Source : Webster's Revised Unabridged Dictionary (1913)

Undergrow \Un`der*grow"\, v. i.
   To grow to an inferior, or less than the usual, size or
   height. --Wyclif.

Undergrow \Un`der*grow"\, a.
   Undergrown. [Obs.] --Chaucer.

Source : WordNet®

undergrow
     v : grow below something; "The moss undergrew the stone patio"