rose bed vs bed of roses

rose bed and bed of roses both are nouns.

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

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

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