Lacking the showy flowers that attract insects to pollinate them, grasses rely on the wind to spread their pollen. Of the 1,200 or so species of grass in North America, though, only about 20 cause allergic reactions. The most common are:
- Perennial Rye
- Salt grass
- Sweet Vernal
Of the 650 trees native to the U.S., the pollens of about 100 are allergens. Some of the most common culprits are: Alder, Ash, Beech, Birch, Chestnut, Cypress, Japanese Cedar, Western Red Cedar, Elm, Hazel, Heath, Hickory, Maple, Myrtle, Mulberry, Oak, Olive, Pine, Plane, Poplar, Ti, Wattle, Walnut, and Willow.
During tree season, keep windows closed tightly and use the air conditioner rather than window or attic fans. Most people are allergic to a single species, but people may be allergic to several members of certain families, particularly these:
Moving from one city to another can actually trigger seasonal allergies in many people. HowStuffWorks explains the details behind why.