Planting a rock garden

Whether in a traditional rock garden, raised bed or container, alpine and rock garden plants can recreate the beauty of a wild, rugged terrain in miniature.

Rock garden plants mainly come from mountainous and rocky regions or the world but other compact perennials, biennials, bulbs and shrubs can also be grown in rock gardens. Most rock garden plants are hardy and tolerate thin, stony soils and exposure and, given free-draining soil, are easy to grow. True ‘alpine’ plants are native to mountainous regions between the upper limits of tree growth and the permanent snow line. Examples include androsaces, certain gentians and primulas. However, some alpines, though hardy, hate winter wet combined with heavy soils and low temperatures – in the wild they would be covered with an insulating carpet of freezing but dry snow through the winter. Such alpines need the conditions of an alpine house – an unheated greenhouse with extra ventilation – or other form of winter protection.

Some of the most popular rock garden plants, such as yellow alyssum, aubrieta and perennial candytuft, thrive in most open, sunny, well-drained spots and, with their easy-going natures and long-lasting displays of brilliant flowers, are an asset to any sized garden. Such plants can be grown in borders, sink gardens, troughs or other ornamental containers, cracks between paving slabs and dry-stone walls as well as in traditional rock garden settings. Those with dense, spreading, weed-proof growth will also provide excellent ground cover.

English: Rock garden A quiet spot in the Botan...

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Types of rock garden plants Perennial plants constitute the bulk of rock garden subjects. Those with tight basal leaf rosettes, such as houseleeks (Sempervivum), lewisias, primulas and ramondas are especially attractive but often vulnerable to rotting and need very gritty, well-drained soil; planting them in vertical or sloping crevices where water won’t accumulate in their hearts also helps.

Cushion- and mat-forming plants usually produce a mass of tiny blooms and include bellflowers (Campanula), dwarf cranes-bills (Geranium), drabas and rock Jasmines (Androsace), gentians, pinks (Dianthus), rock phlox, sandworts (Areiuiria), saxifrages, stonecrops (Secluin), sun roses (Helianthemum) and thymes. These are ideal on flat or gently sloping surfaces.

Shrubs Whether evergreen or de-ciduous, these have a valuable, year-round presence in rock gar- dens. Shrubs can be arching or ground hugging – such as Cytisus x kewensis or the fishbone co-toneaster – and are ideal for cascading over rocks; or they may be bushy and rounded – Japanese maples, whipcord hebes, or dwarf rhododendrons, for example.

Slow-growing, dwarf conifers -false cypress, thuja, spruce, pine, 11 fir and juniper varieties – range from rich green to brilliant yellow to blue-grey, and from prostrate to upright, pencilthin flame shapes, ideal for adding architectural punctuation to a rock garden. Biennials and bulbs You can always pop wallflowers into a rock garden for bright splashes of spring colour. Alpine species of tulip, iris, crocus, cyclamen and narcissus are smaller and more delicate than their showy, hybrid cousins, and by choosing them with care, you can have bulbs in flower from autumn through to late spring.

Rock garden alternatives

As well as traditional, multi-layered rock gardens, a similar’effeer can be achieved quite cheaply-using the ‘outcrop’ principle – part burying a layer of rocks in the soil, angled to give the impression that the bulk of the rocks lie beneath the surface.

Scree beds are slightly sloping expanses of small stones interspersed with a few rocks, simulating conditions at the base of a cliff. You can also create a screelike effect between the base of a rock garden and surrounding lawn. Building a scree involves re-moving a layer of soil, replacing it with a drainage layer of broken bricks, topped with a layer of sand or gravel, then adding a special soil mixture and a top dressing of stone chippings.

English: Cragside Estate, rock garden

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Rock plants can he grown between paving slabs, laid on sand and the gaps between filled with gritty soil. A collection of thymes would be ideal, since they emit a powerful aroma when trodden on.

Raised beds, 15-90cm (6-36in) high, can provide the drainage and exact soil type needed by rock plants, and also make them easier to see and care for. Built of stone or reconstituted stone, bricks, logs or wooden railway sleepers, separate raised beds can each accom-modate the needs of specific plants. Raised peat beds, for example, built using railway sleepers or peat blocks and filled with a special lime-free soil mixture, can provide a home for plants such as heathers, dwarf rhododendrons and autumn-flowering gentians that cannot tolerate lime.

