The Collaborative International Dictionary of English v.0.48:

nutgrass \nut"grass`\, nut grass \nut" grass`\, n.(Bot.),
   A widely distributed perennial plant of the Sedge family
   (Cyperus rotundus, var. Hydra), which has slender
   rootstocks bearing small, edible, nutlike tubers, by which
   the plant multiplies exceedingly, especially in cotton
   fields.

   Syn: nutsedge, nut sedge, Cyperus rotundus.
        [1913 Webster + WordNet 1.5]

WordNet (r) 3.0 (2006):

nutgrass
    n 1: a widely distributed perennial sedge having small edible
         nutlike tubers [syn: nutgrass, nut grass, nutsedge,
         nut sedge, Cyperus rotundus]