PI will not laugh at Roundup, if you make it twice stronger that it should be. Buy concentrate and mix yourself. Spray till point of run off, try to cover all leaves. In few days come back - spray what is not wilted. Do it for couple weeks. They will die eventually. Now the tough part. You will need to somehow remove that dry PI material(they are still poisonous, DO NOT BURN!!!) and PLANT something on the spot. PI is earth bandage, it will fill in empty spots. The easiest way would be cover whole thing with a planter, or lay heavy landscaping fabric and bring some soil to place on top and plant something. If you brave enough to remove it, leave it till fall covered with cardboard, it is easier to remove when it is cool - you will need full body protection - I used rubber boots, heavy pants and washable jacket , heavy rubber gloves. Make sure you do not wipe sweat while you working there… Bag the derbies from top of the soil in trash bags. When soil is clean, dig the rest of the roots with the same precaution. After you done, ask somebody wearing gloves to help you to undress and place the close in the washing machine. Take lukewarm shower, with soap. And hope, you didn’t get it .