underbrush vs undergrowth

underbrush and undergrowth both are nouns.

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

As nouns, underbrush and undergrowth are synonyms defined as:

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

Other synonyms of underbrush include underwood.

underbrush (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