The Collaborative International Dictionary of English v.0.48:

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.
       [Webster 1913 Suppl.]

The Collaborative International Dictionary of English v.0.48:

dallis grass \dallis grass\, dallisgrass \dallisgrass\n.
   a tall tufted perennial tropical American grass (Paspalum
   dilatatum) naturalized as pasture and forage grass in the
   southern U.S.

   Syn: paspalum, Paspalum dilatatum.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

Paspalum dilatatum
    n 1: tall tufted perennial tropical American grass naturalized
         as pasture and forage grass in southern United States [syn:
         dallisgrass, dallis grass, paspalum, Paspalum
         dilatatum]