The Collaborative International Dictionary of English v.0.48:

overgrowth \o"ver*growth`\, n.
   1. Excessive growth.
      [1913 Webster]

   2. A growth that has covered somethin over.
      [PJC]

WordNet (r) 3.0 (2006):

overgrowth
    n 1: excessive size; usually caused by excessive secretion of
         growth hormone from the pituitary gland [syn: giantism,
         gigantism, overgrowth]
    2: a profusion of growth on or over something else

Moby Thesaurus II by Grady Ward, 1.0:

56 Moby Thesaurus words for "overgrowth":
   abandon, boundlessness, egregiousness, enormousness, exaggeration,
   excess, excessiveness, exorbitance, exorbitancy, extravagance,
   extravagancy, extreme, extremes, extremism, extremity,
   fabulousness, giantism, gigantism, gluttony, hyperbole,
   hypertrophy, immoderacy, immoderateness, immoderation,
   incontinence, inordinacy, inordinance, inordinateness,
   intemperance, intemperateness, inundation, monstrousness, nimiety,
   outrageousness, outsize, overdevelopment, overflowing,
   overgreatness, overindulgence, overlargeness, overmuch,
   overmuchness, overpass, overrun, overrunning, overspreading,
   overweight, radicalism, superiority, surplus, too much,
   too-muchness, unconscionableness, undueness, unreasonableness,
   unrestrainedness