Ranunculus: Dwarf Buttercup has a number of varieties suitable for the rock garden but you should make your choice with care. Despite being closely related to some rampant weeds, a few are hard to grow outdoors. At the other end of the scale are the invasive varieties of R. ficaria which can overrun nearby choice specimens. Varieties: R. montanus 'Molten Gold' (Mountain Buttercup) is a showy miniature, height four inches, spread four inches and shiny yellow flowers, one inch wide in spring. R. crenatus is another tiny plant growing only three inches high with pure white flowers in May. R. calandrinioides (height six inches, spread six inches) and bears its white flowers as early as January which makes it a useful out of season plant. For twelve-inch tall Buttercups there are R. gramineus (yellow blooms from May to July) and some R. ficaria varieties, aurantiacus (orange), cupreus (copper), albus (white) and 'flore pleno' (double, yellow). Site and Soil: Requires well-drained gritty soil in full sun. Propagation: Sow seeds in spring or divide clumps in autumn.

Raoulia: Scabweed can grow into three feet high mounds in their native home in New Zealand, but the ones grown in British rock gardens are carpets with a pile which is no higher than half inch when in full flower. The tiny leaves and tiny blooms form a useful ground-hugging cover for areas in which dwarf bulbs have been planted. A very hard winter can be fatal, and they should be covered from late autumn to spring. Varieties: The best known variety is R. australis basic details are height half inch, spread twelve inches and the flowering period is May. It is grown mainly for its silvery foliage rather than its minute pale yellow flowers. R. tenuicaulis is easier to grow; the green mossy mat is covered with fine wool. The best floral show comes from R. lutescens, height half inch, spread twelve inches. From April to June the flower heads turn the surface into a bright yellow carpet. Site and Soil: Requires well-drained gritty soil in full sun. Propagation: Divide the mats in autumn.

Rhodohypoxis: is a South African alpine. It is not quite as delicate as some books claim, and there is no reason why you should not try this small beauty in a sheltered site. Add peat at planting time, top dress around the crown with grit and water in the growing season during periods of dry weather. Varieties: Choose a variety of R. baurii, height two inches, spread three inches, and the flowering period is from April to September. White, pink, rose and purple are the colours available. The corm-like roots produce tufts of narrow leaves and the flowers are unusual. There is an outer ring of three petals and an inner ring of three petals. The attractive flower form and the exceptional length of the flowering season make Rhodohypoxis an excellent subject for a pot or tiny rock garden. Site and Soil: Requires well-drained gritty soil which is lime-free. Full sun is essential. Propagation: Divide clumps in autumn.

Sanguinaria: Bloodroot is a woodland plant from N. America and is easy to grow in the rock garden. The basic needs are peaty, acid soil and some shade. As you would expect from its common name, red sap oozes from the root if it is severed. Sanguinaria is a wide spreading perennial which belongs to the Poppy family, and unfortunately inherits the drawback of having flowers with a very brief life span. The flowers may be fleeting but the large blooms nestling amongst the greyish lobed leaves are extremely attractive. Varieties: There is a single species, S. canadensis. The basic details are height six inches, spread eighteen inches and the flowering period is from April to May. The single white flowers with a central boss of golden stamens look like Anemones, but they last for less than a day. Choose instead the double form 'Flore Pleno' the plants are easier to grow and the flowers are larger, showier and longer lasting. The leaves start to die down once flowering has finished. Site and Soil: Requires humus-rich soil which is lime-free. Thrives best in light shade. Propagation: Not easy, divide clumps carefully in the spring.

Saponaria: Soapwort is a reliable low growing plant for the rock garden. It spreads widely enough to fill bare patches of earth and clothes rock faces. They must also be free-flowering enough to form a sheet of colour during the flowering season. For spring we rely on Aubrietia, Arabis and Phlox. For summer Saponaria is a good choice. For maximum display avoid over-rich soil and shade, and cut back the stems after flowering. Varieties: S. ocymoides is the favourite species, height four inches, spread two feet and the flowering period is from May to August. The flat-faced tubular flowers are about half inch across and are borne in loose clusters. The usual colour is pale pink. Not all Saponarias are vigorous trailers. S. ocymoides 'Rubra Compacta' is a non-invasive plant with reddish-pink flowers, S. 'Bressingham' is another slow-spreading type which bears white-eyed bright pink blooms. S. olivana is worth looking for, six-inch wide cushions with one inch wide pink flowers. Site and Soil: Requires well-drained soil in full sun. Propagation: Plant stem cuttings in summer or divide clumps in spring.

Saxifraga: comes from the mountainous regions (the word Saxifraga means 'stone breaker') and the usual growth pattern of these perennials is small and spreading. A group of leafy rosettes or a mossy mat is formed and from this arise upright flower-stalks bearing loose clusters of starry flowers. Spring or early summer is the usual flowering season and the blooms may be white, pink, red, yellow or purple. But these are generalizations and all sorts of variations occur. Botanists divide the Saxifragas into sixteen groups, but for ordinary garden purposes only four need to be considered. Varieties: The Border group consists of large and invasive types which are more suited to the flower border than a modest rockery. S. urbium (London Pride) is the best known example height twelve inches, spread eighteen inches and the flowering period is from May to July. The Encrusted or Silver group contains plants which have rosettes of lime-encrusted leaves and star shaped flowers from May to July. Examples are S. aizoon (S. paniculata) with its sprays of white flowers and S. cochlearis (height eight inches, spread nine inches) with clusters of white flowers on red stalks. S. cotyledon is very tall, choose instead one of the compact Encrusted hybrids such as S. 'Whitehill' (white) or S. 'Esther' (cream). The Mossy group contains plants which form moss-like hummocks and flower from April to May. Popular ones include S. 'Hi-Ace' (pink), S. 'Pixie' (rose red), S. 'Cloth of Gold' (white, leaves golden) and S. 'Peter Pan' (pink). Finally, there is the Cushion group with a low mound of lime-encrusted leaves and early flowers from February to April. S. burseriana (two inches, white) is a popular beauty which blooms in late winter. S. apiculata (four inches, yellow) blooms a little later. Other popular Cushion Saxifrages include S. 'Jenkinsae' (pale pink) and S. 'Cranbourne' (pink). Site and Soil: All require well-drained soil. Provide a moist spot with some shade from the midday sun, only the Encrusted group will thrive in full sun. Propagation: Separate rosettes or detach part of a clump and plant in early summer.

