Answer: Cottonwoods are notorious for suckering - in the same league with apple and cherry trees. If you snip the suckers off, they'll regrow right away. Try pulling (or yanking) them out, taking a heel of stem/root tissue along with the sucker. Usually, if you can get the cells that encourage sucker growth, no new suckers will grow from that particular area of roots or trunk of the parent tree.
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