garden plant vs plant life

garden plant and plant life both are nouns.

Word	Noun	Adjective	Verb	Adverb
garden plant	Yes	No	No	No
plant life	Yes	No	No	No

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

garden plant: any of a variety of plants usually grown especially in a flower or herb garden
plant life: (botany) a living organism lacking the power of locomotion

Other hypernyms of garden plant include flora, plant.

garden plant (noun)	plant life (noun)
any of a variety of plants usually grown especially in a flower or herb garden	(botany) a living organism lacking the power of locomotion