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Using a Flame Weeder in Vegetable and Fruit Crops

A walk-behind propane flame weeder
at the UMN Southern Research and Outreach
. Photo: Annie Klodd
Authors: Charlie Rohwer and Annie Klodd. Flame weeders can be a helpful addition to an integrated weed management program in many vegetable and fruit crops. We discuss how a walk-behind propane flame weeder is used at the University of Minnesota Southern Research and Outreach Center (SROC) in Waseca, and how growers can incorporate flaming into their own operations.

Uses and advantages of flame weeding

Flame weeding is a "thermal" technique that works by killing weeds with heat (not fire). Cell membrane function is disrupted by the heat, either killing the weed or restricting its ability to compete. Multiple models and types are available to fit the scale of the operation, from backpack single-torch models to multi-burner or even tractor-mounted implements. 

Flaming is a relatively non-disruptive tool that is organically approved, does not necessitate herbicide application, and leaves the soil undisturbed. Disturbing soil can enhance weed germination by bringing seeds closer to the soil surface. Flaming can also be used as an alternative to cultivation if the soil is too wet to cultivate.

It is most effective when used in an integrated program with other management tools, and is rarely utilized as the sole technique in a field. As a physical method of control, it can help to manage populations of herbicide-resistant weeds in conventional systems. 

While it has been used since the turn of the 20th century, research to-date on flame weeding has not been nearly as extensive as other non-chemical weed management methods like cultivation and cover cropping. However, previous research has improved the technology, documented differences how it affects different weed species, and tested how it can fit into weed management programs with other techniques. 

For instance, previous studies have shown that incorporating flaming and cultivation can decrease weed density and increase yield better than cultivation alone. Additionally, a study in late cabbage found that two applications of flaming per season to provide weed control as good as two applications of propachlor. A study in Ohio found that a low-speed flaming pass can be effective at controlling weeds in a tomato crop, but weather patterns and weed species are critical considerations.

Reliable control with a single flaming pass is not a reasonable expectation. Other research in Canada showed that onions are tolerant of repeated selective flame weeding, but as in other systems, flame weeding alone did not result in maximum yield.

Example uses:

  • Applied immediately before cultivation, twice per season
  • Applied after direct-seeded crops but before crop emergence.
  • Applied three weeks after cultivation (depending on weather and soil) and immediately before planting, to create a stale seed bed (non-selective flaming).
  • In-crop, between rows or between vegetable transplants
  • After harvest to terminate winter annual weeds
  • Spot-applied to individual weeds
  • Within rows of fruit trees or grapevines
  • In asparagus, on the trench edges during the establishment year or on row edges
Research has found that flaming is more effective on broadleaf annual weeds and small weeds; and is less effective on grasses, perennials, and large weeds. But that can be used to your advantage: non-broadleaf crops like sweet corn and popcorn have shown tolerance to post-emergence flame weeding.

Damage to corn from flaming very early in the season  might be similar to frost damage. Early in the season, the growing point of corn is below-ground while the leaves are expanding above ground. As the season progresses, the growing point is protected within the stem. For similar reasons, weeds like shepherd’s purse, with a strong rosette habit and protected growing point, may be difficult to kill with heat.

Additionally, a 2012 study found that weeds were more susceptible to flaming in the afternoon than the morning. This is likely due to lower leaf water content in the afternoon. 

Before planting

Flaming can be useful for terminating weeds prior to planting, to create a stale weed seed bed to plant into. At SROC, we used a 5-nozzle walk-behind propane weeder (pictured) in a pepper planting in 2018. Flaming was done in early June, and the pepper plants were transplanted the same day. 

In that experiment, flaming followed pre-plant cultivation by two different time intervals: 4 and 2 weeks after, in order to establish a stale weed seed bed technique. Cultivation was used to stimulate weed seed germination in the spring, before the crop, and then flaming was used to terminate the weeds.

