old witchgrass vs witchgrass

old witchgrass and witchgrass both are nouns.

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

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

old witchgrass and witchgrass: 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, witch grass.

old witchgrass (noun)	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