Pecan trees like this one make more than just sturdy timber and nutritious nuts. They're also major sources of allergenic pollen, and they aren't alone: At least 11 other hickory trees also cause severe pollinosis, including the bitter-nut, mockernut and shellbark hickories.
Habitat: Eastern and central U.S., Mexico, Canada, China, India and Indochina
Description: Tall trees with wide, straight trunks that can reach 100 feet high
Allergy season: Late spring (April to June)
Tips: Hickory trees are monoecious, which can make their pollen hard to escape. Fortunately, the large particles don't travel very far and are less prevalent in the afternoon.