Potamogeton nodosus

Potamogeton nodosus Poiret (Potamogetonaceae)

Loddon pondweed, Dyfrllys Rhwydog

Status in Britain: LOWER RISK - Near Threatened.

Status in Europe: Not threatened.

This species is confined to a few calcareous and moderately but not excessively eutrophic rivers in lowland England. It is found in both shallow and relatively deep water, growing in stretches of moderately rapid flow with species such as Elodea nuttallii, Lemna minor, Myriophyllum spicatum, Nuphar lutea, Potamogeton crispus, P. pectinatus, Sagittaria sagittifolia, Schoenoplectus lacustris and Sparganium emersum. In the Loddon it is often particularly abundant in well aerated stretches below weirs and sluices (Archer 1987). In Dorset it grows in fairly shallow water over a gravelly substrate, and cannot be found where there are soft clay sediments.

P. nodosus is a rhizomatous perennial which seems to vary in abundance from year to year. It flowers freely in summer when growing in shallow water. Plants do not appear to set fruit in the wild but will do so if cultivated in small containers, suggesting that the apparent sterility is caused by environmental factors. Buds develop on short stolons in the leaf axils in late summer and act as vegetative propagules. Established populations overwinter as buds on the rhizome, which develop in response to short days (Spencer & Anderson 1987). In southern Europe this species grows in a much wider range of habitats than it does in England, and fruits freely.

This species was first discovered in England in the Loddon by G.C.Druce in 1893 and named P. drucei by Fryer in 1898 (Preston 1988). It was subsequently discovered in the Bristol Avon, Stour (Dorset) and Thames. However, its correct identity continued to puzzle British botanists until Dandy & Taylor (1939) established that it was the widespread continental species P. nodosus. The species has apparently become extinct in the Thames, where it was once locally frequent (Lousley 1944a) and perhaps disappeared as a result of eutrophication and increasing boat traffic in the 1950s. Recent reports of its rediscovery in the Thames have not been confirmed. It cannot now be found in the Warwickshire Stour at Alderminster, where it is known from a single 19th century specimen. However, it survives as vigorous stands in the Loddon and the Dorset Stour, and has recently been planted at additional sites in the Loddon and the nearby Whitewater and Blackwater. 

This is the most frequent broad-leaved Potamogeton in the Mediterranean region, and extends northwards in Europe to England, the Netherlands and Estonia. It also occurs in Africa south to Angola and Madagascar, temperate and tropical Asia, and North and South America.

C. D. Preston, adapted from an account in Preston & Croft (1997).