Birch

If it’s spring and you’re sneezing, these trees might be part of the problem. They grow throughout the continental U.S., except in the southernmost and westernmost states, and their pollen is very likely to trigger allergies. They’re often planted in yards and other landscaping because people like the way they look — the telltale white bark makes them easy to spot.

Elm

This tree grows in all but the coldest northern parts of the continental U.S. and makes pollen in the fall. The American elm, once common in the East and Midwest, has been steadily dying out since the outbreak of Dutch elm disease in Ohio in 1930. But the sturdy Chinese elm has stepped in. It grows to 40 to 60 feet tall with a full oval crown.

Cedar

There are about 70 different kinds of these trees and bushes, including juniper and cypress, and some of them can cause major allergy issues. Because there are so many types — and they’re fairly common — it can be hard to know which ones are your troublemakers. And their pollen season is long, too. It starts in January, and some of these trees and bushes can make pollen until May or June.

Oak

This tree grows all over the country and makes pollen in the spring. Many people are allergic to oak pollen, and because the trees are common in residential areas and parks, there can be a ton of it floating through the air. That can cause serious reactions in some people.

Ragweed

This grows everywhere and may be the most common allergy trigger in North America. Its pollen season runs from August to November — levels are highest around mid-September. It can be worse on hot, dry, windy days.

Bluegrass

Pollen from the different types of this — Kentucky bluegrass is a common one — can cause serious allergies, especially in the summer months when there’s more of it. It’s found in most Northern, Western, and Southern states, but grows best in the cooler regions north of Georgia and west of Texas.

Nettle

his weed found throughout the U.S. makes pollen for most of the year, though more in late summer and fall. It has upright, woody stems that grow 2 to 7 feet tall. You also may have seen “stinging” nettles in the woods. They can stick to you and cause an allergic reaction on your skin, too.

English Plantain

This weed sheds pollen from May to November, and it makes a lot of it. It’s thought to be a major cause of allergies in the Northwest, on the East Coast, and in California.

article: medicinenet.com