ricegrass vs rice grass

ricegrass and rice grass both are nouns.

Word	Noun	Adjective	Verb	Adverb
ricegrass	Yes	No	No	No
rice grass	Yes	No	No	No

As nouns, ricegrass and rice grass are synonyms defined as:

ricegrass (noun)	rice grass (noun)
any grass of the genus Oryzopsis	any grass of the genus Oryzopsis