Arrhenatherum vs tall oat grass

Arrhenatherum and tall oat grass both are nouns.

Word	Noun	Adjective	Verb	Adverb
Arrhenatherum	Yes	No	No	No
tall oat grass	Yes	No	No	No

Arrhenatherum (noun)	tall oat grass (noun)
oat grass	coarse perennial Eurasian grass resembling oat; found on roadside verges and rough grassland and in hay meadows; introduced in North America for forage