French sorrel vs sour grass

French sorrel and sour grass both are nouns.

Word	Noun	Adjective	Verb	Adverb
French sorrel	Yes	No	No	No
sour grass	Yes	No	No	No

As nouns, sour grass is a hypernym of French sorrel; that is, sour grass is a word with a broader meaning than French sorrel:

French sorrel: low perennial with small silvery-green ovate to hastate leaves
sour grass: any of certain coarse weedy plants with long taproots, sometimes used as table greens or in folk medicine

Other hypernyms of French sorrel include dock, sorrel.

French sorrel (noun)	sour grass (noun)
greens having small tart oval to pointed leaves; preferred to common sorrel for salads	any of certain coarse weedy plants with long taproots, sometimes used as table greens or in folk medicine
low perennial with small silvery-green ovate to hastate leaves