old witchgrass vs witch grass

old witchgrass and witch grass both are nouns.

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

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

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

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

old witchgrass (noun)	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