black knapweed vs hardheads

black knapweed and hardheads both are nouns.

Word	Noun	Adjective	Verb	Adverb
black knapweed	Yes	No	No	No
hardheads	Yes	No	No	No

As nouns, black knapweed and hardheads are synonyms defined as:

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

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

black knapweed (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