tall buttercup vs butterflower

tall buttercup and butterflower both are nouns.

Word	Noun	Adjective	Verb	Adverb
tall buttercup	Yes	No	No	No
butterflower	Yes	No	No	No

As nouns, butterflower is a hypernym of tall buttercup; that is, butterflower is a word with a broader meaning than tall buttercup:

tall buttercup: perennial European buttercup with yellow spring flowers widely naturalized especially in eastern North America
butterflower: any of various plants of the genus Ranunculus

Other hypernyms of tall buttercup include butter-flower, buttercup, crowfoot, goldcup, kingcup.

tall buttercup (noun)	butterflower (noun)
perennial European buttercup with yellow spring flowers widely naturalized especially in eastern North America	any of various plants of the genus Ranunculus