A prostrate perennial herb of sand, shingle or shell beaches, growing above the limit of the highest tides with other strand-line and foredune plants. Lowland.
The 1962 Atlas recorded this species from just four post-1930 sites in S.W. England (W. Cornwall, N. Devon and the Channel Islands). Records during the 1990s have considerably extended the known range. Its recent spread in England correlates with a run of mild winters and hot summers. In Ireland it persists at the site where it was discovered in 1973.
Light (Ellenberg): 9
Moisture (Ellenberg): 3
Reaction (Ellenberg): 5
Nitrogen (Ellenberg): 4
Salt Tolerance (Ellenberg): 3
January Mean Temperature (Celsius): 6
July Mean Temperature (Celsius): 16.2
Annual Precipitation (mm): 950
Height (cm): 20
Perennation - primary
Life Form - primary
Comment on Life Form
Clonality - primary
Count of 10km squares in Great Britain: 16
Count of 10km squares in Ireland: 1
Count of 10km squares in the Channel Isles: 4
Atlas Change Index: 0.21
Plantatt Conservation Status
RDB Species Accounts
Polygonum maritimum L. (Polygonaceae)
Sea knotgrass, Canclwm Arfor
Status in Britain: ENDANGERED. WCA Schedule 8.
Status in Europe: Not threatened.
This species occurs on beaches above the high spring-tide mark in a generally very open community, mainly on fine shingle and coarse sand. In some places it may occur without close associates, but in others it accompanies typical plants of this habitat, including Atriplex glabriuscula, A. laciniata, Beta vulgaris, Cakile maritima, Calystegia soldanella, Catapodium marinum, Glaucium flavum, Honkenya peploides, Polygonum oxyspermum ssp. raii, Rumex crispus, Salsola kali, Senecio vulgaris and Tripleurospermum maritimum.
P. maritimum is a procumbent perennial. The stems become woody with age, but in mainland Britain, most plants do not survive long enough to become large and woody. Flowering extends mainly from June to October, though a few flowers may persist until late November. It seems likely that seed can remain viable for many years, as shown by the reappearance of plants in known sites after long absences.
This species has apparently always been rare in Britain, often with a pattern of sporadic occurrences followed by periods when it appears to be absent (e.g. Clapham, et al. 1987). It has occurred only on the south coast of England, having been first recorded at Christchurch in 1836 by W. Borrer. It has made brief appearances in Devon and Somerset, but currently seems to be absent in these counties, as also in the Isles of Scilly, where it was first recorded in 1852. Populations are currently extant in Cornwall, Dorset (in vice-county 11), Hampshire, the Isle of Wight, West Sussex and East Sussex (Harmes & Spiers 1993). The largest recent counts are at Gunwalloe (250 plants in 1993), and at Hengistbury Head (144 plants in 1993, though only fifteen in 1996). Most populations are, however, much smaller. Only fourteen plants were recorded, for example, in East Sussex in 1993, and fifteen plants at Sandy Point, Hayling Island in 1995.
The fate of most populations seems likely to depend largely on natural events such as storms shifting beach material. For instance, at Hengistbury Head, where P. maritimum reappeared in 1990 after an absence of 88 years, it spread to a beach built up by storms only two years before. Conversely, it was not seen in 1995 at Lantic Beach, Cornwall, though it could reappear there. However, human activity may be significant, for example, in East Sussex where plants appeared on a beach created by recent 'engineering' works. Accidental uprooting of plants on popular beaches such as Gunwalloe and Brighton is likely, though this may constitute only a minor threat. On the other hand, over-enthusiastic beach maintenance by local authorities could be much more damaging.
P. maritimum is found throughout Europe on Atlantic, Mediterranean and Black Sea coasts, extending northwards to the Channel Islands, England and Belgium.
R. M. Walls and M. J. Wigginton
Atlas text references
1990. Flora dels Països Catalans, II. Crucíferes-Amarantàcies.
1988. The Irish Red Data Book. 1. Vascular Plants.
1974. Polygonum maritimum L. new to Ireland. Irish Naturalists’ Journal. 18:95.
1993. Polygonum maritimum L. in East Sussex (v.c. 14). Watsonia. 19:271-273.
Jalas & Suominen (1979)
1981. Docks and knotweeds of the British Isles. Botanical Society of the British Isles Handbook no. 3.
1999. British Red Data Books. 1. Vascular plants, edn 3.