Source : Webster's Revised Unabridged Dictionary (1913)

Undergrowth \Un"der*growth`\, n.
   That which grows under trees; specifically, shrubs or small
   trees growing among large trees. --Milton.

Source : WordNet®

undergrowth
     n : the brush (small trees and bushes and ferns etc.) growing
         beneath taller trees in a wood or forest [syn: {underbrush},
          {underwood}]