rush-grass vs rush grass

rush-grass and rush grass both are nouns.

Word	Noun	Adjective	Verb	Adverb
rush-grass	Yes	No	No	No
rush grass	Yes	No	No	No

As nouns, rush-grass and rush grass are synonyms defined as:

rush-grass and rush grass: grass having wiry stems and sheathed panicles

rush-grass (noun)	rush grass (noun)
grass having wiry stems and sheathed panicles	grass having wiry stems and sheathed panicles