Torching the Weeds
When someone asks about using a propane torch to burn off weeds, I think about a story that appeared many years ago about a back yard strawberry patch covered with straw and someone wanting to burn their lawn to rejuvenate the growth. You can guess what happened, the fire got into the strawberry patch straw and killed the strawberry plants. Plus the grass didn't grow that much better.
So before you fire up the old blow torch to take care of your weed problem there are a few things to consider.
Don't use in an area that has either organic mulch or plastic mulch that can catch fire.
Don't use next to any wood structure or the fire department may be your next call.
The torch flame will work very well on small broad-leafed annual weeds (less than two inches in height). The flame only needs to touch the leaf long enough to change it's color to do its job you don't need to burn the plant to a crisp.
The torch does not affect perennial grasses like quack grass since their root reserves will easily replace any grass blades burned by the flame. Other grasses growing points are protected under ground so the flame may not be as effective on grasses.