[1] : The Collaborative International Dictionary of English v.0.48

ironheads \i"ronheads`\, 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
see also:
centaurea nigra 

[2] : 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
see also:
centaurea nigra bullweed 

[3] : The Collaborative International Dictionary of English v.0.48

matfelon \mat"felon\, n. w. madfelen. bot.
   the knapweed centaurea nigra.
   1913 webster
see also:
centaurea nigra 

[4] : WordNet (r) 2.0

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
see also:
lesser knapweed black knapweed hardheads 

Nigra >> N Words
Nigra, definition of term: Nigra
nigra_pag1.html

Centaurea >> C Words
Centaurea, definition of term: Centaurea
centaurea_pag1.html