knapweed

Any of various weedy thistlelike plants in the composite family having flowers grouped in a head with a spiny involucre, especially several species of the genus Centaurea and the Eurasian species Acroptilon repens.

A general name for plants of the genus Centaurea of the composite family, as C. Calcitrapa, the star-thistle, and C. Cyanus, bachelor’s-buttons.

Specifically, C. nigra, also called buttonweed, hardhead, loggerhead, and by various other names.

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

Any of various common weeds of the genus Centaurea

any of various plants of the genus Centaurea having purple thistlelike flowers