Hand weeding won't work on these guys because you'll never get 100 percent of the roots out of the ground, so they keep coming back. The only effective solution is a systemic herbicide, such as Roundup. "The problem is, weeds are usually mixed in with garden plants and the spray will kill anything it touches," says Roger Cook. "So I like to cut the weed down to the ground and, as soon as it leafs out again, paint the chemical onto the leaves using a foam brush." Coat all of its leaves, and it will finally die, roots and all.