Aquilegia: Alpine Columbine attains a height of approximately one and a half to three feet which is too tall for the average rock garden. Choose one of the attractive Alpine Columbines with flowering stems that attain a height of three to twelve inches. Each pendant flower bears five spurs and the foliage is finely divided. One or two problems, Aquilegias are not long-lived, they require copious watering in dry weather and cross-fertilization means that seeds may produce plants you don't expect. Flowers appear in June and July, deadhead when they fade. Varieties: Some experts consider the A. flabellata (A. akitensis) pumila to be the queen of the crop. The white and violet flowers are borne on six-inch stalks above bluish-green leaves. Another six-inch dwarf is the blue-flowered A. bertolonii. For red and gold flowers choose A. canadensis, a variable species producing stems between six and eighteen inches. A choice but difficult species is A. jonesii with three inch stems and blue flowers. Site and Soil: Thrives in well-drained moist soil and light shade. Propagation: Sow seeds in April. Mature Aquilegias dislike transplanting.

Arabis: Rock Cress is a very easy plant to grow with leaves which are usually jagged and hairy. Masses of small, four petalled flowers appear in loose clusters in the spring. Common Rock Cress is useful for covering a large area of bare rock, but all to often it is allowed to run wild in a small rock garden. The foliage is generally evergreen, but stems can die back in wet and cold winters. To keep the plant in check, cut back after flowering. The fruit pods are long and narrow. Varieties: The Common Rock Cress is A. albida (A. caucasica), attaining a height of nine inches and a spread of two feet. The leaves are downy and grey-green and the flowering period is March to April (flower colour white) with sporadic blooms until June. The more compact double form ('Flore Pleno') is popular, the blooms are larger but the plant is less free flowering. White is not the only colour, there are pink varieties such as 'Pink Pearl' and 'Rose Frost'. There is also a slow growing variety with cream splashed leaves, A. albida 'Variegata'. Avoid A. albida if you want a low growing compact plant, choose instead a prettier and less rambling species. Examples include A. blepharophylla (height three inches, spread nine inches, leaves grey-green, flower colour deep rose), A. Ferdinandi-coburgii 'Variegata' (height four inches, spread one foot, fleshy leaves brightly variegated all year round, flower colour white) and A. alpina (height six inches, spread one foot, flower colour white or pink). Site and Soil: Any well-drained garden soil will do, thrives in sun or light shade. Propagation: Divide clumps in autumn or plant cuttings in summer. Species can be raised from seed sown under glass in spring or summer.

Arenaria: Sandwort has prostrate stems that form a mat of foliage with small white flowers borne on upright stalks. This is a good choice for covering rocks or filling cracks between paving slabs. The leaves are evergreen and although the plant can be left to spread over a wide area it can easily be kept in check. Varieties: For covering damp or dry rocks which receive little or no sun choose A. balaerica (height one inch, spread eighteen inches, flowering period March to July). The tiny green leaves give the plant a mossy appearance. For rocks which face the sun choose instead A. montana (height four inches, spread eighteen inches, flowering period May to June). A. caespitosa 'Aurea' (Sagina glabra 'Aurea') bears golden grassy leaves. A. purpurascens is the odd man out, pale purple instead of white flowers above green leafy cushions. Site and Soil: Requires well-drained moist soil. Sun or shade requirement depends on the species. Propagation: Divide clumps in autumn or spring.

Armeria: Thrift, Sea Pink can be found growing wild around the seashore and in countless rock gardens. The evergreen grassy leaves form dense mounds and tiny flowers in ball-like papery heads appear on top of thin stalks, a good plant for a sunny spot. It will naturalize on stone walls and in rocky crevices. The only problem is a tendency for the middle of the clumps to die out. Varieties: A. maritima attains a height of eight inches, spreads twelve inches and flowers from May to July. The flower heads are about one inch in diameter. The colour of the species is pink and so are some of the named varieties such as 'Merlin', 'Laucheana' and 'Perfection'. Other colours are available, look for 'Dusseldorf Pride' (crimson), 'Vindictive' (bright red), 'Bloodstone' (deep red) and 'Alba' (white). For a more compact Thrift choose A. caespitosa (A. juniperifolia) attaining a height of three inches, spread of nine inches and flowers from April to May. The pink or white flower heads are almost stemless. Site and Soil: Any well-drained soil will do, thrives in sun. Propagation: Divide clumps or plant basal cuttings in summer.

Artemisia: Silver Mound includes an extensive group of herbaceous perennials and shrubs that are grown for their silver feathery foliage rather than their small, button like yellow flowers which appear between July and September. Most are large and belong in the herbaceous border, although A. stellariana (Dusty Miller) is sometimes grown in large rockeries. There are a few dwarf and compact species for the average rock garden. Like the other species they may lose their leaves in winter, but are quite hardy if the roots are not waterlogged. Varieties: A. schmidtiana 'Nana' is a good choice for a sunny area or a pocket between the rocks. It forms a mound of silvery-grey ferny leaves about six inches high and twelve inches across. The young growth is particularly attractive. A. pedemontana (also sold as A. lanata, A. assoana and A. caucasica) is a tiny shrub which grows only two inches high and bears finely divided leaves clothed with white hairs. Other alpine Artemisias such as A. glacialis and A. mutellina are harder to grow. Site and Soil: Required well-drained soil and thrives in full sun. Propagation: Plant stem cuttings in spring.

