bluegrass vs June grass

bluegrass and June grass both are nouns.

Word	Noun	Adjective	Verb	Adverb
bluegrass	Yes	No	No	No
June grass	Yes	No	No	No

As nouns, June grass is a hyponym of bluegrass; that is, June grass is a word with a more specific, narrower meaning than bluegrass:

bluegrass: any of various grasses of the genus Poa
June grass: valuable meadow and pasture grass in Europe and especially central United States having tall stalks and slender bright green leaves; a chief constituent in lawn grass mixtures

Other hyponyms of bluegrass include Kentucky blue, Kentucky bluegrass, Kentucy blue grass, Poa pratensis.

bluegrass (noun)	June grass (noun)
a type of country music played at a rapid tempo on banjos and guitars	valuable meadow and pasture grass in Europe and especially central United States having tall stalks and slender bright green leaves; a chief constituent in lawn grass mixtures
any of various grasses of the genus Poa