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Polygonum maritimum (sea knotgrass)

Sea knotgrass is a coastal plant and is the rarest of the knotgrasses in Britain.
Flower and leaves of sea knotgrass

Polygonum maritimum (Photo: Sarah Griggs)

Species information

Common name: 

sea knotgrass

Conservation status: 



Sandy beaches

Key Uses: 

Dressing for burns. Potential as an indicator species for climate change.

Known hazards: 

Not known


Genus: Polygonum

About this species

This species belongs to the knotgrass family, Polygonaceae. The common name, knotgrass, refers to the swollen nodes of the stems which appear as joints or knots.


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Geographyand distribution

Polygonum maritimum is native to a few coastal sites in southern England, the Channel Islands and one locality in southern Ireland, but it is also found on the west coast of France southwards to the Mediterranean region, and Macaronesia.

Illustration of Polygonum maritimum, from Sowerby & Smith, English Botany.


Overview: The sea knotgrass is a perennial, with herbaceous weak stems that are woody below and grow up to 50 cm in length; the stems are procumbent.

Leaves: The leaves are somewhat oblong and leathery in texture with inrolled margins. The leaf stipules are a very conspicuous and characteristic part of the plant; they are silvery, forming a sheath at the base of leaves and cover part of the stem.

Flowers: The flowers are in inflorescences in the axils of leaves and are reddish or pink with five perianth lobes about 4 mm long.

Fruits: The fruit (achene) is dark brown, shiny and is longer than the perianth.

The flowering and fruiting period is July-September.

Threats and conservation

Polygonum maritimum is the rarest of the knotgrasses in Britain. It is sporadic along the coast of Cornwall, Devon, Hampshire (including the Isle of Wight) and the Channel Islands. Since the 1960s its range has extended in England, its recent spread correlated with a run of mild winters and hot summers.


The leaves of the sea knotgrass are used as a dressing for burns. Perhaps in view of the recent extension of its range in England which is reported to be correlated with changes in the climate, it could act as an indicator species for climate change.

Millennium Seed Bank: Seed storage

Kew's Millennium Seed Bank partnership aims to save plant life world wide, focusing on plants under threat and those of most use in the future. Seeds are dried, packaged and stored at a sub-zero temperature in our seed bank vault.

Number of seed collections stored in the Millennium Seed Bank: Six
Seed storage behaviour: Orthodox (the seeds of this plant survive being dried without significantly reducing their viability, and are therefore amenable to long-term frozen storage such as at the MSB)
Germination testing: Successful

References and credits

Clapham, A.R., Tutin, T.G. & Warburg, E.F. (1962). Flora of the British Isles. Cambridge University Press, Cambridge.

Preston, C.D., Pearman, D.A. & Dines, T.A. (eds) (2002). New Atlas of the British and Irish Flora: An atlas of the vascular plants of Britain, Ireland, the Isle of Man and the Channel Islands. Oxford University Press, Oxford.

Uphof, J.C. Th. (1959). Dictionary of Economic Plants. Engelmann, New York.

Kew Science Editor: Shahina A. Ghazanfar
Kew contributors: Sustainable Uses Group
Copy editing: Kew Publishing

While every effort has been taken to ensure that the information contained in these pages is reliable and complete, the notes on hazards, edibility and suchlike included here are recorded information and do not constitute recommendations. No responsibility will be taken for readers’ own actions.

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