The Collaborative International Dictionary of English v.0.48:

Knotgrass \Knot"grass`\, n. (Bot.)
   (a) a common weed with jointed stems (Polygonum aviculare);
       knotweed.
   (b) The dog grass. See under Dog.
       [1913 Webster]

   Note: An infusion of Polygonum aviculare was once supposed
         to have the effect of stopping the growth of an animal,
         and hence it was called, as by Shakespeare, "hindering
         knotgrass."
         [1913 Webster]

               We want a boy extremely for this function,
               Kept under for a year with milk and knotgrass.
                                                  --Beau. & Fl.
         [1913 Webster]

WordNet (r) 3.0 (2006):

knotgrass
    n 1: low-growing weedy grass with spikelets along the leaf stems
         [syn: knotgrass, Paspalum distichum]