couch grass vs wheatgrass

couch grass and wheatgrass both are nouns.

Word	Noun	Adjective	Verb	Adverb
couch grass	Yes	No	No	No
wheatgrass	Yes	No	No	No

As nouns, wheatgrass is a hypernym of couch grass; that is, wheatgrass 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
wheatgrass: a grass of the genus Agropyron

Other hypernyms of couch grass include wheat-grass.

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