Source : Webster's Revised Unabridged Dictionary (1913)

Ironweed \I"ron*weed`\, n. (Bot.)
   A tall weed with purplish flowers ({Vernonia
   Noveboracensis}). The name is also applied to other plants of
   the same genus.

Source : WordNet®

ironweed
     n : any of various plants of the genus Vernonia of tropical and
         warm regions of especially North America that take their
         name from their loose heads of purple to rose flowers
         that quickly take on a rusty hue [syn: {vernonia}]