black knapweed vs Centaurea nigra

black knapweed and Centaurea nigra both are nouns.

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

As nouns, black knapweed and Centaurea nigra are synonyms defined as:

black knapweed and Centaurea nigra: a weedy perennial with tough wiry stems and purple flowers; native to Europe but widely naturalized

Other synonyms of black knapweed include hardheads, lesser knapweed.

black knapweed (noun)	Centaurea nigra (noun)
a weedy perennial with tough wiry stems and purple flowers; native to Europe but widely naturalized	a weedy perennial with tough wiry stems and purple flowers; native to Europe but widely naturalized