The pollen count tells us how many grains of plant pollen were in a certain amount of air (often one cubic meter) during a set period of time (usually 24 hours). Pollen is a very fine powder released by trees, weeds and grasses. It is carried to another plant of the same kind, to fertilize the forerunner of new seeds. This is called pollination. The pollen of some plants is carried from plant to plant by bees and other insects. These plants usually have brightly colored flowers and sweet scents to attract insects. They seldom cause allergic reactions. Other plants rely on the wind to carry pollen from plant to plant. These plants have small, drab flowers and little scent. These are the plants that cause most allergic reactions, or hay fever.
When conditions are right, a plant starts to pollinate. Weather affects how much pollen is carried in the air each year, but it has less effect on when pollination occurs. As a rule, weeds pollinate in late summer and fall. The weed that causes 75 percent of all hay fever is ragweed which has numerous species. One ragweed plant is estimated to produce up to 1 billion pollen grains. Other weeds that cause allergic reactions are cocklebur, lamb’s quarters, plantain, pigweed, tumbleweed or Russian thistle and sagebrush.
- Trees pollinate in late winter and spring.Ash, beech, birch, cedar, cottonwood, box, elder, elm, hickory, maple and oak pollen can trigger allergies.
- Grasses pollinate in late spring and summer.Those that cause allergic reactions include Kentucky bluegrass, timothy, Johnson, Bermuda, redtop, orchard, rye and sweet vernal grasses.
Much pollen is released early in the morning, shortly after dawn. This results in high counts near the source plants. Pollen travels best on warm, dry, breezy days and peaks in urban areas midday. Pollen counts are lowest during chilly, wet periods.