Cotswold pennycress (Thlaspi perfoliatum)
|Size||Stem height: 5 - 25 cm|
Classified as Vulnerable in the UK, and protected under Schedule 8 of the Wildlife and Countryside Act, as amended 2002.
Perfoliate, or 'Cotswold' penny-cress is an unusual member of the cabbage family, with one or more smooth stems. It has a rosette of waxy grey-green leaves at the base of the plant. These leaves have a distinct stalk to them and are roughly oval in shape with scalloped edges. The leaves on the stem are un-stalked and much narrower in shape, the leaves partially encircling the stem. The flowers are white, about two millimetres in diameter and form a cluster at the top of the plant. As they ripen, they form heart-shaped seed pods below the flower cluster.
As its name suggests, this plant is very local in its range, being found at eight sites in the Cotswolds in Gloucestershire and Oxfordshire, but has also become established at other sites inside and outside this area, often being blown along railway lines by trains, or introduced with the limestone railway ballast. It is found throughout Europe apart from the far north, and its range extends into North Africa and the Near East. It has been introduced into North America.
Cotswold pennycress shows a marked preference for habitats found on the limestone characteristic of the Cotswold region in the south midlands of England. This type of limestone is termed oolitic, from the Greek for egg and stone, which describes the appearance of this rock under the microscope. Oolitic limestone dates from the Jurassic period, from 200 to 135 million years ago, and has been extensively quarried for building use.
This plant is a spring-flowering species, recorded as growing in pasture, scree, walls and in quarries. It grows as an annual and can over-winter. The flowers appear in April and May. It does not disperse its seeds very effectively, and in order to germinate successfully, the seeds need to land on recently disturbed ground. It also prefers conditions where the young plants are not over-shadowed by taller vegetation.
The increased use of herbicides threatens the future of the perfoliate pennycress, as does the removal of the walls, hedges and banks that it has traditionally used. Being a plant of marginal land, the clearance or neglect of this easily over-looked habitat also puts this species at risk.
This plant is one of the few to benefit from 'overgrazing', as this practise prevents the grass sward from closing over and reducing the pennycresses' opportunities to colonise bare patches.
Perfoliate pennycress is listed in the UK Biodiversity Action Plans and included in English Nature's Species Recovery Programme. The wild plant conservation charity, Plantlife, has also included this species in their 'Back from the Brink' project. As well as overseeing the management of four sites where it already occurs, a survey in 1992 provided information on the status of the pennycress and helped to identify sites where the plant could be re-introduced.
Because of the low dispersal rate of the wild plants, there is a danger that sites where it occurs become increasingly isolated from one-another, thereby reducing the opportunities for healthy populations to form. One way to overcome this risk (and many other species of plant and animal would benefit from management to overcome problems of isolation) would be to manage suitable 'corridors', whereby separate populations have the chance to cross-fertilise. This might well involve the development of railways and, where appropriate, the opening of new quarries or the continued use of existing sites.
An unusual side-effect from the management for the pennycress has been a request for the Countryside Stewardship scheme to allow 'overgrazing' where this plant is found. This runs counter to the usual specification for a Countryside Stewardship grant but has been included as the traditional level of disturbance by grazing cattle was insufficient for the plant to thrive.
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- Scree: small loose rock debris covering a slope.