Ragweeds are flowering plants in the genus Ambrosia in the aster family, Asteraceae. They are distributed in the tropical and subtropical regions of the Americas, especially North America, where the origin and center of diversity of the genus are in the southwestern United States and northwestern Mexico. Ragweeds are annual and perennial herbs and shrubs. Species may grow just a few centimeters tall or exceed four meters in height. The stems are erect, decumbent or prostrate, and many grow from rhizomes.
Ragweed pollen is a common allergen. A single plant may produce about a billion grains of pollen per season.