Fire Your Weeds! 

Growing For Market Article

By Stephen Meyer   

Darkness was descending on the carrot patch, but we could not stop. It was the magic moment for flame weeding. Tomorrow morning would be too late. That afternoon our hands-and-knees investigation had verified that the carrot plants were just about to break the soil surface in our one fourth acre of fall carrots. The Colorado wind had died down - it was now or never. As the blue flame of the propane torches glowed, the apprentices were excited that our prototype flame weeder would end the yearly ritual of days of hand-weeding in the carrot patch. The next morning we were pleased to see the carrots starting to come up in weed-free rows. 

Flame weeding gives pre- or post-emergent weed control in a variety of horticultural and field crops. In contrast to broad-spectrum herbicides, it does not pollute the soil or water, and there are no worries about spray drift or weed resistance. My first exposure to flame weeding was the previous year in Europe while farming, teaching and touring organic produce farms with apprentices. Along with the care of the soil and the outstanding quality of the produce I was impressed with the innovative implements, especially the flame weeders. They came in all sizes and shapes. Back in Colorado, the ten-acre market garden that my brother managed for Eden Valley Institute became the testing ground for these ideas. After many prototypes we settled on a four-torch walk-behind model that our apprentices could operate easily. With several hundred foot beds seeded to carrots, beets and onions every two weeks, April through June, flame weeding proved to be a life saver. With this technique we have been able to direct-seed our storage onions instead of transplanting them, saving time and yielding onions with smaller necks and better storage quality. Pre-emergence flame weeding can greatly reduce hand labor on slow germinating crops ranging from parsnips to Larkspur.

    The best pre-emergence weed control is achieved with a “stale bed”. This means that the soil is prepared for planting ten to fourteen days before the crop is planted to let the weeds start to germinate. In dry climates water should be applied to promote weed germination. Floating row covers can speed up weed growth on beds that have not been prepared far enough ahead of time. Then without tilling the soil again the crop is planted into the “stale bed” and flamed just before emergence.

    To find the right timing for flame weeding small panes of glass are placed on the row in several places. This creates a greenhouse effect, causing the crop in that part of the row to germinate one to two days sooner than the rest. When the crop starts coming up under the glass it is time to flame, or at least check the weather so you don’t get caught by a rain. We often plant beets and carrots at the same time. When the beets are up we know that it is time to flame the carrots, since beets come up about a day before carrots under average circumstances. I still like to dig several places in the row and inspect the sprouting seeds to be sure to get the right timing. One becomes an expert in small seed excavation and biology. First the seed starts putting down a root, and then the seedling leaves start pulling out of the seed coat as the plant moves up toward the surface of the soil. This is the time to flame – just before emergence. We tend to plant the crops for flame weeding slightly deeper than usual so that we have more leeway from germination to seedling emergence. It is better to flame a little too early than to late. If the crop seedlings have emerged, the flame weeding will kill the crop as well as the weeds. There are exceptions. Most beets are multigerm and can afford to be thinned. If a few onion plants are up when you flame, it is not a problem since their heart is still under the surface at that point and they will come back.

    Although soil moisture is not critical, flame weeding works best when the weeds are dry and there is no wind. Guards over the burners are nice for those less then ideal days. In his book, The New Organic Grower, Elliot Coleman states that a metal hood over the burners can lower fuel consumption an average of 20-25 % compared to uncovered burners.

    It is not necessary to burn the weeds. The flame only needs to overheat the tissues and rupture the cells of the plants. They will have a glassy look and will wilt in a few hours. Annual weeds can be flamed effectively up to two inches tall, but I try to hit them in the truly seedling stage for best control and fuel efficiency. This is especially true of grasses, since they develop a protective sheath, which can shield the growth point from the heat, allowing regrowth. Established perennial weeds like thistles are a different story, since they will regrow from an extensive root system. 

    To enhance flame weed control, organic grower Paul Harlow of Westminster, VT, uses a modified spiral basket roller from a field cultivator to brake the crust and destroy two or three cycles of germinating weeds over a three or four week period before planting his crops and pre-emergent flaming. When the carrots and parsnips are off to a good start he comes through with Buddingh basket weeders followed by sweep cultivators and a hand-hoeing pass. Danny Lutz who grows 50 acres of organic produce in Yale, MI, flames one or two cycles of seedling weed growth before transplanting or seeding salads and salad mix. I have seen Swiss farmers seed salads into weedy beds, irrigate to perk up the weeds, and flame the following day.

    In addition to pre-emergence flaming of carrots, parsnips and direct seeded onions, Rich de Wilde uses his four-torch row-crop flamer for in-row weeding on his seventy-acre organic market garden in Viroqua, WI. Once the direct seeded onions are about 4-5 inches tall he cross flames the rows, directing the flame at the base of the plants. “Transplanted onions that are 8-10 inches tall can take a lot of heat, so you can burn out even the hardest weeds, like purslane. The onions will get a waxy-green look but they will come right back.”, he says. “Garlic is pretty tough, you can burn the tip off and it will still come back. On potatoes you are not going to loose really any yield at all up to 8 inches tall.” He bases this on Michigan Sate University studies, which also show that flame weeding can help to control potato beetles. There are two windows for flaming sweet corn. “When it is still a spike before the leaves open up you can burn it right off. It will delay it a couple of days but it will come right back. In fact, we sometimes flame half the sweet corn field, just to spread out the harvest. Then wait until it is at least 10-12 inches tall. Of course we don’t rely on it exclusively. But you can clean up some pretty weedy corn.” The corn may look bad for a few days, but will take off again. Eggplant and brassicas can even be flamed once the plants are good sized. “You have to direct the flame down on the ground and keep moving. You may singe some lower leaves.” At current prices (2007) de Wilde estimates his propane cost at $15-16 per acre for his tractor-mounted Thermal Weed Control unit. This year on 9 acres of parsnips he estimated that pre-emergent flaming saved $200 per acre on hand-weeding cost.

    Organic farmer Richard Wiswall of Plainfield, VT, flames his potatoes up to 2 inches tall. He says, “It’s nice, because the first hilling doesn’t always cover the weeds in the row. Flaming sets back the weeds but the potatoes pop right back.” According to his records, he has been able to reduce his hand labor on carrots and parsnips from 200-300 hours per acre down to around 80 hours per acre with pre-emergence flame weeding. Wiswall: “ It is not only economical, but also ecological if you look at getting the number of people out there that it takes for that many hours, and the transportation involved.” Flame weeding is not a cure-all, but it is often the non-toxic treatment of choice on hard-to-weed crops. If properly managed, the difference in weed control between flamed and un-flamed beds can be like night and day.

    For more information on flame weeding contact ATTRA , PO Box 3657, Fayetteville, AR 72702, 1-800-346-9140. Ask for Flame Weeding for Vegetable Crops or Flame Weeding for Agronomic Crops. These materials may also be obtained online at

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