hardheads vs black knapweed

hardheads and black knapweed both are nouns.

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

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

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

Other synonyms of hardheads include Centaurea nigra, lesser knapweed.

hardheads (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