I had them a few years ago and pulled them up two years in a row only to have them come back like the world's fair, bigger and better every year. I got rid of them by using the tiller over the winter to dig them up. You don't have to go deep at all but they have running roots that have to be pulled up. I had to stop a time or two to clear the tines but I haven't had a weed since. Two years ago I helped a neighbor do his the same way and he is still weed less. IMO any spray that is safe to use in water won't kill them. I tried for several years before resorting to brute force.