bitterweed

from WordNet (r) 3.0 (2006)

bitterweed
    n 1: widespread European weed with spiny tongue-shaped leaves
         and yellow flowers; naturalized in United States [syn:
         {oxtongue}, {bristly oxtongue}, {bitterweed}, {bugloss},
         {Picris echioides}]
    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
       [syn: {ragweed}, {ambrosia}, {bitterweed}]

from The Collaborative International Dictionary of English v.0.48

Bitterweed \Bit"ter*weed`\, n. (Bot.)
   A species of {Ambrosia} ({Ambrosia artemisi[ae]folia}); Roman
   worm wood. --Gray.
   [1913 Webster]