While yellow is the typical color you think of and see when you come across pollen, some trees produce flowers that have white pollen. If your allergies act up when you see a lot of pollen around, that white color can make it easier to identify what's annoying you, even in spring months like May, when many plants are releasing pollen. However, sometimes what you're seeing isn't pollen but seeds.
One tree that has white pollen in May is the cherimoya, Annona cherimola, which produces edible fruit with segmented green rinds and pale inner flesh. The flowers bloom in late spring or early summer. The cherimoya tree is from South America but grows in the warmer regions of the United States, such as Hawaii and Southern California. Look for droopy, whitish flowers with three long petals that hang downward. The outsides of the petals may be brownish. Leaves are large, up to three-quarters of a foot long, but the tree tends to drop leaves before forming flowers.
Pollen vs. Seeds
Be sure that the "pollen" you see is actually pollen. Cherimoya pollen doesn't travel on the wind and tends to stay in the flower; this is why the tree needs pollinating insects or hand-pollination. Not to mention you won't find cherimoya trees growing freely in colder climates like the northern half of the United States. If you are outside the cherimoya's growing area or are surrounded by falling or floating white pollen, you may actually be looking at seed pods from poplar trees, especially cottonwoods. In order to propagate, the trees have to send seeds away from the original location, either through birds and animals carrying them away, or by releasing seeds so the wind can carry them.
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.
Several poplars such as the silver and balsam poplars also send fluffy seeds flying in late spring, although these poplars tend to release the seeds a little later into June. However, if you are sure you don't have cottonwoods in the area but are seeing seeds in May, it's possible you just have a tree that's ahead of schedule. The city of Edmonton, Alberta, notes poplars release fluff for about two weeks.