lesser knapweed vs hardheads

lesser knapweed and hardheads both are nouns.

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

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

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

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

lesser 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