witch grass vs wheat-grass

witch grass and wheat-grass both are nouns.

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

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

witch 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 witch grass include wheatgrass.

witch grass (noun)	wheat-grass (noun)
North American grass with slender brushy panicles; often a weed on cultivated land	a grass of the genus Agropyron
European grass spreading rapidly by creeping rhizomes; naturalized in North America as a weed