undergrowth vs underbrush

undergrowth and underbrush both are nouns.

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

As nouns, undergrowth and underbrush are synonyms defined as:

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

Other synonyms of undergrowth include underwood.

undergrowth (noun)	underbrush (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