old witch grass vs witch grass

old witch grass and witch grass both are nouns.

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

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

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

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

old witch grass (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