lesser knapweed vs black knapweed

lesser knapweed and black knapweed both are nouns.

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

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

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

Other synonyms of lesser knapweed include Centaurea nigra, hardheads.

lesser knapweed (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