witchgrass vs old witch grass

witchgrass and old witch grass both are nouns.

Word	Noun	Adjective	Verb	Adverb
witchgrass	Yes	No	No	No
old witch grass	Yes	No	No	No

As nouns, witchgrass and old witch grass are synonyms defined as:

witchgrass and old witch grass: North American grass with slender brushy panicles; often a weed on cultivated land

Other synonyms of witchgrass include old witchgrass, Panicum capillare, tumble grass, witch grass.

witchgrass (noun)	old witch grass (noun)
North American grass with slender brushy panicles; often a weed on cultivated land	North American grass with slender brushy panicles; often a weed on cultivated land
European grass spreading rapidly by creeping rhizomes; naturalized in North America as a weed