Weeding can be tedious and time consuming-–and the worst thing is that you can't be sure that you have removed all the weed seeds and runners that are there, in the soil. One option for areas where there are no plants you want to keep is a chemical weed killer. But people are becoming more reluctant to spread that kind of heavy-duty chemical into the environment, and you have to wait a long time to be able to plant anything else in the affected area. That is why weed torches are gaining in popularity.
Choose the Right Fuel
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Consider the intended uses for your weed torch and how complex you want the system to be. For a small torch, with enough power to kill weeds in the cracks in the driveway and the precision to get into small corners, a single-fuel system powered with MAAP gas may be all you need. The hottest burning torch would be a dual-fuel system of propane and oxygen (favored by serious welders and glassblowers) in which the oxygen accelerates the heat given off by the propane. Most commercial weed torches seem to go for a middle-of-the-road single-fuel propane system.
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Assemble the Torch System
You will need a torch head and a length of hose with appropriate fittings to connect it to the fuel source. For a small flame weeder, you may be able to use a simple soldering torch head. For a larger, hotter flame, it will be safer to attach a length of pipe so that you can angle the torch head accurately while keeping yourself farther back from the flame. To be effective, a torch weeder needs to be mobile, so consider mounting it to a dolly. Backpack systems are also an option, but this may not be the safest choice, especially when testing a homemade torch.
Other Uses
Once you're done weeding, you can use your torch for other things around the house and yard. One thing it can do is burn paint and other residues off your driveway. It can also melt ice on sidewalks and thaw frozen pipes (water only – never use a torch near gas pipes). This kind of torch can dry out a sand pit for your kids, or kill germs in metal pet cages. When thinking of other uses, just make sure that the part you want to keep is non-flammable, and that hot metal may retain heat for some time.
Concerns
If you live in a city, you will need to make sure it is legal to operate a flame weeder. No matter where you live, be careful around fences, trees and other flammable surfaces, and have a contingency plan (water buckets, fire extinguisher, etc.) for what to do if you should accidentally set something on fire. Also remember that root systems can smolder underground, so make sure the fire is completely out if you take out brush or saplings with your torch.