The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Knapweed \Knap"weed`\, n. (Bot.)
   The black centaury (Centaurea nigra); -- so called from the
   knoblike heads of flowers. Called also bullweed.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Matfelon \Mat"fel*on\, n. [W. madfelen.] (Bot.)
   The knapweed (Centaurea nigra).
   [1913 Webster]

WordNet (r) 3.0 (2006):

Centaurea nigra
    n 1: a weedy perennial with tough wiry stems and purple flowers;
         native to Europe but widely naturalized [syn: lesser
         knapweed, black knapweed, hardheads, Centaurea
         nigra]