The front of a border can be attractively filled with rock garden plants and many taller rock plants associate well with herbaceous perennials.

Preparing the soil mixture For information on building rock gardens, see Garden. Construction 2S-2S. Fill pockets between rocks with a free-draining mixture of two parts, by volume, weed-free, good-quality loam, one part grit or stone chippings and one part peat or leafmould. Finish flush with the top of the rocks, gently firm and rake level.

After about ten days, or earlier after heavy rain, top up with extra soil mixture as necessary. Finally, top dress with a layer of gravel -6mm (’/lin) rock chippings – 1.2-2.5cm (VS-lin) deep, raking them flat. The chippings prevent loss of soil moisture, protect plants from rotting, prevent rain splashing soil on the flowers and leaves, and deter weeds.

For screes, mix, by volume, one parr weed-free, good-quality top-soil, one part leafmould or pear, and three parts gravel.

For peat beds, mix two parrs loam with a neutral or acid pH, two parts sphagnum peat and one part stone chippings.

A rock garden in an apartment complex in Edmon...

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Planning a layout

Aim to create a mountain landscape in miniature, with plants arranged according to colour and time of flowering, but also overall height and habit of growth. As with an ‘ordinary1 garden, plant groups of several of one type, rather than individual, different plants dotted about, with the occasional shrub as a focal point. Relate the planting to the rocks, perhaps planting an upright conifer at the base ot a rock, with a cascading plant above it.

Young rock plants are invariably small and it is tempting to plant them closer together than the recommended spacing, but try to follow the guidelines. For a bit more cash, you can buy larger, containerized ‘specimen’ rock plants.

Avoid rampant plants, such as snow-in-summer (Cerastinm tomentosum), which can swamp choice neighbouring plants. Plants which self-seed freely and produce deep tap roots – the Welsh poppy (Meconopsis cambrica), for example – are also best avoided.

Containerized rock plants, home grown or bought from a garden centre, can be planted at any time of year, but avoid extremes of heat, cold or wet. Always water the plant while still in its pot, then allow to drain thoroughly. Some people like to keep their rock gardens tidily lahelled, but try to avoid using obtrusive labels which detract from the display. Alternatively, keep a note of plant names on a labelled plan. Planting on the flat Using a hand trowel, dig a hole generously wider than the root-ball. Remove the plant from its pot, and any crocks entangled in the roots, then centre the plant. Infill the hole with the soil mixture and firm the plant in with your fingers. Recover the area with chippings, and water moderately. Crevice planting Ideally, crevice plants should be planted when the rocks are being positioned but this is not always practicable and planting may have to be left until later. If the roots don’t fit in a crevice, flatten the root-ball between the palms of your hands -provided the compost is moist and the roots well developed, you can do this without causing damage.

Wedge the root-ball into the crevice and firm in with soil underneath the plant. Finally, fill in the crevice from above, using a mixture of soil and stone chippings.

Cracks in dry-stone walls/paving These can be difficult to plant, unless you do so during the course of construction. It is often easier to scatter seeds of the desired alpine perennials where you want them to grow.

Rock Garden, Darjeeling.

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Routine maintenance

A well-planned rock garden needs little maintenance but until established in their new site, water all rock garden plants in hot, dry weather – although they need sharp drainage, water is essential for healthy growth.

Regular weeding is also vital, es-pecially when newly planted -tough-rooted perennial weeds are-difficult to eradicate once established, especially from between tiny or mat-forming alpines. Hand weed or use a trowel or hand fork. Paint-on herbicide gels can be used against large-leaved weeds, but never use herbicide sprays or granules. Scatter slug pellets in the spring, and spray with insecticide if necessary.

Once plants are established, apply an annual, early spring feed of balanced fertilizer, such as bone meal, Growmore or other granular fertilizer, followed by a top dressing of rock chippings. Top dress peat beds with leafmould or peat and hoof and horn. Remove any fallen leaves that cover the plants in autumn, otherwise the rock plants may rot.

Some rock garden plants benefit from being clipped after flowering, to keep growth compact. Others need winter protection from the wet: when cold, wet weather sets in, rest a pane of glass on bricks so that it is above -but not touching – the plant.

Lift, divide and replant established plants to rejuvenate growth and prevent rampant types overwhelming slow-growing ones.

