Glyceria fluitans

Glyceria fluitans (syns Festuca fluitans, Poa fluitans, Panicularia fluitans), known as floating sweet-grass and water mannagrass, is a species of perennial grass in the genus Glyceria native to Europe, the Mediterranean region and Western Asia and occurring in wet areas such as ditches, riverbanks and ponds. It has a creeping rootstock, a thick stem which rises to one metre. The leaves are long, narrow and pale green, rough on both sides, often folded at the keel which lies on the surface of the water. The species epithet fluitans is Latin for "floating"....read more on Wikipedia.