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Cornish Red Data (2009)

The descriptive text, below the map, is from the Cornish Red Data Book (2009). The map on this web page depicts the organisms distribution and shows the records made pre-2000 and those made since.

Polygonum maritimum - Sea Knotgrass

Range & Status

Native; a perennial, Mediterranean-Atlantic coastal plant. In Britain it grows in Cornwall and the Isles of Scilly, Channel Islands, East Sussex, Dorset, and Waterford (Ireland). Formerly recorded in Devon and Somerset.

Regional Distribution

At the northern limit of its range, Polygonum maritimum is erratic in its appearance in Cornwall. First recorded for the Isles of Scilly in 1852 (Davey, 1909) and later along Cornwall' s south coast, by 1939 it had been recorded in five 10km squares. Then, apart from one unconfirmed record for 1966, it disappeared and was not seen again until 1972, this time at Lantic Bay (SX15), then the only site in the British Isles. Further plants were found here over the next few years until it disappeared in 1982. After severe storms in 1990 a single plant appeared in a depression in the shingle at Lantic Bay and three plants at Gunwalloe Church Cove (SW62). At Gunwalloe Church Cove there are now hundreds of plants and the plant has since re-appeared at Lantic Bay, and at three other sites; including Poldhu Cove (SW61 and SW62), Par Beach (SX05) and Marazion (SW53). On

the Isles of Scilly the plant has been very erratic in appearance; the most recent records being from Bryher (2003) and St. Martin' s (2007).

Habitat & Ecology

A strand-line species, growing in fine shingle and sand above high-tide mark.


Vulnerable to trampling and tourist pressure on beaches. Can also be buried under layers of shingle and sand by storms, although conversely storms can also expose buried seed allowing it to germinate.


The sites at Gunwalloe Church Cove, Poldhu Cove, Lantic Bay and part of the Marazion shore are managed by the National Trust, and Par Beach is part of the Local Nature Reserve.

Click here to see Aphotoflora images by David Fenwick


I.J. Bennallick, S. Board, C.N. French, P.A. Gainey, C. Neil, R. Parslow, A. Spalding and P.E. Tompsett. eds. 2009. Red Data Book for Cornwall and the Isles of Scilly. 2nd Edition.Croceago Press.

The Cornish Red Data Book Project was led by the Cornwall and Isles of Scilly Federation for Biological Recorders (CISFBR). The full text and species accounts (minus the maps) are available on the CISFBR website.