The correct name for this species is Grasswrack Pondweed, Potamogeton compressus L., but the name P. compressus has, in the past, sometimes been used to refer to what is now called P. friesii, so care should be taken with literature records prior to about 1950. Older books often used the name P. zosterifolius Schumacher (Clapham, Tutin & Moore 1989).
Chromosome No.: 2n = 26 (Stace 2010).
Confusions about the identification of P. compressus were largely resolved by J.E. Dandy & Sir George Taylor in the mid-20th century, and definitive descriptions are given by Preston (1995). The keys in older works are unreliable. It is easy to confuse P. compressus with either P. friesii or P. acutifolius, and specimens should be collected whenever it is legal and responsible so to do.
Grasswrack Pondweed occurs throughout Europe and Asia as far east as Japan, according to reports on the internet, but not in North America (Preston & Croft 1997). In Britain it occurs in numerous sites in England, but in just one lake in Scotland and one canal in Wales. A report compiled from the Threatened Plants Database, listing its known sites in 2008 is available for downloading here: (pdf 246kb).
- Origin: native. It was first recorded in Britain by John Ray in the River Cam in Cambridge in 1660 (Crompton 2001).
- Rarity: Nationally Scarce in Britain (Stewart, Pearman & Preston 1994)
- Threat: in the New Atlas (Preston, Pearman & Dines 2002) it is described as having declined over the last 150 years, and it is currently listed as Endangered (Cheffings & Farrell 2005). However, analysis of the data on the TPDB shows that the decline is less severe than previously thought. It turns out that P. compressus moves about, creating the impression of a decline. It was slightly more abundant in the mid-20th century than before or afterwards, due to the abandonment of canals, but it has never been a common plant.
- Conservation: it is a Priority Species on the UK Biodiversity Action Plan. Most counties that have it consider it an axiophyte, as it only occurs in clear water.
Preston & Croft (1997) describe its habitat as still or slowly flowing, calcareous, mesotrophic water. It is found in rivers, canals, ditches and lakes in the lowlands. Rodwell (1995) lists it as occurring in A11 Potamogeton pectinatus- Myriophyllum spicatum vegetation.
Because of its ability to colonise new sites, it is important to continue to search for it and record it carefully. Studies into its ecology are needed, especially for the sites where it has persisted for many years, as these appear to be exceptionally rich and important sites for many other aquatic species. As a BAP species, it often receives ‘interventions’ of various sorts, including introductions and translocations. These should be documented on Conservation Evidence (see Markwell & Halls (2008) for an example).
- Crompton, G. 2001. Catalogue of Cambridge Herbarium and Flora of Cambridgeshire. www.mnlg.com/gc/ (accessed 2001).
- Markwell H.J. & Halls J.M. 2008. Translocation of a nationally scarce aquatic plant, grass-wrack pondweed Potamogeton compressus, at South Walsham Marshes, Norfolk, England. Conservation Evidence 5, 69-73.
- Preston, C.D. 1995. Pondweeds of Great Britain and Ireland. Botanical Society of the British Isles, London.
- Preston, C.D. & Croft, J.M. 1997. Aquatic Plants in Britain and Ireland. Harley Books, Colchester.