ragweed

from WordNet (r) 3.0 (2006)

ragweed
    n 1: widespread European weed having yellow daisylike flowers;
         sometimes an obnoxious weed and toxic to cattle if consumed
         in quantity [syn: {ragwort}, {tansy ragwort}, {ragweed},
         {benweed}, {Senecio jacobaea}]
    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

Ragweed \Rag"weed`\ (r[a^]g"w[=e]d`), n. (Bot.)
   A common American composite weed ({Ambrosia artemisiaefolia})
   with finely divided leaves; hogweed.
   [1913 Webster]

   {Great ragweed}, a coarse American herb ({Ambrosia trifida}),
      with rough three-lobed opposite leaves.
      [1913 Webster]