ragweed, any plant of the genus Ambrosia, coarse, weedy herbs belonging to the family Asteraceae (aster family), most of which are native to America. They have inconspicuous greenish flowers and soft subdivided leaves. Ragweeds are regarded as especially troublesome because their pollen is acknowledged as the primary cause of hay fever—especially the pollen of A. artemisiifolia (common ragweed) and A. trifida (great ragweed), the two most prevalent species in North America. The leaves of the common ragweed were formerly used as an astringent and hemostatic; they sometimes impart a bitter taste to milk if eaten by cattle. One variety ( elatior ) of this species has become widely naturalized in Europe. Ragweeds are classified in the division Magnoliophyta, class Magnoliopsida, order Asterales, family Asteraceae.