witch grass vs wheatgrass

witch grass and wheatgrass both are nouns.

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

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

Other hypernyms of witch grass include wheat-grass.

witch grass (noun)	wheatgrass (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