WordNet sense 1 (widespread European weed having yellow daisylike flowers; sometimes an obnoxious weed and toxic to cattle if consumed in quantity):

ragwort, benweed, Senecio jacobaea, tansy ragwort

WordNet sense 2 (any of numerous chiefly North American weedy plants constituting the genus Ambrosia that produce highly allergenic pollen responsible for much hay fever and asthma):

ambrosia, bitterweed