ironweed

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Any of various plants of the genus Vernonia of the composite family, having alternate leaves and clusters of purplish flower heads.

from The Century Dictionary.

• noun Same as flattop.
• noun Also, any of the species of the genus Vernonia, handsome composite plants of North America. See Vernonia.
• noun The blue vervain, Verbena hastata.
• noun The black knapweed, Centaurea nigra. Also called ironhead.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Bot.) A tall weed with purplish flowers (Vernonia Noveboracensis). The name is also applied to other plants of the same genus.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Any of certain species of the genus Vernonia of forbs and shrubs.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun 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