undershrub vs shrub

undershrub and shrub both are nouns.

Word	Noun	Adjective	Verb	Adverb
undershrub	Yes	No	No	No
shrub	Yes	No	No	No

As nouns, shrub is a hypernym of undershrub; that is, shrub is a word with a broader meaning than undershrub:

Other hypernyms of undershrub include bush.

undershrub (noun)	shrub (noun)
a low shrub	a low woody perennial plant usually having several major stems