Centaurea nigra vs knapweed

Centaurea nigra and knapweed both are nouns.

Word	Noun	Adjective	Verb	Adverb
Centaurea nigra	Yes	No	No	No
knapweed	Yes	No	No	No

As nouns, knapweed is a hypernym of Centaurea nigra; that is, knapweed is a word with a broader meaning than Centaurea nigra:

Centaurea nigra: a weedy perennial with tough wiry stems and purple flowers; native to Europe but widely naturalized
knapweed: any of various plants of the genus Centaurea having purple thistlelike flowers

Centaurea nigra (noun)	knapweed (noun)
a weedy perennial with tough wiry stems and purple flowers; native to Europe but widely naturalized	any of various plants of the genus Centaurea having purple thistlelike flowers