I recently had a paulownia tree clump of three trunks removed from near my house. I cut them down because one trunk had fallen in a storm and the other 2 became unstable, threatening to fall on the house. Within a short time, many shoots began popping up from the roots of the trees. These shoots grew with amazing speed. So I had the stump ground out but the shoots keep coming up with a vengance, sometimes growing 6-8 inches a day! I have sprayed them with herbacide to no avail. Is there any way to kill these roots once and for all?