Source : Webster's Revised Unabridged Dictionary (1913)

Water grass \Water grass\
      (a) A tall march perennial grass ({Paspalum dilatatum}) of
          the southern United States and the American tropics.
      (b) Manna grass.
      (c) The grass {Chloris elegans}.
      (d) [Dial. Eng.] (1) Velvet grass. (2) The water cress.
          (3) One of various horsetails.

Source : WordNet®

Paspalum dilatatum
     n : tall tufted perennial tropical American grass naturalized as
         pasture and forage grass in southern United States [syn:
         {dallisgrass}, {dallis grass}, {paspalum}]