A diminutive, autumn-germinating annual of barish ground often kept open by standing water in winter and always droughted in summer. It grows around serpentine rock outcrops, on ledges of granite sea-cliffs, in dune-slacks and sometimes in quarries. Lowland.
Populations vary greatly in size from year to year, and reports of extinction have sometimes proved to be premature, notably on Anglesey where it was re-discovered in 1995. The main threat to the Cornish sites is the cessation of grazing.
European Southern-temperate element; widely naturalised outside its native range.
Light (Ellenberg): 8
Moisture (Ellenberg): 6
Reaction (Ellenberg): 5
Nitrogen (Ellenberg): 1
Salt Tolerance (Ellenberg): 0
January Mean Temperature (Celsius): 6.3
July Mean Temperature (Celsius): 16.1
Annual Precipitation (mm): 889
Height (cm): 5
Perennation - primary
Life Form - primary
Clonality - primary
Count of 10km squares in Great Britain: 12
Count of 10km squares in Ireland: 0
Count of 10km squares in the Channel Isles: 10
RDB Species Accounts
Juncus capitatus Weigel (Juncaceae)
Dwarf rush, Corfrwenen
Status in Britain: LOWER RISK - Near Threatened.
Status in Europe: Not threatened.
In Britain, J. capitatus is now known only on the Lizard peninsula and at a single locality in Anglesey. It is typically a plant of open summer-droughted habitats, some of which may be wet or shallowly flooded in winter. On the Lizard it is more or less restricted to soils overlying serpentine, though there is a tiny sporadic colony associated with an outcrop of schist. Populations occur on south-facing coastal slopes (occasionally on steep sites), in short grassy turf particularly on and around rock outcrops, in serpentine rock-pans, occasionally on rock in abandoned quarries and on rock outcrops in heathland. In grassy turf, a rich assemblage of associates may include many notable species including Herniaria ciliolata, Isoetes histrix, Minuartia verna, Moenchia erecta, Trifolium bocconei, T. occidentale, T. ornithopodioides, T. strictum, T. subterraneum, and rare bryophytes including Riccia beyrichiana and R. nigrella. J. capitatus is found, though less frequently, in shallow rock pans on serpentine, in a distinctive community together with Allium schoenoprasum, Herniaria ciliolata, Isoetes histrix, Minuartia verna, Plantago maritima and Radiola linoides. These rock pans are fed in winter by seepages which wash out finer soil particles leaving a coarse gravelly substrate which dries out completely in summer. A few populations also occur on ledges around small abandoned quarries, and may mark the sites of former undisturbed habitat for this species. At the recently rediscovered site in Anglesey, J. capitatus occurs on the margins of a seasonally flooded sand-dune hollow in very open vegetation containing dune-heath species such as Erica cinerea, Lotus corniculatus and Salix repens (Blackstock & Jones 1997).
J. capitatus is a winter annual, germinating in autumn. Capsules can be found as early as late March, but normally mature in May, producing copious seed. Individual populations may fluctuate from year to year, plants being most abundant in warm wet springs, particularly following an intense summer drought the year before. Evidence suggests that dormant seed is likely to be viable for long periods in the soil (Coombe 1987a).
In a typical year there are many thousands of plants of J. capitatus on the Lizard, and more than a hundred separate populations have been recorded. Many of populations are small, some comprising fewer than ten individuals, though others may number many hundreds in good years. The Anglesey population discovered in 1995 was estimated to contain 10-20 plants. There are unreliable records of this species from other parts of Cornwall and the Hebrides, although it might once have occurred in the Isles of Scilly. In its vegetative state, it is very similar to J. bufonius, with which it has sometimes been confused. This species also occurs on the Channel Islands in habitats similar to those of the cove valleys of the Lizard.
On the Lizard, J. capitatus grows in areas of shallow soil and uneven topography which have therefore escaped agricultural reclamation, some now surviving as islands of semi-natural vegetation amidst intensively managed grassland. The continuity of grazing domestic stock is important for the survival of many of its grassland populations, and a cessation of grazing is likely to encourage more vigorous plant competitors in the sward. Losses in the past have been due to agricultural intensification, quarrying, afforestation and building development.
J. capitatus is found throughout most of Europe (including the Channel Islands), though it occurs sparingly through central Europe to southern Sweden, Finland and western Russia. It also occurs in Africa, south-west Asia, South America, Australia, and as a possible introduction in Newfoundland. In view of its wide distribution in Europe, its extreme restriction in western Britain is not easy to explain.
J. J. Hopkins