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]