witch grass vs old witchgrass

witch grass and old witchgrass both are nouns.

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

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

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

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

witch grass (noun)	old witchgrass (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