Source : Webster's Revised Unabridged Dictionary (1913)

Ironheads \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. --Dr. Prior.

Knapweed \Knap"weed`\, n. (Bot.)
   The black centaury ({Centaurea nigra}); -- so called from the
   knoblike heads of flowers. Called also {bullweed}.

Matfelon \Mat"fel*on\, n. [W. madfelen.] (Bot.)
   The knapweed ({Centaurea nigra}).

Source : WordNet®

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}]