In the Outer Hebrides, this rhizomatous perennial grows in a few peaty lochans, in oligotrophic and base-poor water less than 1 m deep. It is also established in the mesotrophic Rochdale Canal and Calder & Hebble Navigation in N. England. Lowland.
This species was first discovered in England near Halifax (S.W. Yorks.) in 1907; although it must be an introduction there, the manner of its arrival from N. America is unclear. It was not found in the Outer Hebrides, where it is native, until 1943, and there is no evidence for any subsequent change in its distribution there.
Oceanic Boreo-temperate element; in Europe restricted to Britain but widespread in N. America.
Perennation - primary
Life Form - primary
Clonality - primary
Clonality - secondary
Count of 10km squares in Great Britain: 2
Count of 10km squares in Ireland: 0
Count of 10km squares in the Channel Isles: 0
Atlas Change Index: 0.11
RDB Species Accounts
Potamogeton epihydrus Raf. (Potamogetonaceae)
American pondweed, Lìobhag Aimeireaganach
Status in Britain: VULNERABLE.
Status in Europe: Occurs only in Britain
In the Outer Hebrides P. epihydrus grows in a few peaty lochans near Loch Ceann a'Bhaigh, where it is found in oligotrophic and base-poor water less than one metre deep. Other species in these lochans include Eleogiton fluitans, Equisetum fluviatile, the aquatic form of Juncus bulbosus, Littorella uniflora, Lobelia dortmanna, Nymphaea alba, Potamogeton natans, P. polygonifolius, Sparganium angustifolium and Utricularia vulgaris sensu lato. There is also a record from Loch an Duin. Submerged leaves which appear to belong to this species are washed up in late summer at the edge of a large, oligotrophic loch on Skye, but rooted plants have never been found here and the identification remains unconfirmed. However, P. epihydrus is locally plentiful in the mesotrophic water of the Rochdale Canal and the Calder & Hebble Navigation, where it grows with Elodea nuttallii, Lemna minor, L. trisulca, Luronium natans, Potamogeton crispus, Sparganium emersum and Nitella mucronata in water 0.4-1.2 metres deep. All the known sites are lowland.
P. epihydrus is a rhizomatous perennial. It has both submerged, linear leaves and broader floating leaves, although floating leaves may not develop on plants in relatively deep water. The species is self compatible and self-pollination occurs once the anthers dehisce if cross-pollination has not taken place (Philbrick, 1983). In the American population studied by Philbrick all ovules developed into fruits, and plants with floating leaves flower and fruit freely in both the Outer Hebrides and northern England. In addition, short stolons bearing buds or small fascicles of leaves can be found in the leaf axils in September. They are easily detached and act as vegetative propagules.
This species was originally discovered in Britain in 1907, at Salterhebble Bridge near Halifax (Bennett 1908). It is well-established in canals on both sides of the Pennines, but it has not been seen recently in the Calder, where it was collected in 1942. The English populations are believed to originate from an introduction, although it is difficult to imagine the source of the material. The native populations in South Uist were discovered by W.A.Clark and J.W.Heslop-Harrison in 1943 and 1944 (Heslop-Harrison 1949; 1950).
P. epihydrus is widespread in North America, where it is found in lakes, pools and streams from southern Alaska and Labrador south to northern California and Tennessee; it is restricted to mountains in the southern edge of its range (Fernald 1932). The British records are the only known occurrences in Europe.
There are similarities between this species and Luronium natans: both have native sites where the water is oligotrophic and also grow in mesotrophic canals. However, L. natans spread into the canal system from its native sites, whereas the canal populations of P. epihydrus presumably originated independently.
C. D. Preston, adapted from an account in Preston & Croft (1997)
PLANTATT - Attributes of British and Irish plants. (.zip 1455KB) This dataset was compiled and published in 2004, and last updated in November 2008. Download includes an Excel spreadsheet of the attributes, and a PDF explaining the background and nomenclature. Note that the PDF version is the booklet as published, whereas the Excel spreadsheet incorporates subsequent corrections. A hardcopy can be purchased from the Centre for Ecology and Hydrology.
Atlas text references
Pondweeds of Great Britain and Ireland. Botanical Society of the British Isles Handbook no. 8,
, London, (1995)
Aquatic plants in Britain and Ireland,
, Colchester, (1997)
British Red Data Books. 1. Vascular plants, edn 3,
, Peterborough, (1999)