pickerel weed vs water plant

pickerel weed and water plant both are nouns.

Word	Noun	Adjective	Verb	Adverb
pickerel weed	Yes	No	No	No
water plant	Yes	No	No	No

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

pickerel weed: American plant having spikes of blue flowers and growing in shallow water of streams and ponds
water plant: a plant that grows partly or wholly in water whether rooted in the mud, as a lotus, or floating without anchorage, as the water hyacinth

Other hypernyms of pickerel weed include aquatic plant, hydrophyte, hydrophytic plant.

pickerel weed (noun)	water plant (noun)
American plant having spikes of blue flowers and growing in shallow water of streams and ponds	a plant that grows partly or wholly in water whether rooted in the mud, as a lotus, or floating without anchorage, as the water hyacinth