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