Pollen is one of the most common triggers of seasonal allergies. Many people know pollen allergy as hay fever. Experts usually refer to pollen allergy as seasonal allergic rhinitis.Each spring, summer and fall, plants release tiny pollen grains to fertilize other plants of the same species. Most of the pollens that cause allergic reactions come from trees, weeds and grasses. These plants make small, light and dry pollen grains that travel by the wind.Grasses are the most common cause of allergy. Ragweed is a main cause of weed allergies. Other common sources of weed pollen include sagebrush, pigweed, lambs quarters and tumbleweed. Certain species of trees, including birch, cedar and oak, also produce highly allergenic pollen.Plants fertilized by insects, like roses and some flowering trees, like cherry and pear trees, usually do not cause allergic rhinitis.