hardheads vs Centaurea nigra

hardheads and Centaurea nigra both are nouns.

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

As nouns, hardheads and Centaurea nigra are synonyms defined as:

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

Other synonyms of hardheads include black knapweed, lesser knapweed.

hardheads (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