couch grass vs wheat-grass

couch grass and wheat-grass both are nouns.

Word	Noun	Adjective	Verb	Adverb
couch grass	Yes	No	No	No
wheat-grass	Yes	No	No	No

As nouns, wheat-grass is a hypernym of couch grass; that is, wheat-grass is a word with a broader meaning than couch grass:

couch grass: European grass spreading rapidly by creeping rhizomes; naturalized in North America as a weed
wheat-grass: a grass of the genus Agropyron

Other hypernyms of couch grass include wheatgrass.

couch grass (noun)	wheat-grass (noun)
European grass spreading rapidly by creeping rhizomes; naturalized in North America as a weed	a grass of the genus Agropyron