old witch grass vs panic grass

old witch grass and panic grass both are nouns.

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

As nouns, panic grass is a hypernym of old witch grass; that is, panic grass is a word with a broader meaning than old witch grass:

old witch grass: North American grass with slender brushy panicles; often a weed on cultivated land
panic grass: any grass of the genus Panicum; grown for grain and fodder

old witch grass (noun)	panic grass (noun)
North American grass with slender brushy panicles; often a weed on cultivated land	any grass of the genus Panicum; grown for grain and fodder