Any of about 15 species of weedy plants (see weed ) that make up the genus Ambrosia in the composite family , 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. hay fever in eastern and middle North America. Since ragweeds are annuals, mowing before pollination season eradicates them.