Ragweed is one of the top seasonal-allergy sources in North America. Climate change has lengthened its pollen season by 27 days since 1995, a trend that's expected to continue.
Habitat: Temperate regions around the world, often near farm fields, roads and rivers
Description: Lobe-leafed shrubs in upright clumps. All 41 species are monoecious, which means every ragweed plant produces pollen.
Allergy season: August to November
Tips: Most trees and grasses have finished pollinating by late summer, but ragweed is just getting started. It releases pollen at dawn, with airborne levels often spiking from 10 a.m. to 3 p.m., depending on wind and dew. (Dry, windy days are worst.) Some natural allergy remedies may help, from butterbur and quercetin to hot peppers and raw local honey.