Certified GKH Gardening Expert
Undiluted Round Up will help kill the roots. You will need to wound the roots (normally, people cut the suckers down and use those wounds) and then paint the undiluted Round Up on the wounds. The bigger or more wounds you paint, the better. The roots will suck the Round Up in and it will kill them. You may have to repeat the process a few times to fully kill the roots if the root system is still strong.