Flame weeding using propane is a USDA approved organic weed removal method. It affords an opportunity for organic farmers to save time and money in their weed removal efforts. Furthermore, propane prices are currently reasonable, as shown by the following ten-year price graph.
There are a wide range of human and tractor powered flame-weeder models, many hand-built by the farmer. Individual propane tanks with hand held wands are quite common, as are human powered push carts which can be wider and span rows.
The flame weeder is used for pre-emergent and post-emergent weeding. It is particularly useful for small crops which are slow to germinate, like carrots and onions. Since it avoids cultivation, it is a no-till method of weed control.
Red Dragon is one company which has designed commercial equipment for industrial thermal weeding. This method uses an average of five gallons of propane per acre, or about half the cost of herbicide application.
Row crop flame weeding was used back in the 1930s, using kerosene. Research using the method has been done on 30 to 40 different crops, with good results. The goal is to rupture the weed plant cell walls, something that can be accomplished in one-tenth of a second with exposure to flame. The gas pressure and ground speed are used to control the heat exposure. It is most successful in use against small broadleaf weeds two-inches tall or at the 3-leaf stage. It works well on morning glories or bindweed. Flame weed control is less successful on grasses and perennial weeds. If necessary, repeat flaming three to five days apart is better than a one time heavy flaming.
The University of Nebraska is undergoing testing using mechanical flamers, and is hinting at a possible growing interest among conventional crop producers due to RoundUp weed resistance. They are working with manufacturers to make four-, six-, eight-, 10- and 12-row units.
Safety is important when using flame weed control. The method should be used on a dry day, but not in extreme dry conditions.
You may also want to check out this video of a tractor mounted flame weeder working a field of carrots.