The Collaborative International Dictionary of English v.0.48:

Overgrow \O`ver*grow"\, v. t. [imp. Overgrew; p. p.
   Overgrown; p. pr. & vb. n. Overgrowing.]
   [1913 Webster]
   1. To grow over; to cover with growth or herbage, esp. that
      which is rank; as, dandelions have overgrown the lawn.
      [1913 Webster +PJC]

            The green . . . is rough and overgrown. --Sir W.
                                                  Scott.
      [1913 Webster]

   2. To grow beyond; to rise above; hence, to overcome; to
      oppress. [Obs.] --Mortimer. "O'ergrown with labor."
      --Beau. & Fl.
      [1913 Webster]

   Note: [Usually in the past participle.]
         [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

overgrown \overgrown\ adj.
   1. covered with growing plants.
      [WordNet 1.5]

   2. abounded in usually unwanted vegetation. [Narrower terms:
      wooded (vs. unwooded); weedy]
      [WordNet 1.5]

WordNet (r) 3.0 (2006):

overgrown
    adj 1: covered with growing plants
    2: abounding in usually unwanted vegetation