bed of roses vs flowerbed

bed of roses and flowerbed both are nouns.

Word	Noun	Adjective	Verb	Adverb
bed of roses	Yes	No	No	No
flowerbed	Yes	No	No	No

As nouns, flowerbed is a hypernym of bed of roses; that is, flowerbed is a word with a broader meaning than bed of roses:

Other hypernyms of bed of roses include bed of flowers, flower bed.

bed of roses (noun)	flowerbed (noun)
a flower bed in which roses are growing	a bed in which flowers are growing