If you are trying to get rid of weeds in an area, and not just single weeds. one way is to wet the area and then put heavy duty clear plastic over it. Weight it down with rocks, bricks, or whatever.
Leave at least until everything turns brown; I think 3 – 4 months is best. The water with the sun burns the weeds; then keeping the plastic on prevents anything from sprouting up and should kill roots of the weeds.