old witchgrass vs old witch grass

old witchgrass and old witch grass both are nouns.

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

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

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

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

old 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