Najas flexilis

Tracheophyta MagnoliopsidaNajadaceaeNajasNajas flexilis


This aquatic plant is an annual which is usually found in deep, clear, mesotrophic lakes where the water receives some base-enrichment from nearby basalt, limestone or calcareous dune-sand. Lowland.



World Distribution

Circumpolar Boreal-montane element, with a disjunct distribution.

Broad Habitats

Standing water and canals

Light (Ellenberg): 6

Moisture (Ellenberg): 12

Reaction (Ellenberg): 7

Nitrogen (Ellenberg): 4


Salt Tolerance (Ellenberg): 1

January Mean Temperature (Celsius): 4.1

July Mean Temperature (Celsius): 13.8

Annual Precipitation (mm): 1338

Length: 30

Perennation - primary


Life Form - primary

Annual hydrophyte (aquatic therophyte)



Clonality - primary

Little or no vegetative spread

Count of 10km squares in Great Britain: 28

Count of 10km squares in Ireland: 28

Count of 10km squares in the Channel Isles: 0

Atlas Change Index: 0.48

Distribution information

JNCC Designations


Scarce Atlas Account

Scarce Atlas Account: 

Najas flexilis (Willd.) Rostkov & W. Schmidt

Slender naiad

Status: rare

WCA Schedule 8 species


N. flexilis is usually found over silty substrates in clear water in mesotrophic lowland lakes. It often occurs in sites which have slight base-enrichment from nearby basalt or limestone rock outcrops, or in machair lochs adjacent to calcareous dune sand. It is occasionally found in water less than 1 metre deep, as at Loch Ballyhaugh, where it grows with Potamogeton rutilus in a bed of Chara aspera. At most sites it grows in water over 1.5 metres deep, where associates can include Callitriche hermaphroditica, Potamogeton praelongus and the charophyte Nitella flexilis

N. flexilis is an annual which reproduces by seed, Male and female flowers are found on the same plant. Pollination occurs under water and most mature plants appear to set seed.

N. flexilis was first discovered in Scotland in 1872 and at its only known English site, Esthwaite Water, in 1914. It is difficult to find as it grows out of sight of a shore-based observer, and it can usually be reached with a grapnel only where the water shelves rapidly. The number of sites from which it has been recorded has increased gradually in recent years, and further localities almost certainly await discovery. It has, however, become extinct at some sites in eastern Scotland as a result of eutrophication, which must be the most serious threat to this species. Although the more remote sites in western Scotland are less likely to receive enrichment from agricultural fertilisers, at least one is threatened by the presence of a fish farm.

N. flexilis has a northern distribution in Europe, extending south to Switzerland. It is rare throughout its European range. It is a circumboreal species, also found in northern Asia and North America. For a map of its world distribution, see Hulten & Fries (1986).

Fossil evidence indicates that N. flexilis was much more widespread in Britain and Europe in the Late Glacial and early Postglacial than it is today (Godwin 1975). As it has only been recorded from 15 British 10 km squares since 1970, it qualifies for inclusion in the Red Data Book under the current criteria. It is one of the species listed in Appendix I of the Bern Convention and Annexes II and IV of the European Community Habitats & Species Directive; the governments of the member states of the Council of Europe and the EC have an obligation to protect it.


C. D. Preston

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

Atlas (311a)
The Irish Red Data Book. 1. Vascular Plants,
Curtis, T. G. F., and McGough H. N.
, Dublin, (1988)

Atlas of north European vascular plants north of the Tropic of Cancer. 3 vols,
Hultén, E., and Fries M.
, Königstein, (1986)

Vergleichende Chorologie der zentraleuropäischen Flora. Volume 1. 2 vols,
Meusel, H., Jäger E., and Weinert E.
, Jena, (1965)

Aquatic plants in Britain and Ireland,
Preston, C. D., and Croft J. M.
, Colchester, (1997)

Scarce plants in Britain,
Stewart, A., Pearman D. A., and Preston C. D.
, Peterborough, (1994)