bed of roses vs flower bed

bed of roses and flower bed both are nouns.

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

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

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

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