Today, May 28, 2017, in beautiful, warm, late-spring weather, the air is filled with snow. The snow is piling up along the edges of my driveway. It wasn't there yesterday. What is this?
Female black cottonwoods are releasing their seeds today. Each seed is encased in white hairs like fluffy wisps of cotton. This allows the seeds to be transported great distances by the slightest breeze. The trees produce cottonwood "snowstorms" in the air and "snowdrifts" on the ground.
Black cottonwood is dioecious, meaning that each individual tree is either female (seed-producing) or male (pollen-producing) but not both. Only female black cottonwoods produce the cotton-covered seeds. Male trees pollinated the nascent seeds earlier in the spring.
Mature seeds, once released (i.e., today), germinate almost at once if they happen to land on moist soil. Because black cottonwood seeds can travel great distances and germinate easily, the tree is an important pioneer species in Pacific Northwest bottomlands.