root celery vs herb

root celery and herb both are nouns.

Word	Noun	Adjective	Verb	Adverb
root celery	Yes	No	No	No
herb	Yes	No	No	No

As nouns, herb is a hypernym of root celery; that is, herb is a word with a broader meaning than root celery:

root celery: grown for its thickened edible aromatic root
herb: a plant lacking a permanent woody stem; many are flowering garden plants or potherbs; some having medicinal properties; some are pests

Other hypernyms of root celery include herbaceous plant.

root celery (noun)	herb (noun)
grown for its thickened edible aromatic root	aromatic potherb used in cookery for its savory qualities
	a plant lacking a permanent woody stem; many are flowering garden plants or potherbs; some having medicinal properties; some are pests