Cottonwood trees belong to the same genus as other poplar trees -- Populus. Cottonwood species exist throughout much of the United States, from the Pacific Northwest south to Arizona and east to the Midwestern states. The seeds have a fluffy, white casing. Cottonwoods tend to release seeds around May and June, and the resulting clouds of fluff are often mistaken by passersby as pollen because all they see is something falling out of the flowers of these trees during pollen season. It doesn't help that the cottony casings can set off allergies in people; the health clinic at Lane Community College in Oregon warns allergy sufferers to stay away from the cottonwood seeds when they float by.