old witchgrass vs panic grass

old witchgrass and panic grass both are nouns.

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

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

old witchgrass: 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 witchgrass (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