Tower mustard (Arabis glabra)

SizeHeight: 30-100 cm

Classified as Vulnerable in the UK.

Tower mustard is a very distinct plant which flowers from May to June. It has an unbranched stem, sometimes tinged with violet, and is slightly hairy towards the base. The base rosette of leaves resemble a dandelion and the flower petals are pale yellow.

Widespread in Europe and across Asia up to a latitude of 70 degrees north, in the UK tower mustard is declining in numbers as a native species. Since 1980 it has only been recorded from some 30 sites in the Breckland regions of Norfolk and Suffolk, Berkshire, Buckinghamshire, Gloucestershire, Hampshire, Middlesex, Surrey, Wiltshire and Worcestershire. There are some records from Wales but none within the last 20 years.

Tower mustard prefers free-draining, sandy soils in grassy and waste places over chalk or limestone. Described as an 'opportunistic species', it seems able to colonise areas which are extensively grazed and occasionally managed as arable land, such as were traditionally found in the Breckland region of East Anglia. It is also found in conifer plantations which are being clear-felled.

This plant is considered to be biennial or, sometimes, a short-lived perennial. Plants germinate in spring and spend at least one season in a vegetative state before flowering.

Tower mustard can produce large quantities of seed which seem to be capable of lying dormant in the soil for years before germinating. This has led to appearances of the plant on sites after long periods of absence.

The factors contributing to the plant's disappearance, include habitat neglect, over-grazing by rabbits and building development. The greatest factor, however, seems to have been the loss of open habitat on heathland or rough sandy terrain in South-east England. This has been caused by agricultural intensification, when areas of 'waste' land have been put under crops. In the plant's main stronghold of the East Anglian Breckland, much of the habitat that would have supported the tower mustard has been put under the plough. On land formally considered unproductive, farmers have been able, using modern nitrate-based fertilisers, to produce regular crops throughout the year. Together with the use of herbicides, this has effectively reduced the species' numbers to dangerous levels.

In partnership with Plantlife's 'Back from the Brink' project, English Nature has placed the tower mustard under its Species Recovery Programme. There is also a UK Biodiversity Action Plan for the plant.

Part of the Action Plan recommended a survey of all the known sites and in 1999 this was carried out. The report's aim was to establish the true status of tower mustard, evaluate the current management of the existing sites and make recommendations for future regimes, and to make an ecological assessment and fill in the gaps about our knowledge of the plant and its requirements.

Actions at some of the sites have concentrated on the removal of scrub, together with increased grazing to reduce the competition from more vigorous plant species, and create areas of disturbed ground. Seed from the plants has been stored and, when conditions at known former sites are more favourable, this seed will be distributed over the site or germinated plants re-introduced.

It is also hoped that a reserve population of tower mustard plants will be established to provide a nursery stock of plants for re-introduction programmes, and from which, by further study, our knowledge of this species can be improved.

For more information on the tower mustard see:

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