undergrow v 1: grow below something; "The moss undergrew the stone patio"
Undergrow \Un`der*grow"\, v. i. To grow to an inferior, or less than the usual, size or height. --Wyclif. [1913 Webster]
Undergrow \Un`der*grow"\, a. Undergrown. [Obs.] --Chaucer. [1913 Webster]