Source : Webster's Revised Unabridged Dictionary (1913)

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

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

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

Source : WordNet®

mellowing
     n : the process of becoming mellow

mellowing
     adj : having or suggesting softness and richness in quality; "a
           mellow sound"; "the mellow air brought in the feel of
           imminent autumn"- Thomas Hardy; "a mellowing sun" [syn:
            {mellow}]