A wide array of ash trees cause allergies, including black, blue, green and white ash (pictured). But while they may spur seasonal misery in humans, many U.S. ash trees are suffering more than we are: They're under siege from the emerald ash borer, an invasive beetle introduced from Asia in the 1990s.
Habitat: North America, Europe, Asia and North Africa
Description: Tall, hardwood trees that grow up to 90 feet high. Most are dioecious.
Allergy season: Varies (winter to spring)
Tips: Avoiding male ash trees is usually enough, but a few species are monoecious, and some even have bisexual flowers. Be wary of places where both ash and olive trees grow, since their pollen can cross-react.