sweet grass vs meadowgrass

sweet grass and meadowgrass both are nouns.

Word	Noun	Adjective	Verb	Adverb
sweet grass	Yes	No	No	No
meadowgrass	Yes	No	No	No

As nouns, meadowgrass is a hypernym of sweet grass; that is, meadowgrass is a word with a broader meaning than sweet grass:

sweet grass: any of several moisture-loving grasses of the genus Glyceria having sweet flavor or odor
meadowgrass: any of various grasses that thrive in the presence of abundant moisture

Other hypernyms of sweet grass include meadow grass.

sweet grass (noun)	meadowgrass (noun)
any of several moisture-loving grasses of the genus Glyceria having sweet flavor or odor	any of various grasses that thrive in the presence of abundant moisture