poisonous plant vs plant life

poisonous plant and plant life both are nouns.

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

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

poisonous plant: a plant that when touched or ingested in sufficient quantity can be harmful or fatal to an organism
plant life: (botany) a living organism lacking the power of locomotion

Other hypernyms of poisonous plant include flora, plant.

poisonous plant (noun)	plant life (noun)
a plant that when touched or ingested in sufficient quantity can be harmful or fatal to an organism	(botany) a living organism lacking the power of locomotion