Knapweed n. : The black centaury (Centaurea nigra); -- so called from the knoblike heads of flowers. Called also bullweed.

Example:
Knapweed, help to stamp it out effectively?

Taxonomy
spotted knapweed, Pasture, North Dakota State University, Management, Rangeland, infestations, plants, seedlings, herbicides, dicamba, 2,4-D, acre, disability, Prevention, picloram, seeds, North Dakota, Montana.

Defination from the web:
... Spotted knapweed is an aggressive, introduced weed species that rapidly invades pasture, rangeland and fallow land and causes a serious decline in forage and crop production. ...