Asperula: Alpine Woodruff has weak stems that form clumps or loose mats and bear the tiny leaves in whorls. These leaves are of two types. There are smooth surfaced species which can be grown fairly easily outdoors and the woolly ones which hate winter rain and need either protection or the comfort of an alpine house. The flowers are borne in clusters and are distinctly tubular. Pink is the usual colour, but white and yellow Woodruffs are available. The flowering season is early summer. Varieties: A. lilaciflora caespitosa is a prostrate plant, forming cushions of glossy green leaves and an abundance of one inch long lilac-pink flowers. Grow in an open spot in the rock garden. An easier plant to grow outdoors is A. gussonii which produces four-inch high woody-based tufts. The flowers are flesh-pink. The most popular woolly leaved species is A. suberosa which produces three-inch high cushions in the alpine house. The flowers are short and pale pink. Site and Soil: Requires well-drained gritty soil in full sun. Propagation: Divide clumps or take cuttings in spring.

Aster: Mountain Aster is well represented in the garden by the Michaelmas Daisies in the herbaceous border, but there are some dwarf species which belong in the rock garden. They are spreading plants with greyish leaves, and in summer the flowers are large and colourful. By far the most popular is A. alpinus which is an easy plant to grow. If you don't mind having a mixture of colours you can raise this plant from seed, but it is better to buy a named variety from your garden centre or catalogue. Varieties: A. alpinus has a basal rosette of hairy leaves above which are the solitary Daisy-like flowers, about two inches across with pale purple ray petals surrounding the golden eye. Basic details are height six inches, spread eighteen inches, flowering period May to July. Choose a good named variety, albus (white), 'Beechwood' (blue) or 'Happy End' (pink). A. natalensis is a sky-blue species which blooms rather later, A. tibeticus (blue flowers is noted for its free flowering habit. Site and Soil: Any well-drained garden soil and thrives in full sun. Propagation: Divide clumps every three years in spring.

Astilbe: is a stately plant with three feet high feathery plumes in summer. There are several rockery varieties which grow less than one foot tall. They differ in a number of interesting ways from the run of the mill alpines. Astilbes thrive best in cool, damp and slightly shady places. They have a flower structure which makes a pleasant change from the standard rockery plant form and these flowers appear in midsummer when so many alpines have passed their flowering season. Varieties: One of the most popular Rockery Astilbes is A. chinensis pumila attaining a height of nine inches and a spread of one foot. The flowering period is July to October. Its miniature ostrich plumes are mauve. The baby of the family is A. glaberrima 'Saxatilis' (height five inches, spread six inches, leaves bronzy, flowers pink in June and July). A number of colourful and highly recommended hybrids of A. simplicifolia are available, 'Aphrodite' (deep rose), 'Willy Buchanan' (creamy white), 'Sprite' (pale pink), etc. A word of warning, Astilbes fail miserably in dry soil. Site and Soil: Requires moist soil and thrives in light shade. Propagation:Divide clumps every three years in spring.

Aubrieta: Rock Cress is perhaps the most widely grown of all rock garden plants. It is vigorous, tolerant of a wide range of conditions and extremely colourful when covering a sloping bank in spring. In the average sized rockery it certainly has a place, but it should not be allowed to take over. Cut back hard after flowering to keep it in check and to induce a second flush of flowers in autumn. Varieties: The basic species is A. deltoidea (height three to five inches, spread two feet, flowering period April to June). The grey green leaves are downy and the blooms about an inch across. The species is not grown, the garden varieties are hybrids and there are many from which to choose. A few popular ones are 'Aureovariegata' (lavender flowers, gold variegated leaves), 'Bressingham Pink' (double pink), 'Doctor Mules' (purple), 'Red Carpet' (red), 'Carnival' (deep violet), 'Gloriosa' (rose-pink), 'Wanda' (double pale red) and 'Dream' (pale blue). Site and Soil: Any well-drained, non-acid soil in full sun. Propagation: Divide clumps in autumn or plant cuttings in summer. Sow seeds in spring for a mixture of colours.

Calceolaria: Slipperwort The large pouched flowers of the half hardy Calceolaria made this plant a favourite in Victorian times, but it is the few hardy species which are grown in the rock garden and peat bed. Yellow is the usual flower colour and the flowering period is June to August. They are short-lived perennials, a few will grow quite happily outdoors if the soil has ample humus and is kept moist, but others need an alpine house. Varieties: The largest Calceolaria for growing outdoors is C. biflora. The one-inch bright yellow flowers are carried on one-foot stalks above the hairy toothed leaves. C. tenella is much more compact, the tiny leaves form a dense carpet and the yellow flowers are borne on two-inch stalks. For something really different grow C. darwinii (height four inches, spread six inches). Each yellow flower is speckled with brown and the lower lip has a broad white band. Unfortunately it is difficult to grow outdoors. Site and Soil: Requires moist soil and thrives in light shade. Propagation: Divide clumps or sow seeds in spring or summer.

Calluna: Heather, Ling have scores of named varieties. Common Heather grows on the moors. These miniature shrubs grow four inches to three feet high and bloom in late summer. Choose one or more of the dwarf types if you have a spot to fill where the soil is infertile and acid. Trim the plants after flowering to keep them compact. Varieties: There is only one species (C. vulgaris) but there is a wide assortment of varieties. Many have coloured foliage, golden, silvery, bronze or red. There is also a range of flower colours from pure white to deep red. Varieties which grow less than eighteen inches high include 'Foxii Nana' (four inches, purple), 'J. H. Hamilton' (eight inches, double pink) and 'Joan Sparkes' (nine inches, double purple). For coloured foliage choose 'Gold Haze' (bright yellow) or 'Blazeaway' (red in winter). Site and Soil: Well-drained acid soil is essential, thrives best in full sun. Propagation: Layer shoots in spring or plant stem cuttings in summer.