underwood vs undergrowth

underwood and undergrowth both are nouns.

Word	Noun	Adjective	Verb	Adverb
underwood	Yes	No	No	No
undergrowth	Yes	No	No	No

As nouns, underwood and undergrowth are synonyms defined as:

underwood and undergrowth: the brush (small trees and bushes and ferns etc.) growing beneath taller trees in a wood or forest

Other synonyms of underwood include underbrush.

underwood (noun)	undergrowth (noun)
the brush (small trees and bushes and ferns etc.) growing beneath taller trees in a wood or forest	the brush (small trees and bushes and ferns etc.) growing beneath taller trees in a wood or forest