Each plant has a period of pollination, which does not vary greatly from year to year, though weather conditions can affect the amount of pollen in the air at any one time. The pollinating season starts later in the spring the further north you travel. The pollen season can begin as early as January in the southern areas of the U.S., but generally lasts from February or March through October. Trees pollinate earliest, from February or March to April or May, though this may fluctuate in different locations, starting in April in the northern United States and varying to as early as January in the South. Grasses follow next in the cycle, with pollination beginning in May and continuing until mid-July.
Weeds usually pollinate in late summer and early fall.
Molds are parasitic, microscopic fungi without stems, roots or leaves. They contain no chlorophyll, and their spores float in the air like pollen. Mildew is caused by molds. Outdoor mold spores begin to appear after a spring thaw and reach their peak in either July, August, September, or October in the northern United States. Outside molds can be found all year long in the South and on the West coast.