Each spring, summer, and fall, the season is filled with lush colors of blooming trees, grasses, and weeds, which release tiny particles. The tiny particles are known as pollen. Pollen has several vectors it uses for mobility; anemophily (movement via wind), entomophily (movement via insects), ornithophily (movement via birds), hydrophily (movement via water), chiropterophily (movement via bats), and zoophily (movement via other animals). The pollen grains proceed by hitching a ride on the currents of the air. Then, nature does its part to generate variation and speciation through cleistogamy (self-pollination), and allogamy (cross-pollination). Although the main purpose of pollen is to fertilize other plants, many times it never succeeds in making it to its intended target. Instead, pollen enters human noses and throats, triggering a type of seasonal allergic rhinitis typically called pollen allergy or hay fever.
Trees, grasses and weeds have a very distinct period of pollination that typically do not vary from year to year. Generally, the entire pollen season lasts from February through October with pine having an elevated pollen production throughout. Even though it is abundant, pine pollen is seldom an important allergen. The pollinating season, however, starts later in the spring the further north one goes. In warmer places, pollination can occur year-round. In most southern states, tree-pollinating season commences in late December and ends in May. In the South Texas region, a unique fall pollination of Ulmus (elm) and evergreens such as Juniperus (junipers). Also in Texas, mountain cedar pollen (Juniperus ashei) is another unique pollination period that occurs in December and January. Typically, grass pollen begins in late May followed by the weed season in June and July. Starting in August, weed pollen increases in the environment and by the end of August, ragweed pollen begins to dominate the air.
Seasonal allergic rhinitis is often caused by tree pollen in the early spring. The chemical makeup of pollen is the basic factor that determines whether the pollen is likely to cause any type of allergic symptom. During the late spring and early summer, grasses often cause symptoms. Hay fever is caused by weeds in the late summer and early fall. In the late fall, unique to Central Texas, is a seasonal allergic rhinitis known as cedar fever. Trees that produce allergenic pollen include oak, ash, elm, hickory, pecan, box elder, and mountain cedar. Among North American plants, weeds are the most prolific producers of allergenic pollen. Usually ragweed is the major culprit, but others of importance are sagebrush, redroot pigweed, lamb’s quarters, Russian thistle (tumbleweed), and English plantain. Grasses are known to be a significant source of allergenic pollen. Timothy grass, Kentucky bluegrass, johnsongrass, Bermuda grass, redtop grass, orchard grass, and sweet vernal grass are all known to produce highly allergenic pollen.
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