bed of roses vs rose bed

bed of roses and rose bed both are nouns.

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

As nouns, bed of roses and rose bed are synonyms defined as:

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