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Flaming Stale Seedbeds for Weed Control

by Vern Grubinger
Vegetable and Berry Specialist
University of Vermont Extension

Flame weeding is the killing of weeds with intense heat produced by a fuel-burning device, either hand-held or tractor-mounted. Flame weeding usually relies on propane gas burners to produce a carefully controlled and directed flame that briefly passes over the weeds. The goal is not to set plants on fire, but rather to damage the cell structure of their foliage. Brief exposure to intense heat causes the cell sap to expand and that in turn disrupts cell walls. The flamed weeds don’t keel over immediately, but within several hours or days they wilt and then die.

Weeds are most susceptible to flaming while still small, ideally as seedlings less than a couple of inches high. Broadleaf weeds are more readily killed by flaming than are grasses. Many grasses have their growing point below ground, or they may have a protective sheath around it, so they can usually re-grow after flaming. Several passes with a flame weeder, a few days or weeks apart, may be needed to adequately suppress grasses. Repeated flaming can likewise be used to suppress perennial weeds that have stored energy in their roots or stems, allowing them to re-grow after a single flaming. For control of many perennial weeds, cultivation will be needed in addition to, or instead of flaming. Other weeds, such as purslane, appear to be relatively tolerant of flaming unless it is done at very slow speed, which increases the exposure to heat.

The stale seedbed technique is well suited to the use of flame weeding. Stale seedbeds are used primarily for early- to mid-season weed control in direct-seeded crops. When a seedbed is prepared and soil is stirred, whether by plowing, rototilling and/or bed-forming, there is always a flush of weeds that follows because new weed seeds have been brought up neat the surface of the soil. With the stale seedbed technique, instead of sowing vegetable seeds into freshly prepared soil, planting is delayed. The aim is to first allow weeds to germinate and be killed, and then plant, while minimizing soil disturbance that would bring new weed seeds to the surface. A stale seedbed can also be used to reduce weed pressure on ground that is being readied for setting out transplants.

When preparing stale seedbeds it’s important to get the soil completely ready for crop planting. Apply nutrients according to soil test recommendations, thoroughly incorporate fertilizer and organic residues, and prepare a very smooth seed bed. Irregularities in the surface of the soil can sometimes deflect heat from the flame weeder, giving some protection to weed seedlings. After soil preparation is complete, if growing conditions are not optimal, as with dry or cool weather, you may want to encourage weed growth by applying irrigation or floating row cover prior to flame weeding.

Stale seedbeds have been used for many years, most often in conjunction with shallow tillage or non-selective herbicides to kill that first flush of weed. Flaming is an alternative technique that has been growing in popularity, especially among organic growers. It is relatively quick, inexpensive and safe if proper precautions are followed. Generally, flaming focuses on getting in-row weed control because one can rely on mechanical cultivation after crop emergence for between-row weed control.

Depending on when the soil is prepared, when the weeds are germinating, and when the crop is to be planted, stale seedbeds may be flamed once or perhaps several times. Some growers like to prepare many beds in advance and keep them clean with regular flaming until they are needed for planting. Other growers do not have enough extra land to do that, and they prepare beds, flame, and plants in as short a time span as possible.

Early in the season when it is cool stale seedbeds may not work as well because warm season weeds won’t germinate fully. But once the weeds are coming on strong don’t wait too long to flame them. Broadleaf weeds that reach the 3-leaf stage they should be flamed to prevent them from getting too large and becoming difficult to kill.

Unlike with shallow cultivation of a stale seed bed, flame weeding can continue even after a crop is direct seeded, so long as the crop has not emerged. You can usually flame right up until the seedlings are starting to push the ground up without sustaining crop damage. For the longest-lasting weed control effect, the final flaming of weeds should be done as late as possible: ideally after seeds have germinated but before crop seedlings have emerged. To identify that ideal timing usually requires some digging around in the row, and checking the ground to see if crop seeds have sprouted. Some growers place a glass or plastic plate over a row end to speed crop emergence in one small area to give them a heads-up. Others have tried planting a few feet of row to a slightly faster-emerging crop, which also provides a signal that it’s time to flame.

Flame weeders come in a range of human- and tractor-powered models. Market-farming equipment options include handheld single-torch flamers, as well as push-wheeled multiple-torch flamers mounted under a flame hood.  These small-scale units are easy to operate and very convenient for flaming on farms with many small, sequential plantings of crops. Flaming is especially useful with small crops that are slow to germinate, like onions and carrots, which are not competitive with weeds and are not well-suited to moving a lot of soil into the row during cultivation.

Tractor-powered flaming kits are available in multiple-row models, with or without a flame hood; other options include a complete toolbar setup with accompanying cultivator attachments for between-row mechanical cultivation. Farmers that have learned how to establish stale seed beds and use flame weeders in a timely fashion have been positive about the effectiveness of this method for weed control.

Safety should be a serious concern with flame weeding, especially with tractor-mounted units. Always consult with a gas professional if constructing your own flaming unit. Do not mount propane tanks intended for stationary use onto tractors. Be careful to flame against the breeze, and to avoid areas with dry residues or dry hedgerows. Liability concerns may hinder the use of flaming for some farmers.

For much more detailed information on flame weeding, and a list of flame weeding equipment suppliers, contact ATTRA at 1-800-346-9140 or see their web site:  http://attra.ncat.org/attra-pub/flameweedveg.html.

Published: May 2004
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