hardheads vs knapweed

hardheads and knapweed both are nouns.

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

As nouns, knapweed is a hypernym of hardheads; that is, knapweed is a word with a broader meaning than hardheads:

hardheads: a weedy perennial with tough wiry stems and purple flowers; native to Europe but widely naturalized
knapweed: any of various plants of the genus Centaurea having purple thistlelike flowers

hardheads (noun)	knapweed (noun)
a weedy perennial with tough wiry stems and purple flowers; native to Europe but widely naturalized	any of various plants of the genus Centaurea having purple thistlelike flowers