WordNet sense 1 (widespread European weed with spiny tongue-shaped leaves and yellow flowers; naturalized in United States):

bugloss, oxtongue, bristly oxtongue, Picris echioides

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, ragweed