Potamogeton compressus is a species of aquatic plant known by the common names grass-wrack pondweed, flatstem pondweed and eel-grass pondweed. Potamogeton compressus is native to northern Europe (Austria, Britain, Denmark, Finland, France, Germany, Ireland, Netherlands, Poland, Russia, Spain, Sweden, Switzerland, Ukraine), Asia (China (Yunnan), Japan, Kazakhstan, Mongolia, Russia). In North America P. compressus / zosteriformis occurs in northern USA and Canada. There are isolated populations in the Balkans, Oregon and the Rockies. There is considerable undertainty regarding the exact distribution of this species in Siberia and Canada. Ecology and Grass-wrack pondweed grows in still or slow-flowing, lowland, calcareous and often rather nutrient-rich water bodies such as backwaters in rivers, ponds and slow-moving streams, usually on fine substrates such as sand, silt, clay or peat, usually in less than 1. 5 m water depth. However, Japanese populations have been reported growing in 5 m of water. It is also capable of colonising artificial habitats such as canals and drainage ditches, so long as these are not heavily boated. Grass-wrack pondweed is intolerant of turbid water and prefers some shade.