Chapter 13

Weed Management for Organic Crops

Flame Weeding

Many organic farmers have included propane (LP) flame-burners as an additional tool in their weed management toolbox (See Figure 14.27). Heat from the flamer kills the plant by rupturing cell walls, not burning the plant tissue. After flaming, weeds that have been killed change from a glossy to a matte finish. The length of time the flame is applied depends on the age, size, and tenderness of the weed. Flaming is used particularly during times of high field moisture when tillage with large machinery is not feasible. In drier weather, flaming is used in conjunction with cultivation. Flaming is most effective on annual broadleaf plants that have relatively thin (non-succulent) leaves and aboveground or unprotected growing points. In general, younger plants, especially newly emerged seedlings, are more susceptible to flaming than older plants. Flaming is least effective on weeds that can effectively avoid or tolerate high temperatures—for example, those with pubescent leaves (common purslane) or those that can initiate new growth after flaming, such as those with below-ground growing points (annual grasses). Weeds that have germinated, but are not yet emerged, will also not be affected by flame weeding. Because flaming does not control grasses or perennials well, rotary hoeing or harrowing may be a better option.

Advantages and Disadvantages of Flame Weeding

Flame weeding is an increasingly attractive weed control method because it provides multiple advantages over chemical and mechanical weed management methods used in both conventional and organic farming operations. Compared with the use of chemical herbicides with conventional crop systems, flame weeding does not leave chemical residues in or on plants, soil, air, or water.

Timing of Flame Weeding

Timing is probably the most important factor that influences the success of flaming, and it must be timed to balance weed damage with crop damage. In general, it is best to flame weeds when they are newly emerged or still very small in stature and when crops have yet to emerge or are large enough to withstand flaming without significant damage.

Flame Weeding Treatments

Two basic approaches are used to control weeds with flaming. 1) Non-selective flame weeding or across crop rows (“cross” flaming) so that flames are in contact with both weed and crop plants, and 2) selective flame weeding (“parallel” and “middle” flaming) and often with the crop protected by a metal shield or a fan of water sprayed between the flame and crop row.

Non-selective Flame Weeding

During non-selective flame weeding treatments (also known as broadcast treatments), everything in the treatment path—weeds and crops—is fully exposed to heat.

Stale Seedbed Technique. The stale seedbed technique is mostly reserved for high value, direct sown crops, which are poor weed competitors such as carrots and onions. It can be used on other direct-sown crops, e.g., cereals, but it is rarely cost-effective in such situations. The stale seedbed technique is a form of early-season weed control in direct-seeded crops.

Peak Emergence Technique. In the peak-emergence flaming technique, seeds are promptly sown after seedbed preparation. Just before the seedlings emerge, the bed is flamed to kill seedling weeds (which tend to sprout faster).

Selective Flame Weeding

Selective flame weeding treatments (also known as banded flame weeding) are done after the crop has emerged and aims to treat weeds without damaging the crop. This selectivity is usually achieved through torch configuration or by shielding the torches with various hoods. Banded flame weeding only treats widths of about 12 inches (30 cm) in the center of the crop row, in contrast to the treatment of the full row width (e.g., 30 in, 76 cm) with broadcast flame weeding.

Equipment Used for Flame Weeding

Flame weeders come in a range of configurations. 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.

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