Source : Webster's Revised Unabridged Dictionary (1913)

Mulch \Mulch\, n. [Cf. mull dirt, also Prov. G. mulsch, molsch,
   rotten, soft, mellow, as fruit.]
   Half-rotten straw, or any like substance strewn on the
   ground, as over the roots of plants, to protect from heat,
   drought, etc., and to preserve moisture.

Mulch \Mulch\, v. t. [imp. & p. p. {Mulched}; p. pr. & vb. n.
   {Mulching}.]
   To cover or dress with mulch.

Source : WordNet®

mulch
     n : a protective covering of rotting vegetable matter spread to
         reduce evaporation and soil erosion
     v : cover with mulch; "mulch the flowerbeds"