(Editor's Note: This article was originally published on December 12, 2009. Your comments are welcome, but please be aware that authors of previously published articles may not be able to promptly respond to new questions or comments.)
Among the most popular rock garden plants are Arabis and Aubrieta, commonly known as rock cress or wall cress. These genera are often considered a beginners plant in the rock garden world but besides rockeries, this group of plants is ideal for growing as a groundcover on slopes, cascading over walls or as an edging plant for borders. Rock cress are considered harbingers of spring as they bloom about the same time as many of the popular spring bulbs. Both genera have evergreen leaves and form trailing mats topped with short racemes. Their flowers are distinct being 4-petaled in the shape of a cross.
The genus Arabis contains about 120 species distributed across the northern hemisphere. Arabis is Latin for Arabia, an area that contains several species of rock cress. In the garden, we only grow a select few species and cultivars. White is the most popular colour but pink to purplish cultivars exists. Aubrieta are very similar to Arabis differing slightly in petal and seedpod shape. This latter genera is named after Claude Aubriet (1668-1743) a French botanical artist. This genus has only 12 species found in Europe and central Asia. While Arabis blooms are most commonly white, Aubrieta come in pink, reddish to purple-blue shades.
The wild form of Arabis alpina growing in northern Newfoundland
Culture is generally quite easy. In the wild they grow in full sun on gravelly, limestone soils often in alpine zones. In the garden, provide them with well-drained soil that is not too acidic. Full sun is best but in hot regions, protection from midday sun is often advantageous. Shear the spent flowers to keep the plants more compact as well as to curb their enthusiasm to self-seed. Older plants can be divided but they germinate easily from seed (do not cover as they need light to germinate) and root easily from herbaceous cuttings. Rock cress are quite hardy, suitable for areas as cold as zone 4, even colder for some species.
Among the Arabis, the most popular species is A. alpina subsp. caucasica. Arabis alpina has a holarctic distribution but the standard species has rather small flowers and wiry stems. The subspecies caucasica has much larger flowers with a more compact habit thus is more desirable as a garden ornamental. There are many cultivars among this subspecies but in North American nurseries ‘Snowcap' (white) and ‘Spring Charm' (pink) are perhaps the most popular. Others to look for include ‘Little Treasure Deep Rose', ‘Pink Sequins' and ‘Pixie Cream'. Among the Arabis there are several very attractive variegated types. Arabis alpina subsp. caucasica ‘Variegata' has striking cream-edged foliage. Arabis procurrens ‘Variegata' has leaf edges widely edged in white. The foliage is much smoother on this species and the flower stems a bit taller than the common rock cress. Considered a synonym of A. procurrens in some books, but as a separate species in others, Arabis ferdinandi-coburgi has several variegated versions; ‘Old Gold' has yellow-edged foliage, ‘Reversed' has green leaves with a white center while ‘Variegata' has white-edged foliage. All of these take on purple tints in winter.
Arabis alpina subsp. caucasica 'Snowcap' and 'Variegata'
Arabis procurrens 'Variegata' and 'Old Gold'
The main Aubrieta grown in gardens is either A. deltoidea and A. X cultorum; the two names are often interchanged. Overall, Aubrieta is smaller in stature than Arabis. There are several named selections including ‘Whitewell Gem', ‘Novalis Blue', 'Purple Gem', ‘Cascade Red', ‘Royal Red' and the ‘Axcent' series. Like Arabis, Aubrieta also have variegated forms which include ‘Variegata' with white-edged leaves and ‘Aureovariegata' whose leaves are yellow edged or even pure yellow. These variegated forms are particularly striking with the purplish blossoms.
Popular cultivars of Aubrieta include 'Royal Red', 'Purple Gem' and 'Cascade Red'
Above are A. X cultorum 'Variegata' and 'Aureovariegata'
I would like to thank the following people for the use of their pictures: altagardener (Aubrieta 'Royal Red'), Baa (Aubrieta X cultorum 'Aureovariegata), Calif_Sue (Arabis alpina subsp. caucasica 'Variegata'), growin (Arabis procurrens 'Variegata'), Happenstance (Aubrieta X cultorum 'Variegata), henryr10 (Arabis 'Snowcap'), Kell (Aubrieta 'Purple Gem') and staceysmom (Arabis 'Old Gold').