The Collaborative International Dictionary of English v.0.48:

manna grass \manna grass\ n. (Bot.),
   Any of several tall slender grasses of the genus Glyceria.
   They have long loose panicles, have a sweet flavor or odor,
   and grow in moist places. Nerved manna grass is Glyceria
   nervata, and Floating manna grass is Glyceria fluitans.
   [1913 Webster + WordNet 1.5]

WordNet (r) 3.0 (2006):

manna grass
    n 1: any of several moisture-loving grasses of the genus
         Glyceria having sweet flavor or odor [syn: manna grass,
         sweet grass]