Centaurea nigra vs hardheads

Centaurea nigra and hardheads both are nouns.

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

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

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

Other synonyms of Centaurea nigra include black knapweed, lesser knapweed.

Centaurea nigra (noun)	hardheads (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