Madagascar periwinkle vs herb

Madagascar periwinkle and herb both are nouns.

Word	Noun	Adjective	Verb	Adverb
Madagascar periwinkle	Yes	No	No	No
herb	Yes	No	No	No

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

Madagascar periwinkle: commonly cultivated Old World woody herb having large pinkish to red 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 Madagascar periwinkle include herbaceous plant.

Madagascar periwinkle (noun)	herb (noun)
commonly cultivated Old World woody herb having large pinkish to red 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