tall buttercup vs butter-flower

tall buttercup and butter-flower both are nouns.

Word	Noun	Adjective	Verb	Adverb
tall buttercup	Yes	No	No	No
butter-flower	Yes	No	No	No

As nouns, butter-flower is a hypernym of tall buttercup; that is, butter-flower 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
butter-flower: any of various plants of the genus Ranunculus

Other hypernyms of tall buttercup include buttercup, butterflower, crowfoot, goldcup, kingcup.

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