garden plant vs plant life

garden plant and plant life both are nouns.

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 plant, flora.

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