1. Family: Polygonaceae Juss.
    1. Genus: Polygonum L.
      1. Polygonum maritimum L.

        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.


    Kew Species Profiles

    General Description
    Sea knotgrass is a coastal plant and is the rarest of the knotgrasses in Britain.

    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.

    Species Profile
    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.


    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: SixSeed 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

    France, Ireland, United Kingdom
    Sandy beaches

    Not known

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



    Found In:

    Albania, Algeria, Azores, Baleares, Bulgaria, Canary Is., Cape Provinces, Corse, East Aegean Is., Egypt, Falkland Is., France, Great Britain, Greece, Ireland, Italy, Kriti, Krym, Libya, Madeira, Morocco, Netherlands, Portugal, Romania, Sardegna, Sicilia, Spain, Tunisia, Turkey-in-Europe, Yugoslavia

    Common Names

    Sea knotgrass

    Polygonum maritimum L. appears in other Kew resources:

    Date Identified Reference Herbarium Specimen Type Status
    Lewis, R. [540], Falkland Is. K000299181
    s.coll. [s.n.], Massachusetts K000831005

    First published in Sp. Pl.: 361 (1753)

    Accepted in:

    • [1] (2013) Index synonymique de la flore d'Afrique du nord 5: 1-451. Éditions des conservatoire et jardin botaniques, Genève
    • [2] Akeroyd, J.R. (2013) Docks and Knotweeds of Britain and Ireland . Botanical Society of Britain and Ireland
    • [3] (2011) Saussurea; Travaux de la Société Botanique de Genève 41: 131-170
    • [4] (2009) Lagascalia 29: 105-257
    • [5] (2005) Journal of the Torrey Botanical Society 132: 115-148
    • [6] (2004) Nouvelle flore de la Belgique du G. D. de Luxembourg, du Nord de la France et des régions voisines , ed. 5: 1-1167. Edition du Patrimoine du Jardin botanique national de Belgique
    • [7] (2003) Strelitzia 14: 1-1231. National Botanical Institute, Pretoria
    • [9] Boulos, L. (1999) Flora of Egypt 1: 1-419. Al Hadara Publishing, Cairo
    • [10] Tutin, T.G. & al. (eds.) (1993) Flora Europaea ed. 2, 1: 1-581. Cambridge University Press


    • [8] 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.
    • [11] Clapham, A.R., Tutin, T.G. & Warburg, E.F. (1962). Flora of the British Isles. Cambridge University Press, Cambridge.
    • [12] Uphof, J.C. Th. (1959). Dictionary of Economic Plants. Engelmann, New York.


    Kew Names and Taxonomic Backbone
    The International Plant Names Index and World Checklist of Selected Plant Families (2017). Published on the internet at http://www.ipni.org and http://apps.kew.org/wcsp
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    Kew Species Profiles
    Kew Species Profiles
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