tallgrass vs tall-grass

tallgrass and tall-grass both are nouns.

Word	Noun	Adjective	Verb	Adverb
tallgrass	Yes	No	No	No
tall-grass	Yes	No	No	No

As nouns, tallgrass and tall-grass are synonyms defined as:

tallgrass and tall-grass: any of various grasses that are tall and that flourish with abundant moisture

tallgrass (noun)	tall-grass (noun)
any of various grasses that are tall and that flourish with abundant moisture	any of various grasses that are tall and that flourish with abundant moisture