Floating manna grass

from 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]