Arabis scabra (Bristol Rock-cress)
A short-lived perennial herb of Carboniferous limestone, growing in shallow soils, on scree, and on rock ledges. It spreads by seed. Lowland.
A. scabra has been known in the Avon Gorge since 1686. Although invasion of scrub and wire-netting to prevent rock falls have eliminated some sites, and reduced the open ground needed for germination, populations are still healthy. Introductions elsewhere have proved to be short-lived failures, except those at Combwich (S. Somerset).
Count of 10km squares in Great Britain: 1
Count of 10km squares in Ireland: 0
Count of 10km squares in the Channel Isles: 0
RDB Species Accounts
Arabis scabra All. (Brassicaceae)
Arabis stricta Huds.
Status in Britain: VULNERABLE. WCA Schedule 8.
Status in Europe: Not threatened.
A. scabra is a plant of highly calcareous soils derived from Carboniferous limestone. It grows on very shallow soil in open turf, on loose rubble and scree, and directly on the limestone rock, where it roots into crevices. It is intolerant of competition, and cannot establish itself from seed in closed turf. A. scabra accumulates strontium in its leaves, and it may be favoured by strontium-rich soils (Bowen & Dymond 1955). In the Avon Gorge it is usually found on south-facing rocky slopes and crags, though some plants occur low down near tide level. Nearby at Shirehampton it grows in two disused quarries. Its capacity to withstand drought enables it to survive in open sites which become very dry in summer. In open communities, where it grow best, it is often associated with such species as Bromopsis erecta, Catapodium rigidum, Festuca ovina, Helianthemum nummularium, Pilosella officinarum and Sanguisorba minor. It is able to survive in more enclosed areas where scrub has encroached (Cotoneaster species, Crataegus monogyna, Hedera helix, Quercus cerris) but will succumb if too densely shaded or overgrown.
It is a perennial species, flowering from late March to May, the seeds maturing in midsummer and shed from July onwards, even as late as October. Germination normally takes place in early autumn, the seedlings overwintering as small rosettes. The slightly protandrous flowers are highly self-fertile and a high proportion set seed. Though seeds are freely produced, they are not usually dispersed more than a few centimetres, and seedlings develop in a cluster around the parent plant. However, most seedlings do not survive, and mature plants typically occur singly or in very small groups. There is little or no vegetative spread.
This species has long been known in Britain, having been first recorded in the Avon Gorge by John Ray in 1686 "in rupe S. Vincentii prope Bristolium", where it is still found. The largest populations are in the Avon Gorge, where it occurs in at least twelve sub-sites, on both sides of the river, in Somerset and Gloucestershire. Numbers appear to vary from year to year, and counts of the whole population between 1977 and 1989 ranged from about 500 to 5,400 individuals (Taylor 1990f). There has been a serious decline in some areas because of scrub growth and rock stabilisation work, but several thousand individuals occur in the Gorge at the present time. The Shirehampton population is much smaller: in 1989, only four sub-populations with a total of 146 individuals were recorded (Taylor 1990f). Attempts in the 1950s to establish the plant in suitable habitats elsewhere in the district by sowing seed or planting rooted plants have been largely unsuccessful (Hope-Simpson 1987). An old record for Radnorshire is now regarded as unreliable.
Conservation management mainly entails the control of scrub which has vigorously encroached at some sites and is threatening some populations. Grazing would be an ideal means of control, but may not be practical in a largely urban situation. It has been suggested that trampling may be a problem, but the threat from this activity appears to be minimal; indeed, it may increase the amount of bare ground and marginal habitat favoured by the plant. Of prime concern to the local authorities is the prevention of rock-falls from the cliffs of the Avon Gorge which might endanger traffic below. This has involved the extensive netting of rock faces, which traps litter and leads to the accumulation of humus and the development of closed communities. This has adversely affected populations of A. scabra, which requires open ground for germination. Debris from shot-blasting the Clifton Suspension Bridge by copper smelter slag in 1995, leading to toxic levels of copper, zinc and lead in the Gorge, may have had some deleterious effect on the plant.
In continental Europe the species is rather rare, being recorded from mainly montane areas of eastern and southern France, Switzerland and Spain.
Further details are given in Pring (1961) and Thompson (1928).
M. J. Wigginton
PLANTATT - Attributes of British and Irish plants. (.zip 1455KB) This dataset was compiled and published in 2004, and last updated in November 2008. Download includes an Excel spreadsheet of the attributes, and a PDF explaining the background and nomenclature. Note that the PDF version is the booklet as published, whereas the Excel spreadsheet incorporates subsequent corrections. A hardcopy can be purchased from the Centre for Ecology and Hydrology.
Atlas text references
Jalas & Suominen (1994)
Vergleichende Chorologie der zentraleuropäischen Flora. Volume 1. 2 vols,
, Jena, (1965)
Biological Flora of the British Isles. No. 78. Arabis stricta Huds,
, Journal of Ecology, Volume 49, p.431-437, (1961)
Crucifers of Great Britain and Ireland. Botanical Society of the British Isles Handbook no. 6,
, London, (1991)
British Red Data Books. 1. Vascular plants, edn 3,
, Peterborough, (1999)