quack grass vs wheat-grass

quack grass and wheat-grass both are nouns.

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

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

quack 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 quack grass include wheatgrass.

quack 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