The Collaborative International Dictionary of English v.0.48:

Mellow \Mel"low\, v. t. [imp. & p. p. Mellowed; p. pr. & vb.
   n. Mellowing.]
   To make mellow. --Shak.
   [1913 Webster]

         If the Weather prove frosty to mellow it [the ground],
         they do not plow it again till April.    --Mortimer.
   [1913 Webster]

         The fervor of early feeling is tempered and mellowed by
         the ripeness of age.                     --J. C.
                                                  Shairp.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

mellowing \mellowing\ adj.
   pr. p. of mellow.
   [WordNet 1.5]

The Collaborative International Dictionary of English v.0.48:

mellowing \mellowing\ n.
   The act or process of acquiring desirable qualities by being
   left undisturbed for some time.

   Syn: ripening, aging, ageing.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

mellowing
    n 1: the process of becoming mellow