willowherb vs herbaceous plant

willowherb and herbaceous plant both are nouns.

Word	Noun	Adjective	Verb	Adverb
willowherb	Yes	No	No	No
herbaceous plant	Yes	No	No	No

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

willowherb: a plant of the genus Epilobium having pink or yellow flowers and seeds with silky hairs
herbaceous plant: a plant lacking a permanent woody stem; many are flowering garden plants or potherbs; some having medicinal properties; some are pests

Other hypernyms of willowherb include herb.

willowherb (noun)	herbaceous plant (noun)
a plant of the genus Epilobium having pink or yellow flowers and seeds with silky hairs	a plant lacking a permanent woody stem; many are flowering garden plants or potherbs; some having medicinal properties; some are pests