Centaurea nigra vs black knapweed

Centaurea nigra and black knapweed both are nouns.

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

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

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

Other synonyms of Centaurea nigra include hardheads, lesser knapweed.

Centaurea nigra (noun)	black knapweed (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