Tower Mustard

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.

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.

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.

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.

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.

Classified as Vulnerable in the UK.

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.

Foodplant / parasite
Erysiphe cruciferarum parasitises live Arabis glabra

In Great Britain and/or Ireland:
Foodplant / saprobe
scattered, covered then bursting through a slit pycnidium of Phomopsis coelomycetous anamorph of Phomopsis cruciferae is saprobic on dead stalk of Arabis glabra

Herbs biennial, rarely perennial, (30-)40-120(-150) cm tall, sparsely to densely pilose basally with simple and short-stalked, forked trichomes, glabrous and glaucous above. Stems erect, simple basally, often branched above. Basal leaves rosulate, petiolate; leaf blade spatulate, oblanceolate, or oblong, (4-)5-12(-15) × 1-3 cm, pubescent or rarely glabrous, margin pinnatifid, sinuate, dentate, repand, or rarely entire, apex obtuse. Cauline leaves sessile, lanceolate, oblong-elliptic, or ovate, 2-9(-12) × (0.5-)1-2.5(-3.5) cm, base sagittate or auriculate, margin dentate or entire, apex acute. Fruiting pedicels erect, (0.6-)0.7-1.6(-2) cm, slender, appressed to rachis, glabrous. Sepals oblong or oblong-linear, (2.5-)3-5 × 0.5-1.2 mm, glabrous. Petals pale yellow, creamy white, or rarely pink, linear-oblanceolate, narrowly spatulate, or rarely linear, 5-8.5 × 1.3-1.7 mm. Filaments slender, lateral pair 2.5-4.5 mm, median pairs 3.5-6.5 mm; anthers narrowly oblong, 0.7-1.5 mm. Fruit linear, (3-)4-9(-10) cm × 0.7-1.5 mm, erect, appressed to rachis, subterete-quadrangular; style 0.5-0.8(-1) mm. Seeds brown, oblong or suborbicular, 0.6-1.2 × 0.5-0.9 mm. Fl. Apr-Jul, fr. May-Aug. 2n = 12, 16, 32.

Annual or biennial, 30-120 cm tall, simple, erect, glabrous, ± glaucous, sometimes turning violaceous in appearance. Rosette leaves 5-15 cm long, 1.5-3.5 cm broad, entire or toothed, stalked to subsessile, ± hairy with branched hairs rarely glabrous; cauline leaves 2-8 cm long, 0.5-3 cm broad, oblong ovate, sagittate-amplexicaul, entire, acute, subapressed, glaucous, glabrous, rarely margin sparsely hairy. Racemes many flowered, up to 30 cm long in fruit. Flowers c. 5 mm across, pale yellow, turning whitish when dried; pedicels up to 10 mm long in fruit, erect, subappressed. Sepals 2-4 mm long. Petals 4-6 mm long, 1.5 mm broad. Stamens 3 :4 mm long. Siliquae linear somewhat terete or sub-quadrangular, 30-90 mm long, c. 1.5 mm broad, erect, straight, glabrous, often subappressed; valves with a conspicuous mid-rib; seeds many, ± biseriate, c. 0.7 mm long, ovate-orbicular; septum with deep depressions.

Jiangsu, Liaoning, Shandong, Xinjiang, Zhejiang [Afghanistan, India, Japan, Kashmir, Kazakhstan, Korea, Kyrgyzstan, Mongolia, Nepal, Pakistan, Russia, Tajikistan, Turkmenistan, Uzbekistan; N Africa, SW Asia, Europe, North America; naturalized in Australia].

Distribution: Europe and Temperate Asia, introduced elsewhere.

Fl. Per.: May-August.

Mountain slopes, forest margins, valleys, fields, meadows, woods, fields, river banks, roadsides; 100-3500 m.

Arabis glabra (Linnaeus) Bernhardi; A. perfoliata Lamarck; A. pseudoturritis Boissier & Heldreich; Turritis glabra var. lilacina O. E. Schulz; T. pseudoturritis (Boissier & Heldreich) Velenovsky.

Arabis glabra, commonly known as tower rockcress or tower mustard, is a tall, slim, grey-green plant with small creamy flowers at the top of the stem. It usually grows on poor chalky or sandy soils, in open situations. It is native to Europe, Asia, and North Africa, and it is widespread in North America where it is also probably native. It can be found in many other parts of the world as an introduced species.

It is classified as an endangered species in the UK and is considered to be facing a very high risk of extinction in the wild. It is listed as a Priority Species under the UK Biodiversity Action Plan. Only 35 sites are recorded by Plantlife mostly in Norfolk, (where 100 plants were found at a new site in 1999) but includes 6 sites near Kidderminster in Worcestershire.