Sedum: Stonecrop is a low growing plant with succulent leaves and can be seen in rockeries everywhere but far too often is one of the weedy varieties such as S. acre and S. album. There are many well-controlled and much more attractive types so choose with care. The general features are a prostrate mat of fleshy leaves from which arise five-petalled starry flowers, borne either singly or in flat heads. Most of them are evergreen and they are amongst the easiest of rock garden plants, flourishing in dry and poor soils where little else will grow. They are also very easy to propagate and so deserve their popularity as carpeters for cracks and crevices. Varieties: Only grow the Common Stonecrop S. acre if you have a large area to clothe, height two inches, spread over two feet and the flowering period is from June to July. The variety aureum has yellowish leaves. S. album is another invasive species, but it does have several well-behaved varieties such as 'Coral Carpet' and murale which have colourful leaves. S. spathulifolium is an old favourite, height three inches, spread twelve inches and the flowering period is from June to July. The leafy mound is made up of silvery rosettes and the yellow flowers are borne in flat heads. The variety 'Purpureum' has large purple leaves and 'Cappa Blanca' has foliage which is almost white. The flowers of S. spurium (height three inches, spread twelve inches) are white, pink or red a change from the usual yellow. There are other unusual ones. S. cauticolum is deciduous, with grey leaves in summer and red flowers in autumn. S. humifusum bears solitary yellow flowers in late spring above reddish leaves, and S. kamtschaticum 'Variegatum' is an eye-catcher with yellow and golden flowers above cream and red-edged leaves. S. ewersii is a trailer with paired leaves and pink or red flowers. Site and Soil: Requires well-drained soil and thrives best in full sun. Propagation:Very easy, divide clumps in autumn or spring.

Sempervivum: Houseleek can grow in the tiny amount of earth in the cracks in old walls and the small crannies between close-fitting rocks. Thick flower-stalks bearing a number of multi-petalled flowers in yellow, red or purple appear in summer or autumn, but Sempervivum is usually grown for its foliage. The leaves are fleshy and grouped into a ball-like rosette. There is a wide variation in colour, texture size, hairiness, etc. This rosette quickly produces offsets at the end of short runners, the mother rosette dies when flowering comes to an end. Varieties: The Common Houseleek S. tectorum is the best known one, height three inches, spread twelve inches or more and flowers in July. The flowers are deep pink or pale purple and are borne on twelve-inch flower-stalks. The leaves are purple tipped. For a smaller plant with hairy leaves but similar flowers choose S. montanum. The decorative foliage ones include S. arachnoideum (small rosettes covered with web-like hairs). S. 'Jubilee" (green/red rosettes) and S. calcaratum (purple-tipped rosettes). Site and Soil: Requires well-drained soil and full sun is essential. Propagation: Plant offsets in autumn or spring.

Silene: Rockery Campion is a lover of dry, sandy soil and will grow in the cracks and crevices of a stone wall. The stems and soft, hairy leaves spread slowly to form a low carpet. In summer or autumn the flat-faced tubular flowers appear. The petals are blunt ended and often notched. Silene is a useful carpeter as it provides late summer colour when many of the more popular mat-forming plants have passed their flowering season. You will be disappointed if you grow it in a shady, poorly drained spot. Varieties: S. acaulis is the popular Moss Campion attaining a height of two inches and a spread of twelve inches. The flowering period is from May to June. The tiny green leaves form a cushion and the notched pink flowers are almost stemless. S. alpestris is a creeping plant with sticky, lance-shaped leaves and white flowers in loose clusters between June and August. 'Flore Pleno' is a double-flowered variety. S. maritima is a prostrate grey-green spreading plant and you can grow the white double-flowered 'Flore Pleno'. Best of all; choose the bright pink S. schafta for a July to October display. Site and Soil: Requires well-drained soil in full sun. Propagation: Plant stem cuttings in summer.

Sisyrinchium: has clumps of Iris-like leaves and open star-shaped flowers. It is not a common sight in the rock garden. The blooms are not long-lived but they appear in regular succession throughout the flowering season. It is not difficult to grow in a shade-free site but there is a problem. Some of the species produce self-sown seedlings very freely; the answer is to remove flowers when they fade. Varieties: The smallest one is S. brachypus, height four inches, spread six inches and the flowering period is from June to September. The blooms are yellow, for a similar but taller plant choose S. californicum. Two species have the common name Blue-eyed Grass. They are S. angustifolium and S. bermudianum, height nine inches, spread twelve inches and the flowering period is from June to September. Both bear blue flowers and are not easy to tell apart. The key is to look for the yellow eye in the bloom of S. bermudianum. S. 'Mrs Spivey' has white flowers; S. douglasii has purple bell-shaped blooms. Site and Soil: Requires well-drained, humus-rich soil in full sun. Propagation: Divide clumps in autumn or spring.