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Construction : Rock Gardens

Rockery | Alpine bed | Scree bed

Rockery

Most rockery plants are sun-lovers – do not place a rockery in the shade of buildings or trees. A rock garden should resemble a natural rock outcrop and because rock normally outcrops from sloping ground, it helps greatly to have a rock garden built into a slope.

The rocks must not be simply dispersed over the area of the rockery but seem to actually come ‘out’ of the ground. They must be in the ground, not on top of it. Some of the rocks should be almost completely buried, as they would be in a natural outcrop, giving the impression of underlying rock.

Choose suitable stone and do not mix different types. Start by clearing the site of all existing vegetation, especially perennial weeds. On a flat site, build the rockery in stages – do not just heap up the soil and dot it with stones. Lay a tier of stones with the big ends outwards and a slight backward tilt to trap rainwater.

Fill in behind with soil and firm well. Mix some broken bricks, small stones or rubble into this fill-in material. Lay another tier of stones in the same way and fill in behind – repeating the procedure until all the rocks and soil are used up. Remember to hold onto some nice stone for the top of the rockery.

On a slope, dig out a low terrace, about 20to 30 cm deep and set the stone against the earth. Fill in behind and then move up to the next terrace. When placing rocks, try not to make the rock structure too regular. Group some stones together to give bulk and space them out a little in other places, as they would occur naturally.

Before planting, it is a good idea to make a special planting mixture for the top 10 centimetres, into which the plants will go. Even if some soil has to be moved to make room for this layer, it is worthwhile doing so. A mixture of 3 parts soil (2 parts, if the soil is heavy), 1 part coarse sand and 1 part peat is ideal. Use a 2.5 centimetre layer of shingle or stone chippings over the soil as a mulch to prevent weed germination.

Alpine bed

A raised alpine bed is an ideal way to grow alpine plants, giving the excellent drainage that they need. The bed is raised by at least 30 centimetres and looks best when the supporting sides are made of rock. A concrete block wall can be built first as support for the natural stone.

Stone walls can be built up without supporting blocks behind but they tend to ‘belly’ out and can even collapse in time, unless they are well built. But this irregularity can be part of their charm, and unsupported stone walls have the advantage that plants can be grown between the stones of the walls, rooting back into the soil behind.

If this attractive effect is required in an alpine bed with a concrete backing wall, some gaps will have to be left between the blocks during building. A drainage pipe should be laid beneath the bed to remove excess water, then a 10 cm layer of stone for drainage and finally the soil mixture when should be roughly 3 parts soil, one part shredded leaf mould or peat and one part coarse sand, grit or fine gravel. After planting, the surface can be dressed with 4-5 cm of grit.

Scree bed

A scree bed is built in the same way as a raised bed; it needs good drainage and it not simply a question of spreading gravel over the existing soil. A scree bed can be level with the existing soil or slightly raised above it to improve drainage. It can have a slight slope across it, but should meet the existing ground level and not have kerbs or raised sides.

Dig out the area to at least 30 cm and lay a base of rubble or coarse rock, use the soil for making the rockery soil mixture and use the surplus elsewhere in the garden. A layer of Plantex or similar material can be laid down to keep the rubble free of fine material. Then a layer of gravel about 5 cm deep is laid over and then the soil mixture of roughly 3 or 4 parts soil, one part shredded leaf mould or peat and one part coarse sand, grit or fine gravel.

A light coloured gravel, grit or coarse sand looks best in a scree bed in a layer of about 5 centimetres. One or two carefully placed large stones nicely set off the sand or gravel surface. They should be set well down into the surface layer.

When planting a scree bed, or indeed a rock garden or alpine bed, try to avoid planting too many plants. Alpine plants can grow quite large in a few years, spreading out by means of overground, or underground, stems. When the surface of a scree bed, rockery or alpine bed becomes cluttered with plants it loses much of its attractiveness.

It is the combination of plants, rock and sand or gravel, that is interesting. Plants are less likely to smother a rockery than a flat scree bed, but this problem should be guarded against by restricting the number of plants and their size.

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