gardenia vs shrub

gardenia and shrub both are nouns.

Word	Noun	Adjective	Verb	Adverb
gardenia	Yes	No	No	No
shrub	Yes	No	No	No

As nouns, shrub is a hypernym of gardenia; that is, shrub is a word with a broader meaning than gardenia:

gardenia: any of various shrubs and small trees of the genus Gardenia having large fragrant white or yellow flowers
shrub: a low woody perennial plant usually having several major stems

Other hypernyms of gardenia include bush.

gardenia (noun)	shrub (noun)
any of various shrubs and small trees of the genus Gardenia having large fragrant white or yellow flowers	a low woody perennial plant usually having several major stems