old witch grass vs old witchgrass

old witch grass and old witchgrass both are nouns.

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

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

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

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

old 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