water fennel vs poisonous plant

water fennel and poisonous plant both are nouns.

Word	Noun	Adjective	Verb	Adverb
water fennel	Yes	No	No	No
poisonous plant	Yes	No	No	No

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

water fennel: European poisonous herb with fibrous roots
poisonous plant: a plant that when touched or ingested in sufficient quantity can be harmful or fatal to an organism

water fennel (noun)	poisonous plant (noun)
European poisonous herb with fibrous roots	a plant that when touched or ingested in sufficient quantity can be harmful or fatal to an organism