dog mercury vs herb

dog mercury and herb both are nouns.

Word	Noun	Adjective	Verb	Adverb
dog mercury	Yes	No	No	No
herb	Yes	No	No	No

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

dog mercury: European perennial weedy plant with greenish flowers
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 dog mercury include herbaceous plant.

dog mercury (noun)	herb (noun)
European perennial weedy plant with greenish flowers	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