Carpets of rock cress for rockeries

The Latin name for rock cress Arabis derives from the word Arabia. This is a plant for sandy, stony and infertile places, which thrives planted in rock gardens and stone walls. The genus consists of some 100 species, 4 of which are native to Lithuania. They are sand rock cress (Arabis arenosa), Arabis gerardii, tower rock cress (Arabis glabra) and hairy rock cress (Arabis hirsuta).

These are mostly prostrate plants with creeping and rooting stems. Their leaves are densely hairy; white, pink, lilac or cream flowers are up to 1.5cm wide. They are single or double, held in small and dense racemes. Some species, such as alpine and wall rock cress, have long been cultivated in the gardens; while others – A.procurrens, A.x arendsii, A.ferdinandii-coburgii  – are less common.

Alpine rock cress (Arabis alpina L.) is native to the Ural Mountains, the Far East, North Scandinavia and higher elevations of Western Europe and North America. This perennial plant grows to 35cm tall, and produces cushion-shaped mounds, which stay evergreen over the winter. The leaves in the basal rosette are ovoid, although the stem leaves are oblong and grey. Flowers are white or pink, up to 1cm, fragrant, and held in racemes. It flowers in April and May for 25-30 days. Seeds ripen in July.

`Schneehaube` alpine rock cress grows 10-25 cm tall, with white flowers up to 2cm wide, held in 15cm long racemes. It flowers in April for 25-30 days.

`Flore Plena` rock cress is very similar in appearance to the species, but flowers are twice the size, racemes are longer and resemble those of snowflakes (Leucojum). It flowers profusely from early May to mid-June.

`Rosea` rock cress grows to 20cm tall, with pink flowers up to 2cm wide. Racemes are up to 12cm long. It starts flowering in late April, and continues for 30-35 days.

Most rock cress grow best in full sun, some species tolerate light shade. In favourable conditions rock cress flower abundantly, and spread quickly to form loose mats of dense rosettes. They dislike standing water, heavy or very fertile soil, and half-rotted organic matter. When planting on heavy garden soil, replace part of it with a mixture of compost and sand (1:1 or 2:1), and lay some draining material at the bottom. These are perfect plants for rock gardens, stone walls, etc.

In an exposed location, especially on a hillside, rock cress are susceptible to winter injury in snowless winter, and need to be covered with conifer branches or leaves. If snow melts early in spring, but the earth is still frozen over, rock cress need to be sheltered from direct sunlight, just like conifers or rhododendrons would.

Species plants are propagated by seeds, while the double-flowered and variegated cultivars are usually propagated by division or softwood cuttings. Seeds are sown in spring or in late autumn, and seedling start flowering in the second year. The best time to divide plants is in late summer. About 20 new plants can be easily produced from 3 or 4 mature plants. These should be planted at the 30-35cm intervals. Softwood cuttings are best taken from mid-May to mid-June. Cut terminal shoots from new growth, about 6-8 cm long. Remove 2-3 bottom leaves and plant into nursery beds. The propagation beds should be shaded and thoroughly watered. Cuttings break roots within 2 to 3 weeks. Move plants to their permanent position in August.

Wall rock cress is more widely planted than the alpine one. Some authors ascribe them to separate species, while others describe them as a subspecies or a variety of the alpine rock cress. The main difference between alpine and wall rock cress is in the flowering time – the wall rock cress flowers later in the season.

© Giedra Bartas, 2009

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