However, weather conditions can affect the amount of pollen in the air at any given time. The pollinating season starts later in the spring the further north one goes. Generally, the pollen season lasts from February or March through October. Trees pollinate earliest, from late February through May, although this may fluctuate in different locations starting in April in the northern United States to as early as January in the south. Grasses follow next in the cycle, beginning pollination in May and continuing until mid-July.
Weeds usually pollinate in late summer and early fall.
Molds Molds are microscopic fungi related to mushrooms and mildew but without stems, roots or leaves. Their spores float in the air like pollen, and are present throughout the year in many states. Unlike pollens, molds do not have a specific season, but are affected by weather conditions such as wind, rain or temperature. Outdoor mold spores begin to appear after a spring thaw and reach their peak in July in warmer states and October in the colder states. Molds can be found all year long outdoors in the South and on the West coast.