The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

ironweed
    n 1: 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: ironweed, vernonia]