Knap·weed n. Bot. The black centaury (Centaurea nigra); -- so called from the knoblike heads of flowers. Called also bullweed.
  ◄ ►
  Mat·fel·on n.  Bot. The knapweed (Centaurea nigra).
  ◄ ►
  I·ron·heads n. Bot. A European composite herb (Centaurea nigra); -- so called from the resemblance of its knobbed head to an iron ball fixed on a long handle.
  ◄ ►
  Centaurea nigra
       n : a weedy perennial with tough wiry stems and purple flowers;
           native to Europe but widely naturalized [syn: lesser
           knapweed, black knapweed, hardheads]