ragweed

(noun) widespread European weed having yellow daisylike flowers; sometimes an obnoxious weed and toxic to cattle if consumed in quantity. Synonyms: ragwort, tansy ragwort, benweed, Senecio jacobaea.

(noun) any of numerous chiefly North American weedy plants constituting the genus Ambrosia that produce highly allergenic pollen responsible for much hay fever and asthma. Synonyms: ambrosia, bitterweed.