pickerelweed vs water plant

pickerelweed and water plant both are nouns.

Word	Noun	Adjective	Verb	Adverb
pickerelweed	Yes	No	No	No
water plant	Yes	No	No	No

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

pickerelweed: 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 pickerelweed include aquatic plant, hydrophyte, hydrophytic plant.

pickerelweed (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