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]

WordNet (r) 3.0 (2006):

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