centaury vs knapweed

Centaurea, a genus in the Asteraceae containing species sometimes called centaury 

Centaurium, sensu stricto 

Gyrandra, a genus in Gentianaceae, formerly included in Centaurium, with species commonly called centaury 

Cheirolophus crassifolius, a species in the Asteraceae commonly known as Maltese centaury 

Schenkia, a genus in Gentianaceae, formerly included in Centaurium, with species commonly called centaury 

Zeltnera, a New World genus in Gentianaceae, formerly included in Centaurium, with species commonly called centaury 

Sabatia, a New World genus in the Gentianaceae containing species sometimes called centaury 

Any of various common weeds of the genus Centaurea 

especially Centaurea nigra.