Favourite Rock-garden plants to grow

Acantholimonglumaceum. A dark green cushion, 15 cm (6 in) high, of spiny leaves and, in summer, short sprays of pink flowers. These remain in an attractive ‘dried’ state for many weeks.

Allium beesianum. A clump-forming, bulbous plant with 23 cm (9 in) stems, each carrying pendent flowers of Wedgwood blue in mid to late summer.

Atitbyllis monlana ‘Rubra’. In summer, rich-red, clover-like flowers lie over a mat of fern-like, grey-green leaves. Artemisia schmidtiana ‘Nana’. Finely-cut leaves of glistening silver form a low hummock during the early summer. An exquisite foliage plant.

Aubrieta ‘Bressingham Fink’. Trailing mats of neat rosettes, with stemless, semi-double flowers of a clear pink. These open in earliest spring.

Campanulagarganica. A clump-forming species 15 cm (6 in) high, producing radiating stems that are loaded with blue, star-shaped flowers during June and July.

Cytisus hirsutus. This mat-forming broom has large yellow flowers in May anil June, each stained mahogany-red as it matures.

Diascia rigescens. Leafy stems, some 30 cm (1 ft) high, carry open flowers of a rich salmon-pink throughout the summer.

Erodium macradenum. Produces a summer-long succession of white, finely-veined flowers, each bearing a dark-purple blotch on the upper petals. Parsley-like foilage of silvery-green. Height 23 cm (9 in).

Euphorbia capitulata. An evergreen mat, 5 cm (2 in) high, of olive-green rosettes. Small heads of bright yellow flowers appear in spring.

Globularia cordifolia. Summer-flowering, this evergreen sub-shrub has glossy, wedge-shaped leaves, and flowers resembling blue powder puffs. It is 5-8 cm (2-3 in) high.

HELIANTBEMUM ‘Ben More’. A low-growing sub-shrub to 15 cm (6 in) with narrow leaves and a mass of coppery-orange flowers for several weeks during early summer.

Haberlea rbodopensis. Thick, dark green leaves slowly form a clump 8 cm (3 in) high. Lipped flowers of pale lilac appear in May. Plant in a moist, shady spot.

PHLOX ‘Kelly’s Eye’. A trailing mat of needle-like leaves, with masses of pale-pink flowers, each having a ‘blood-shot’ eye, in late spring.

Primula vulgaris var. sibthorpii. Crinkled leaves, with 5 cm (2 in) stems carrying lilac-coloured primroses all through the winter months. Needs light shade. Viola ‘Irish Molly’. An old variety with flowers of an unusual mixture of khaki-yellow and bronze. Spasmodic flowering all summer.

List of Plants for growing in raised beds

(T) indicates a trailing habit. (S) indicates plants suitable for the side of a bed.

Asperttla lilaciflora var. caespitosa. (T). Deep green, heath-like foliage. Pink, tubular stars, 12 mm (0.5 in) high, are carried throughout the summer.

Cotoneaster congestus ‘Nanus’. (T). A prostrate, twiggy mat covered in tiny, dark green leaves. Ideal to provide miniature, evergreen ground-cover. Occasional white flowers.

Erinus alpinus ‘Albus’. (S). This adaptable plant is quite happy in the driest crevice, where, in May, it produces pure white flowers on 8 cm (3 in) stems.

Geranium dalmaticum. (S). Given a sunny crevice, this plant rewards with shell-pink flowers in June, and leaves that are attractively coloured in autumn.

Helianthemum lunulatum. A dwarf, shrubby species, some 20 cm (8 in) high. It has grey-green, oval leaves, and masses of yellow flowers during June and July.

Lewisia Trevosia’. (S). It has narrow, fleshy leaves and branched stems, 15 cm (6 in) high, that carry flowers of a salmon-red colour in May.

Primula marginata. (S). Leaves covered with powder (farina), and serrated at the edges. Blue and mauve flowers in earliest spring, scented like primroses.

Salix serpyllifolia. (T). This tiny-leaved willow makes a mat of tangled, trailing stems. Attractive, yellowish bark during the winter.

Saxifraga x apiculata ‘Alba’. A vigorous hybrid, with apple-green and white flowers on 8 cm (3 in) stems during March and April.

