Gardening Articles: Care :: Pests & Problems

Kill Weeds With Heat

by Dan Hickey


Annual weeds, such as lambsquarters, are quickly killed by heat

Our early ancestors discovered fire and invented gardening. And even though farmers for centuries have used controlled burning to improve crops, it has not been until recently that home gardeners began to use mechanical flame torches, or flamers, in the garden. Of course, it's never too late to invent a garden tool that kills unwanted weeds without requiring the gardener to bend and pull, disturb the soil, or lace both soil and crops with herbicides.

Though flaming technology has been around since the 1940s, home gardeners have expressed renewed interest in these weed-fighting tools. Flamers require no chemicals, and don't result in groundwater contamination or chemical residues on garden crops. But safety concerns of another type remain. Never use flame torches around any dry, brown, or otherwise flammable material. Also, their use during dry periods in forested or arid regions is prohibited, or ought to be. Always check with your local fire department or town clerk before investing in a flamer.

Personal safety is another issue. These portable torches use pressurized tanks of propane and, if handled carelessly, can be hazardous. When operated properly, however, flamers are easy-to-use, safe, and timesaving gardening tools.

Using Flamers

Flamers are portable gas torches that produce intense heat (about 2,000&degF). When you pass the flame over and around weeds, it quickly boils the water in the plants' cells, causing them to burst. Once the heat destroys any section of a weed's stem, for instance, water and nutrients cannot reach the leaves, and the top part of the weed dies.

For the home gardener, killing weeds is as easy as holding the flamer and walking slowly (1 to 2 miles an hour) between garden rows. Killing a weed requires heat for only 1/10 of a second.

"You know you're successful when the weed changes from a glossy to a matte finish," says Tom Lanine, weed ecologist at the University of California at Davis. "The weed may not droop immediately but will wilt and die within a few hours. Then you just leave the weed to compost naturally. You don't want to disturb the soil and bring more weed seeds to the top."

For effective weed control, use flamers in spring and early summer as annual and perennial weeds emerge. Killing larger, mature plants requires more heat, so save time and fuel by flaming weeds when they're still young and tender.

For effective control, Penn State vegetable specialist Mike Orzolek recommends a series of flaming attempts, 2 to 3 weeks apart. Flaming kills annual weeds completely (though more annual weeds will pop up), but it doesn't kill the roots of perennial weeds. These will send up new shoots within a week or so after flaming. Additional treatments will eventually deplete the roots' stored energy, and the weeds will die.

Lanine and Orzolek both recommend using flamers as a pre-emergence control. Most viable annual weed seeds are in the top 1/4 inch of soil, and flamers can kill already-germinated seeds with heat.

Lanine recommends watering the soil before applying heat. "Unless the seeds have sprouted, there's no way to kill them with flamers. Even when they've sprouted, the soil is an incredible insulator," he says. "However, if the soil is saturated with water, then you will get some conduction of heat and can kill some seedlings in the soil."

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