From 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]