: any of various chiefly North American weedy herbaceous plants comprising the genus Ambrosia and producing highly allergenic pollen responsible for much hay fever and asthma: as a: an annual weed (A. artemisiifolia) with finely divided foliage that is common on open or cultivated ground in much of North America b: a coarse annual (A. trifida) with some or all of the leaves usually deeply and palmately 3-cleft or 5-cleft—called also great ragweed
Any of about 15 species of weedy plants (seeweed) that make up the genus Ambrosia in the aster family (Asteraceae), most of which are native to North America. They have rough, hairy stems, mostly lobed or divided leaves, and inconspicuous greenish flowers borne in small heads. Common ragweed (A. artemisiifolia) is found across North America. Pollen shed by ragweeds in great abundance in late summer is the principal cause of hay fever in eastern and middle North America. Since ragweeds are annuals, mowing before pollination season eradicates them.