A few weeks each summer, clumps of cottony-coated seeds are shed by several North American tree species belonging to the genus Populus that, as a result, are commonly known as cottonwoods. The trees are dioecious, and only females shed the familiar white fluff, which softly floats through the air for dispersal to new locales. The cotton can be a nuisance and may clog gutters, air conditioners, and other items, but it is the pollen produced by male cottonwoods that is most problematic for individuals that suffer from allergies. Cottonwoods are solely pollinated by the wind, which requires male trees to produce massive amounts of pollen in order to ensure that it will successfully reach a flower on a female tree. Cottonwood blossoms are long, drooping catkins that emerge before the leaves sprout in the spring.