before the crop emerges go through with the flame weeder to kill off emerging weeds. Crops tend to be slower out of the ground than weeds so it can be a very effective control,” he says.

“They can also do selective flaming, similar to a selective herbicide. For example, with onions, the onions survive but the flames kill the fat hen weeds in amongst the onion crop. Carrots and onions are both resistant to flaming once they have four or five true leaves. Mature onions can also be flamed as part of harvest to prevent neck rot.”

Where use of herbicides is an option, conventional growers generally prefer that to flame weeding. However, flaming could be useful where herbicide resistance develops or herbicides are banned, says Merf.

“Foliar flaming needs to be better researched and understood so that when we run into problems with the herbicides we’ve got it as a Plan B,” he says.

“We need to look at alternative techniques. People in vineyards and orchards, especially in Marlborough, are interested in finding non-herbicide ways of controlling vegetation under their vines. Flaming is one option there, particularly if they are on a stony soil or don’t want to cultivate the ground.”

The downside of flame weeding is the high cost of fuel and the risk of setting fire to stems, posts and other flammable materials.

An alternative to fire is the use of steam. Glasshouse soils are often sterilised using steam and although very effective in killing pests and diseases it also kills the beneficial soil organisms and uses large amounts of energy. However, brief steam treatment of cropping soils (for tens of seconds) could be effective in killing off weed seeds if a practical technique can be developed.

“The most important weeds to control in outdoor crops are the ones in the crop row, so if you kill the weed seed bank in the crop row then weeds will not grow. Most weeds emerge from the top 5 cm of soil so you don’t have to heat a great depth of soil, just a cube of soil 7 cm or so across and 5 cm deep,” says Merf.

“Because you heat the soil just before you drill in the crop the technique could be used with any crop, so it means that you can control all the annual weeds that come from seed, and that means virtually all the weeds in vegetable and cereal crops. It is a bit like an ultra selective herbicide that works on any crop and kills all the weeds, which herbicides actually can’t do.”

“And it has a residual effect that lasts at least for the life of the crop. As long as you don’t till the soil and don’t allow any weeds to go to seed, then the crop row will stay free of weeds. For crops such as carrot seed where they can be in the ground for 14 months you can guarantee that strip is going to remain weed free for that entire 14 months.”

That’s the good news. The downside is that the steam generation systems used overseas use huge amounts of fuel – around 400 L of diesel per hectare to steam the crop row – and it involves a large megawatt boiler on the back of a tractor.

Instead of steam, Merf proposes using hot air and developing a process that allows recycling of the hot air.

“It doesn’t require a steam boiler, all you need is to burn some fuel that produces hot air and then you just have to channel it down, which is a bit of an engineering challenge but not insurmountable,” he says. “If we can recycle the heat there is the potential to reduce the amount of fuel used from 400 L of diesel per hectare down to perhaps 40 L.”

Merf has carried out some lab experiments treating soil with hot air and steam with different moisture contents and seeing how effective each was in killing weed seeds and what other issues there were. He found hot air is as effective as steam in heating soil and killing weed seeds, although there are some differences due to soil and seed moisture content.

“From my perspective this technique will almost certainly work using recycled hot air, and we now need to turn the lab studies into a working machine. It will be quite expensive to do the engineering scale up and field research,” he says. “But to me it is one of those techniques where we really should push on and find out how practical and feasible it is because vegetable growers are already faced with complicated herbicide management programmes and this technique offers the prospect of spray- and residue-free control of all weeds, not just some, for the life of the crop.”

“We also have the looming prospect of less herbicide and pesticide use in the future, not more. The Americans have had Roundup-ready crops for a decade or so and now they have a large number of Roundup-ready weeds as well and they are running out of herbicide options. We’ve already had the Roundup resistant ryegrass turn up in North Canterbury, so we need to start developing new control techniques now.”

Merf envisages two possible methods for heat-treating soil in crop rows. One being done in Europe is to drive a tractor down the row pulling the machine that heats the soil in situ. The second is to remove a load of soil from the field before the crop is planted, process it in the machine on the farm and then place it back in the field. The advantage of the second method is an even smaller energy requirement and simpler engineering.

“It may seem a bit way out but the technique is potentially really energy-efficient, and we are talking about volumes of soil of perhaps 80 tonnes per hectare, which is do-able,” says Merf. “And if we can get the energy efficiency really high we could look at treating the top 7 cm of the whole field, and then you would essentially have no weeds at all.”

The technique has possibilities beyond horticulture. Merf has had interest from road construction companies and the civil engineering departments of district councils that dig up large quantities of soil in road construction.

“When they put it back down they have huge weed problems, and they see that if they had a really energy-efficient technique for eliminating weed seeds from that soil, they could put it back on to a landscape and avoid weed control problems afterwards,” says Merf. “So to me it is one of those ideas that could offer a bunch of solutions to problems we didn’t quite realise that we had to start with.”

The FFC is seeking funding of about $250,000 to continue development of the technique. While it is at the very early stages, it offers great prospects for the one-pass control of weeds throughout the life of the crop and the elimination of the need for herbicides.