S. callosa ‘Superba’. (S). Lovely silvery leaves form close-packed rosettes. These bear arching stems clothed with pure white flowers during May.

Sedum spurium ‘Variegatum’. Forms a tangled mass of ground-hugging stems, with fleshy leaves of grey-green, margined with pink. Lovely from spring to autumn.

Thymus x citriodorus ‘E.B. Anderson’. A rounded mat of tiny, lemon-scented leaves with the occasional sprinkling of lavender-pink flowers. Attractive golden foliage during the winter.

Veronica austriaca ‘Trchane’. A plant for both flowers and foliage. Low tufts of golden-yellow leaves, with bright blue flowers on 10 cm (4 in) stems during May.

List of best Scree plants for rock gardens

Achillea clavenae var. integrifolia. A clump-forming plant with narrow, silver-grey leaves that are aromatic. White flowers on 15 cm (6 in) stems in summer.

Anacyclus depressus. Prostrate stems carry ferny leaves and, in spring, white, daisylike flowers that are crimson beneath.

Aquilegia bertolonii. This May-flowering columbine has dark green leaves and large, rich blue flowers, which are spurred, on 8 cm (3 in) stems.

Arenaria tetraquetra var. granatensis. A dense cushion of grey-green, four-angled leaves and stemlcss white flowers. These open in May.

Campanula cochleariifolia. With running, spreading stems, this plant produces masses of bell-like flowers throughout summer and autumn. Colours range through shades of blue to white.

Dianthus ‘Boydii’. Fringed pink flowers are carried on 10 cm (4 in) stems over narrow leaves. Mainly June-flowering, it blooms spasmodically in summer.

Douglasia montana. A loose, cushion-forming species, with greyish-green, pointed leaves. In April, it carries deep pink flowers that are 2.5 cm (1 in) high.

Draba aizoides. In earliest spring, golden flowers on 5 cm (2 in) stems are carried above tufted, bristly rosettes of dark green. This is a British native plant, found on Welsh cliffs.

Gentiana saxosa. A mid to late summer-flowering gem from New Zealand. White flowers, with green veining, are borne on 10 cm (4 in) stems over dark-green, glossy leaf rosettes.

Helichrysum milfordiae. This grey, woolly, rosetted South African plant, 5 cm (2 in) high, is absolutely hardy if the drainage is perfect. Pointed, crimson buds open as white everlasting flowers in June.

Saxifraga grisebachii. From beautiful rosettes of intense silver, each 2.5 cm (1 in) wide, arise bracts of a red-velvet texture. Small flowers open during April.

Plants to grow in tufa

Anchusa caespitosa. A rock-growing plant that consists of strap-shaped leaves, with vivid-blue flowers, each with a white eye, in summer. Height 5-8 cm (2 in).

Androsace villosa. Small grey-green, 5 cm (2 in) high tufts form on red-stemmed stolons, each bearing round, scented white flowers during April.

Asperula suberosa. A delicate-looking carpet of white, woolly foliage, topped with pink tubular flowers, giving a ‘foam’ effect. The flowers appear in May. Height 5-8 cm (2-3 in).

Douglasia vitaliana var. practutiana. Greyish-green rosettes, each 12 mm (0.5 in) across, make a compact plant. During April, it is covered with lemon-yellow, half-open flowers.

Linum salsoloides ‘Nanum’. Slender stems carry needle-like leaves and, in summer, large, pearly-white flowers. The habit is semi-trailing.

Omphalodes luciliae. A beautiful plant, about 15 cm (6 in) high, with blue-grey leaves that provide a low-domed effect. Pink buds open to soft-blue flowers, mainly during the summer.

Physoplexis comosa, syn. Phyteuma Cotnosum. After its winter dormancy, sharply-toothed leaves appear, soon followed by curious claw-shaped, tubular flowers, the colour of lilac. Height and spread about 5 cm (2 in).

Potentilla nitida ‘Alba’. Forms a tight mat of silver-green, hairy leaves. Stems, 5 cm (2 in) high, bear white flowers with orange stamens mainly in May and June. Raoulia lutescens. A minute carpet of grey-green leaves, well covered with tiny, stemless yellow flowers in summer. Saxifraga ‘Jenkinsae’. A 2.5 cm (1 in) hummock of grey-leaved rosettes covered, during March and April, with near-stemless, shell-pink flowers.

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