black knapweed vs lesser knapweed

black knapweed and lesser knapweed both are nouns.

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

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

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

Other synonyms of black knapweed include Centaurea nigra, hardheads.

black knapweed (noun)	lesser 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