ryegrass vs rye grass

ryegrass and rye grass both are nouns.

Word	Noun	Adjective	Verb	Adverb
ryegrass	Yes	No	No	No
rye grass	Yes	No	No	No

As nouns, ryegrass and rye grass are synonyms defined as:

ryegrass and rye grass: any of several annual or perennial Eurasian grasses

ryegrass (noun)	rye grass (noun)
any of several annual or perennial Eurasian grasses	any of several annual or perennial Eurasian grasses