The Collaborative International Dictionary of English v.0.48:

Quitch grass \Quitch" grass`\ (kw[i^]ch" gr[.a]s`). [Properly
   quick grass, being probably so called from its vigorous
   growth, or from its tenacity of life. See Quick, and cf.
   Couch grass.] (Bot.)
   A perennial grass (Agropyrum repens) having long running
   rootstalks, by which it spreads rapidly and pertinaciously,
   and so becomes a troublesome weed. Also called couch grass,
   quack grass, quick grass, twitch grass. See
   Illustration in Appendix.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Quack grass \Quack" grass`\ (Bot.)
   See Quitch grass.
   [1913 Webster]

WordNet (r) 3.0 (2006):

quack grass
    n 1: European grass spreading rapidly by creeping rhizomes;
         naturalized in North America as a weed [syn: dog grass,
         couch grass, quackgrass, quack grass, quick grass,
         witch grass, witchgrass, Agropyron repens]