Image by photofarmer
This is a propane torch. I use it to create a stale seedbed. That's a technique whereby one makes a seedbed and waters it to encourage the weeds to germinate. After a week or two, when the seeds in the upper layer of soil have germinated, you kill them and if you don't disturb the soil and bring up more seeds, the seedbed will be fairly weed free. Of course planting seeds disturbs the soil a bit, but at least there are very few weeds around the seeded line. There's a rule of thumb to the effect that a seed buried more than 7 times its largest dimension will not germinate. I'm sure there are exceptions, but it seems to work fairly well for weed seeds. Once you deplete the seeds in the top thin layer of soil there aren't any more weed seeds to germinate until you do a mechanical cultivation to a deeper level and bring up more seeds.
The propane generates a flame around 2500 F, and this torch puts out about 250,000 BTU/hour, about the same as a furnace for a medium size house (depending, of course, on what you might call medium sized). The 20 lb propane cylinder will last about 45 minutes at that rate, but that will cover about 3/4 acre.
Note the hearing protectors on the backpack frame. This thing is noisy.
I just used it on the carrot bed. I now seed the carrots, wait 5 days, then torch anything that's up. On the 6th or 7th day, the carrots are up. You don't crisp the weeds. All that is necessary is to heat them up to the point where the water in the cells boils and disrupts the cell wall. The plant then dies. At the hottest point of the flame, it takes less than 0.1 seconds to do that, so you can walk along the row at a moderate saunter. The heat doesn't penetrate the 1/8" of soil where the carrot seeds are planted, so they survive. The horizontal range of the flame is about a foot, depending on the size and number of the weeds it encounters on the surface. For a 2 foot wide seedbed I have to count on two passes to get the weeds. Weeds more than 2" high are resistant to the flame. Some people even use this when the crop is up high enough, since the crop will also survive if it is large enough and the flame exposure is brief enough. I don't tempt fate that way.
If you look carefully you will see that the propane cylinder on the backpack is mounted upside down. That's because this particular model of torch is designed to work with liquid propane rather than propane gas. Both types are available, but for large areas the liquid torch is preferable. The propane is a gas when it burns, so it has to be converted from a liquid in the tank to a gas. That conversion (evaporation) reduces the temperature of both the liquid and the gas. After 15-20 minutes of use, a propane tank supplying a gas torch is starting to frost up, and the output pressure is dropping. With the liquid torch, the liquid travels down the pipe to the little nose just in front of the torch (resting on the gravel in the picture) where the flame supplies the heat to evaporate the liquid, then burns the gas. With this system the temperature of the liquid in the tank stays fairly constant, so the pressure doesn't drop with extended use.
This is a specialized piece of equipment, not the garden torch you can find in hardware stores. (i.e. don't try this at home).Weed Torches
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