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

Moby Thesaurus II by Grady Ward, 1.0:

116 Moby Thesaurus words for "overgrown":
   Gargantuan, a bit much, abandoned, abundant, amplitudinous,
   astronomical, awesome, blooming, blossoming, boundless, braky,
   brambly, budding, bulky, burgeoning, colossal, copsy, cosmic,
   covered, crescent, dense, developed, egregious, enormous,
   exaggerated, excessive, exorbitant, extensive, extravagant,
   extreme, exuberant, fabulous, fancy, florescent, flourishing,
   flowering, full-fledged, full-grown, fully developed, galactic,
   gigantic, gluttonous, gross, growing, grown, grown-up, heavy, high,
   huge, hyperbolic, hypertrophied, immeasurable, immense, immoderate,
   impenetrable, incontinent, infinite, inordinate, intemperate,
   jungled, jungly, king-size, large, lush, luxuriant, mammoth,
   massive, massy, mature, monster, monstrous, monumental,
   mountainous, out of bounds, out of sight, outrageous, outsize,
   outsized, overbig, overdeveloped, overfed, overgreat, overlarge,
   overmuch, overrun, oversized, overspread, overweening, overweight,
   prodigious, rank, riotous, sizable, spacious, sprouting, steep,
   stiff, stupendous, thick, thriving, titanic, too big, too much,
   tremendous, unbridled, unconscionable, undue, unreasonable,
   unrestrained, unweeded, vast, voluminous, weed-choked, weed-ridden,
   weedy, weighty