Pollen is a fine, powdery plant dust that seed plants use to reproduce. The light grains from wind-pollinated trees, grasses, and weeds float through the air, and in some people the immune system reacts to their proteins, causing hay fever.
The simplest way to understand pollen is one idea we will call the Wind-Pollination Rule: the pollen that causes allergies is the pollen that travels on the wind. Pollen itself is a fine plant dust that plants produce to reproduce, carrying the cells one plant needs to fertilize another of the same kind. Only a slice of it ever bothers people. This guide explains what pollen is, the three types that matter for allergies, and why ragweed is such a heavy hitter in fall. For the wider picture, start with our pollen allergy overview.
Because pollen is central to plant reproduction, it is everywhere in the growing season. But the plants with big, bright, insect-pollinated flowers rarely trouble allergy sufferers, because their pollen is heavy and moves on insects. The plants that cause allergies are the plain ones you barely notice, whose light, dry grains drift through the air and are easy to breathe in.
For allergy purposes, pollen falls into three broad groups: tree, grass, and weed. These are all wind-pollinated, which is exactly why they matter. Their grains are made to be released in huge numbers and carried on the breeze, so they end up in the air you breathe. Knowing which group is active helps explain why symptoms come and go across the year. To track that daily, see pollen count explained.
The three groups also arrive at different times. Trees release pollen in spring, grasses in late spring and summer, and weeds such as ragweed in late summer through fall. Because the calendar shifts by plant type, allergy season can feel like it never fully ends. Our guide to when is allergy season breaks down the timing in more detail.
Pollen is harmless to the body, so why does it cause so much misery? In people with a pollen allergy, the immune system mistakes proteins in the pollen grains for a threat and mounts a defense. This is an immune reaction to pollen proteins, not an infection. The response releases chemicals that lead to sneezing, a runny or stuffy nose, and itchy, watery eyes. That collection of symptoms is called allergic rhinitis, better known as hay fever, and you can read more in what is hay fever.
This is also where the Wind-Pollination Rule pays off. Because the immune reaction depends on breathing in the grains, the pollens that cause the most trouble are the light, airborne ones from trees, grasses, and weeds. The heavy pollen from bright garden flowers rarely reaches your airways in the same way. Pollen is not the only airborne allergen at home, though; for a fuller view, see allergens in the home.
Pollen allergy can usually be managed, but a specialist can confirm what you react to and guide treatment. Consider seeing an allergist if:
An allergist can identify your triggers with testing and discuss options with you. This article is for general information and is not medical advice.
If one plant deserves special attention, it is ragweed. Ragweed is a weed that releases its pollen in late summer and fall, and it is a major reason so many people flare up in September. What makes it so potent is scale: a single ragweed plant can produce up to one billion pollen grains in a season. That is an enormous amount of allergy-triggering dust from one unremarkable plant.
Ragweed pollen is also built to travel. A single grain can be carried more than 100 miles on the wind, which means you can react to ragweed even if none grows in your neighborhood. Together, the huge output and the long-distance travel make ragweed one of the most far-reaching fall allergy triggers in the United States, and a clear example of the Wind-Pollination Rule at work.