Building a rock garden

Designing and building a rock garden is easy, and once planted, it is simple to maintain and looks good at any time of year.

R ock plants grow quite happily in the cracks between paving slabs, in dry-stone walls, or in raised beds, but the best setting is a rock garden (or rockery). A well-designed rock garden provides conditions close to the plants’ native alpine environment and also looks natural.

Choosing a suitable site

Since most rock plants are sun lovers and enjoy a completely open position, a warm, gentle slope facing south is ideal. The soil should be well drained to prevent the roots rotting. Try to avoid frost pockets.

Aim to site the rock garden in a natural setting, rather than near formal features, such as flowerbeds. If you have no option but to site the rock garden near the house or garden walls, you can cover the walls with climbers. (On a practical note, any rock garden next to a house should be well below the damp-proof course; if building against a garden wall, make sure it is strong enough to stand the pressure.)

Site the rock garden in the open – away from shaded areas, such as under trees, which cast shade and drop leaves that will smother small plants; and water dripping from overhanging branches can lead to rot and decay.

Make access as easy as possible. Remember that you will have to shift heavy stone from the delivery point to the rock garden site.

Size and shape

In many gardens, size and shape will be determined by the site. A good size for a small rock garden is around 3sq m (30sq ft), which gives enough room for two or three tiers of rock.

In a free-standing rock garden, the tiers become progressively smaller as they get higher (this also makes for a sound structure). On a sloping site, irregular semicircular or L-shaped terraces give the best effect. Aim to create a natural-looking rock garden – a simple, irregular shape is better than a rigid, geometric one.

Choosing stones

Stone can be costly to buy and transport – a good reason for getting your supply locally. Buy it from a large garden centre or from a quarry or stone merchant, where it might be cheaper. Local stone will look more natural in your garden than stone imported from another area, and should be com-patible with your soil type. Local suppliers are also more likely to stock it.

Sandstone and limestone both weather well. A porous sandstone suits a wide range of plants, allowing roots to penetrate lor stability and moisture. Limestone weathers more quickly, but has a less pronounced ‘grain’. Tufa, a soft, porous, limey rock, is lightweight and contains many air pockets, so it is easy to transport and handle, and is ideal for rock plants. Unfortunately, it is also rather expensive.

Granite and marble are not suitable because they are non-porous and of no use to plants. White marble also looks stark and conspicuous. Artificial stone is lighter to move than real stone, but does not look nor perform so well. Avoid using different kinds of stone, or mixing artificial stone with real. Choose one type and stick to it.

When estimating the amount of stone you will need, get advice from a reputable supplier, explaining the size of your site and the effect you want to create. Even a small rock garden can use around a tonne (ton) of stone.

Try to select the pieces of stone yourself, otherwise give your supplier a guide to the minimum and maximum sizes you will require. Choose stones that are of slightly different sizes and shapes so that your rock garden will look interesting as well as natural.

Preparing the site

On a flat site, drainage is improved simply by raising the soil level in layers. Improve it further, especially on heavy clay soils, by bedding the rock garden on a firm base of rubble (or hardcore) and gravel. Drainage on a sloping site is unlikely to be a problem.

Ensure the site is free of weeds, especially deep-rooted perennials, and use a weedkiller if necessary, following the manufacturer’s instructions. Weeds can be difficult to remove once they have become established in a rock garden.

Make up the planting medium this is used for bedding in the rocks and for filling in between them. Depending on the natural grittiness or humus content of the soil, an approximate guide is one part (by volume) of sharp sand to one part leafmould or fine-grade moss peat to two parts soil.

You can use any good quality topsoil excavated from the site in the planting medium, but make sure it is weed free.

Positioning the rocks

For moving larger stones, you will need a sack barrow or rubber-wheeled wheelbarrow (narrow-wheeled ones may tip) and, possibly, some wooden log rollers. To position heavy stones, you may need either a crow-bar or a length of wood to use as a lever with a block of wood as a fulcrum.

Position all rocks with the grain running the same way (preferably horizontally) for a natural effect. Bury around one third of each stone in the ground and tilt them slightly backwards (at an angle of around 15°) so rainwater is then directed back towards the plants. Stand back as you work to see how it looks and reposition any stones that aren’t right. Continue adding soil and building until all the rocks are in position, before planting and covering with the final layer of gravel, stone chip-pings or grit.

For information on planting rock gardens see Know-How 57-60.

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