Cottonwood trees are either male or female. Male trees produce no cotton. Nurseries sell male “cottonless” cottonwoods, propagated from cuttings taken from trees known to be male. The seedling in your yard, a female, has now reached an age when it is capable of reproducing. The cotton, so easily airborne, carries small seeds with it.
Ethephon, sold under the trade name Florel, has been used to limit crabapple fruit development. Similarly, when used according to label directions at the time of flowering, it should reduce cotton. Cottonwoods flower before leaves emerge in March. The flowers are about 4 inches long and look like a gray fox tail. Florel is now labeled for this use on cottonwoods. You may need to have a tree service apply Florel, as small home-use sprayers could not reach all parts of a large cottonwood tree.
