What is the best way to get rid of thistles?

The thistle is one of the most difficult perennial weeds to control because its underground stems can travel up to 20 feet a season. After chopping them down, use a plastic mulch to cover the entire area. A straw mulch, laid at least three inches thick, also will smother thistles. (Do not use a leaf mulch.) Your weed-control program must be long-lasting because the roots can lie dormant for several years. In addition to mulching, plowing the garden deeply in the fall will bring the rhizomes (stems) to the surface. But avoid rototilling, because this will cut the rhizomes, and each piece will become a new plant.