The Collaborative International Dictionary of English v.0.48:

meadowgrass \meadowgrass\, meadow grass \meadow grass\n.
   Any of various grasses that thrive in the presence of
   abundant moisture, especially those of the genus Poa,
   common in meadows, and of great value for hay and for
   pasture. See Grass.
   [1913 Webster + WordNet 1.5] meadowlark

WordNet (r) 3.0 (2006):

meadowgrass
    n 1: any of various grasses that thrive in the presence of
         abundant moisture [syn: meadowgrass, meadow grass]