You know instead of roundup, try this:
1 cup salt
2 cups vinegar
1 tsp dish soap
water to make a gallon
if this doesn't kill it nothing will. I just pull it (all the time, out of pots, out of the ground, out of hanging baskets). It has the ability to broadcast its seeds very widely and the flowers are so small you never seem them do it. But don;t get this on anything you want to keep
