The oak's hanging catkins, tassel-like appendages that are a few inches long, bear clusters of male flowers. Once these flowers disperse pollen, the catkins fall off the tree — you may have seen them on the sidewalk or piled up on your car's windshield in the spring. Pollen from the catkins fertilizes the oak's female flower to produce acorns. If you're allergic to oak, you may also be sensitive to beech, birch and alder trees, which are in the same family.
When does it peak? February through May
Where does it grow? Throughout the United States