Potamogeton epihydrus is a perennial aquatic plant known by the common name ribbonleaf pondweed, Nuttall's pondweed (not to be confused with Elodea nuttallii) or in Britain and Ireland American pondweed. It is native to much of North America, where it grows in water bodies such as ponds, lakes, ditches, and slow-moving streams. Ribbonleaf pondweed is predominantly a North American species, and is widespread in boreal and temperate North America. Its centre of distribution is the northeastern USA and southeastern Canada from the Atlantic to the Great Lakes, and on the Pacific seaboard from northern California to British Columbia. However, there are scattered populations in Alabama and Louisiana, Alaska, Wyoming, Montana and central Canada. There are also two isolated populations in Britain, where it was identified as recently as 1944. Ribbonleaf pondweed generally grows in shallow, standing to slow-flowing standing and running waters at up to 1900 m altitude. It tolerates acid waters as low as pH 5 and has a preference for oligotrophic, soft water conditions. Nevertheless, liming experiments suggest that ribbonleaf pondweed is sensitive to severe acidification. P. epihydrus is a poor disperser relative to many other aquatic plants occurring in Connecticut lakes. Ribbonleaf pondweed is generally common and widespread in North America. However, it is listed as Endangered in Indiana and Special Concern in Tennessee. In Britain it is Nationally Rare and listed as Vulnerable. British populations lack detectable genetic variation, suggesting a strong founder effect. P. epihydrus is not in cultivation, but would be worth attempting to grow. Its fairly small size, ribbon-like underwater leaves and scattered floating leaves could be effectively used in a garden pond, tub or stream. Introduced populations in Britain have not proved invasive, so there is little chance of it becoming problematic. Like other pondweeds, it needs to be planted with the root in contact with a suitable substrate such as aquatic compost.