Pollen Pollen consists of tiny, microscopic granules from trees, grasses and weeds that can trigger allergy symptoms and asthma attacks.
Pollen is light enough to be carried by air currents or insects.
Different plants pollinate at different times of the year. In the early spring, trees such as oak, western red cedar, elm, birch, ash, hickory, poplar, sycamore, maple, cypress and walnut pollinate. During late spring and summer, grasses begin to pollinate. Weed pollen from ragweed, sagebrush, pigweed, tumbleweed, Russian thistle and cockleweed takes over in late summer and early fall. The pollen count is the number of grains of a specified variety of pollen present in a cubic meter of air at a specified time and place.