Answer: It sounds like the plant is sending up suckers from the root system. There are several ways to fight this. One is to consistently remove the suckers as soon as they appear, hoping to eventually wear out the root reserves. Another is to treat the suckers with an herbicide containing glyphosate which will be translocated to the roots and kill them, although it may take more than one application. Another is to dig down and grub out the roots, or as much of them as you can find. I suspect it may take a combination of efforts if it is as vigorous as you described. Good luck!
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