bed of roses vs bed of flowers

bed of roses and bed of flowers both are nouns.

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

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

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

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