Answer: Cottonwood trees tend to send out suckers. It's their way of trying to colonize an area. There's nothing you can do to stop the suckering and about the only way to deal with the suckers is to cut them down as soon as they emerge. Even if you dug down and severed the root from the tree to stop suckering in a specific place in the landscape, the tree would likely regrow a root and the suckering would resume. Wish I had a solution for you, but aside from mowing or cutting down the suckers as they appear, there's not way to eliminate them. Sorry!
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