herb garden vs garden

herb garden and garden both are nouns.

Word	Noun	Adjective	Verb	Adverb
herb garden	Yes	No	No	No
garden	Yes	Yes	Yes	No

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

herb garden (noun)	garden (noun)
a garden for growing herbs	a plot of ground where plants are cultivated
	a yard or lawn adjoining a house
	the flowers or vegetables or fruits or herbs that are cultivated in a garden

herb garden (adjective)	garden (adjective)
	the usual or familiar type

herb garden (verb)	garden (verb)
	work in the garden