knapweed

from WordNet (r) 3.0 (2006)

knapweed
    n 1: any of various plants of the genus Centaurea having purple
         thistlelike flowers

from 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]