Another look at thermal technology weed control

There has been a resurgence of flame technology in agriculture in recent years for controlling insect pests and diseases — for frost protection as well as for defoliation, dairy and poultry sanitation.

With renewed interest in thermal technology, there is a need to discuss the successful and economical adoption of the technology, especially in controlling weeds. Thermal technologies, besides flame, being explored for weed control include foam, hot water, steam and quenched hot gases. Regardless of the option employed, there are several best practices for safe, effective thermal weed control technologies.

Burning vs. thermal killing

A common misconception is that flame equipment should “burn” or consume the weeds with fire during treatment. Thermal weed control is based on flash heating to rupture cell membranes within the weed, thus shutting down the plant’s capacity for photosynthesis. When applied correctly to young, vigorous green weeds with minimal dead material there should be very little, if any, smoke from the treated area.

In fact, many air quality districts are approving thermal weed treatments (flame desiccation). As long as only green weeds are being treated, not burned, growers do not need burn permits and do not need to restrict themselves to burn days. However, igniting dead organic matter will certainly raise red flags and fees with local air districts.

Growers often drive too slow (less than 1 mph) with the propane regulator opened too wide (above 65 psi), with the goal of blackening weeds. This not only consumes far more propane than is necessary, but creates a potential fire danger among the crop, stakes and organic matter.

The safest applications of flame with the best results are accomplished within 2-3 mph and at 40-65 psi. These rates can effectively control young weeds while limiting fire issues and avoiding any damage to drip lines and tree or vine canopies. These parameters are most effective on 1-inch to 4-inch weeds in their early stages of growth and can reduce weed pressure by as much as 95 percent.

Some sites need to be prepared prior to flaming by removing the previous years’ dead matter. This will prevent unwanted fires and improves the kill rate of the young weeds.

If dead matter is intertwined with the new growth (between 1 percent and 5 percent coverage), and preparatory work does not seem necessary, then surface irrigating, or waiting for damp conditions can minimize fire danger while effectively killing young, green weeds. In fact, a limited presence of moisture can improve heat transfer and kill success — as we discovered with steam-based applications.

For effective control, a series of flame applications, three to five weeks apart can provide better control than one more intense application. Flaming kills annual weeds completely, however, it cannot kill the roots of perennial weeds. These may send up new shoots in the weeks after flaming. Subsequent treatments can eventually kill perennial weeds by depleting the roots’ stored energy.

No weed, if young enough, can tolerate 1,500-degree thermal treatment (800 degrees for steam). However, the following variables can make a difference.

– Mature weeds with a well-developed central stock are more tolerant of heat.

– Weeds with waxy surfaces may require additional treatments.

– Dense, matted growth at weed base is more difficult to heat sufficiently.

– Weeds with significant root masses such as with Bermuda can survive multiple flame applications.

These conditions may require two applications, slower drive speeds, or higher settings on the regulator. Often, when there is a low canopy or crop present, doubling up on the application is preferable to slower speeds or higher gas pressures to avoid crop damage.

Running several tests on short runs at various speeds, pressure settings and emitter positions before treatment can help ensure optimal results. What you want to do is compare weed kill success with canopy damage to determine the optimal settings. The results may indicate that two runs are necessary to kill the weeds while protecting the canopy. Typically, the options of driving slower and increasing gas flow are viable and effective only if the canopy has not developed, or if it is well off the ground.

Otherwise, two applications on mature (6-inch to 18-inch dense weeds) at 55 psi and 2 mph can do an outstanding job of cleaning the rows and eliminating most weed pressure. Don’t try to eliminate everything in the first pass. Waiting for three to five days, then making a second application of flame provides a safe alternative to trying to burn down weeds all at once.

Using flame as a pre-emergent can be accomplished just as the seeds are beginning to sprout, some an eighth of an inch under the soil. If the soil is moist, either through irrigation or a sprayer with water, then heat from the flame can be transferred down to the seed level and effectively control a respectable percent of the coming weed pressure.


Best results are accomplished when the flame has longer contact with the weeds. Position flame heads roughly 6 inches off the ground (providing for some clearance above the weed tops), with the heads angled to the rear of the applicator. This allows the flame to “brush” the ground, exposing weeds to the flame longer, as opposed to attacking weeds from the top.

Heads can also be angled out to achieve a longer reach or a wider control area. However, the gas pressure must also be increased if heads are angled out. Positioning the height and angles of the heads in the evening makes it easier to see the pattern of the flame and to optimize its coverage.

Adding flame

A single flame treatment in tandem with a chemical regime can have direct, positive economic and environmental benefits to the conventional weed control program.

The Herbicide Resistance Action Committee (HRAC) agrees that mixing treatments in an integrated approach, or interrupting the chemical program with an alternative treatment can help reduce the development of herbicide resistance among weeds.

A common integrated program consists of a pre-emergent herbicide, a post-emergent application of flame, and then (if necessary) another chemical post-emergent later in the season. If weed pressure persists as harvest approaches, another flame application at 30-45 psi and 3 mph can help control weeds.

Fire safety

Precaution and prevention are the cornerstones of fire safety. A small fire extinguisher on board the tractor is always a smart option. Dedicate a shovel to be secured to the trailer. Avoid flaming dry areas, keeping the application to young, green weeds.

Thorough training on the equipment before going into the field can go a long way to ease a crop manager’s mind. Lessons in how to shut down flame equipment while still in the field, or at the end of the day are critical factors contributing to an overall positive experience with flame.

This includes keeping in mind that propane lines can store 30-60 seconds worth of fuel, so flame heads don’t always stop as soon as switches are turned off.

One particular habit I encourage is performing a follow-up check in the evening. Although it’s rare, a surface root may have ignited under a patch of dry weeds. This root can smolder for hours, unnoticed in the daylight. Unchecked, a smoldering root can slowly burn its way to the base of a tree, or larger fuel source. A ride through the field on the quad, just after dusk, can help prevent just such an accident. If there are any embers, they’ll be much more visible.

Start with kits

Manufacturers encourage starting off with complete flaming units, as opposed to kits. This helps to ensure the components are well fitted and mounted together safely. Some distributors/dealers also provide assembly, which can include pressure testing all the valve and fittings for any leaks. This “full service” approach can alleviate most concerns about anything going wrong with the equipment itself.

However, if a grower wants to use an existing tank and/or trailers, kits are available. The best recommendation with this route is that the grower ask his propane dealer to inspect the equipment, paying particular attention to the location of hoses. Hoses will obviously need to be strapped or directed away from flame sources. They should also be well protected from looming branches and tractor tires.

Whether it’s flame weed control, dairy sand bed sanitation, or poultry house sanitation — the propane storage and fuel transfer system should be just as thought out as the technology itself. Your local propane provider should be consulted to help you determine the best size tank and location.

If the storage tank is close to an established electrical supply, then an electric pump can save “a lot” of time in filling the mobile tank. If that’s not an option, then propane compressors can also do the trick. Both of these modes can add $1,000-$2,000 for a turnkey solution, but they are well worth it in the long run.

Site managers should make sure all tanks, hoses and equipment are inspected regularly for rust, leaks, clogged strainers and overall safe operation.

(Kevin Smith conducts research projects for the propane industry. He can be reached at 559-431-3681; [email protected])

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