Between transplants

Pre-emergence herbicides are generally used on brassica transplants at the SROC, but complete control is not possible with herbicides alone. In 2018, we used a walk-behind flame weeder with 3 of the 5 burners on to control early weed growth in broccoli shortly after transplant. 

It was extremely effective against purslane between rows with a slow walking speed. Purslane is succulent and can survive flaming (and cultivation) in the right circumstances. We were able to use the flame weeder two times before the crop was tall enough to prevent the flame from reaching close to the ground. 

We still required a wheel hoe and hand weeding later in the season, mostly for intra-row weeds that the burners didn’t reach, for a small number of broadleaves that escaped flame weeding, and for grasses that are difficult to control with heat.

In hops

At the SROC in Spring 2018, we attempted to remove the previous year’s woody residue within hop rows using our flame weeder. This was attempted when the soil was moist, early in the spring, to help prevent an uncontrolled burn. It was ineffective at reasonable walking speeds. We could start the stems on fire, but only temporarily and by walking extremely slow. 

However, our flame weeder was effective at removing the first flush of shoots, a common practice for controlling powdery and downy mildew. We found no crown damage in 2018 from flame weeding for shoot removal. Later in the season, a lush mat of basal hop growth near the crown is helpful for controlling weeds by excluding light from the soil, but the canopy increases humidity and can enhance disease severity on the hops. 

We found that hop foliage extending into the alleys is extremely susceptible to flaming, and this practice may help reduce disease severity. 

Operating a flame weeder

Walking pace

A general recommendation for propane usage in flaming is 10 gal/acre, in order to achieve desired weed control. This might be achieved at a walking speed of 3 mph, depending on gas PSI and burner configuration. Slower walking speeds or higher PSI may be required for larger or difficult-to-control weeds.

Nozzle spacing

Some types of flame weeders allow for adjustment of torch spacing. On the five-nozzle model pictured below, nozzles can be removed to create larger spacing for between-row flaming.
Two nozzles have been removed from the flamer to
enable it to operate in narrower rows without
injuring the crop. Photo: Charlie Rohwer.

Distance from the ground 

On hand-operated and some walk-behind equipment, there is a learning curve for keeping the nozzles a safe and consistent distance from the ground. A minimum of three inches from the ground is a general recommendations, but if set too high the weeds will not perish. Also, with a multi-burner unit, if one of the flames is accidentally extinguished during use, it may be difficult to notice.
Depending on type of flamer and operator experience,
there may be a learning curve to get the right walking
pace and distance from the ground. Photo: Charlie Rohwer.

Safety Considerations

It is of primary importance to recognize that flame weeding can be dangerous. Observe all safety precautions necessary for your equipment. Despite the common name "flame weeder," a general rule of thumb is that there may be no actual "flame" visible, as flame weeders work by heat, not physical burning.

If orange flames are common during use, the torches are likely too close to the weeds or dry residue, or the walking pace is too slow. The weeds do not need to shrivel and die immediately. The weeds will wilt and perhaps turn a darker green color as cell membrane integrity is compromised, but they should not be lit aflame or charred.

Like most weed management technologies, flame weeders are most effective on smaller weeds. Not only is this an issue of efficacy, but in the case of a flame weeder, it is also an issue of safety. Passing a flamer over large weeds, especially at a slow pace, can cause them to start a fire that can spread.

Additionally, take caution to avoid flaming high-residue areas. For instance, research has shown that while it is possible to use a hand-held flamer in blueberry plantings with wood chip mulch, the operator must take great caution to avoid getting the flame near the mulch under the bushes. If it is not possible to avoid that, it is best not to use a flame weeder on the area.

Avoid flaming during high wind conditions, especially if weed density or residue levels are high, as wind could cause flames could spread outside the target area.

Authors: Annie Klodd, Extension Educator - Fruit and Vegetable Production, and Charlie Rohwer, Scientist at UMN Southern Research and Outreach Center

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