Answer: The roots that remain in the soil are busy trying to sprout new trees so they can live. Each time a root sends up a new sprout, it depletes its energy reserve. If you keep the suckers cut down before they can develop leaves, the roots will eventually run out of stored energy. This may take a very long time, though. 75' trees probably had extremely extensive root systems. Digging them up is probably not an option. In addition to cutting down every single sucker, you can also paint Round Up on the cut stubs. It will move down into the root and kill at least that part of the root. You'll just have to keep at it until the roots die off, or you have them removed. Wish I had better news for you!
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