Rock Garden Construction
A ROCK garden imitates a natural feature — the wild outcrops of stones and boulders found on the slopes of mountains. To recreate the haphazard grandeur of Nature on the small scale suitable for a garden requires careful planning, not only in the choice of site, design and materials, but also to hide the artificiality of the feature.
Choose a site away from the formal parts of the garden, where thecan blend in with its immediate surroundings. To ensure the success of the alpine plants that will clothe it, the site should be in a sheltered position, but well away from prolonged shade cast by large trees, fences or buildings.
Highly effectivecan be constructed on sites that are naturally uneven or banked and, therefore, they often solve the problem of what to do with oddly contoured and terraced areas. But, if they are carefully planned, they can also be constructed on flat ground.
It is worth visiting well-designed rock gardens to see how the various problems of layout have been coped with, and to walk over a natural outcrop of rock in the country, taking careful note of how the rocks lie. For the success of a rock garden is measured by its naturalness.
The size will depend on the size of the garden as a whole. As rock gardens are so distinctive, it is better that they be on the small side; if they are too large, they become dominating. But a way can usually be found to site one in most gardens, however small. A rock garden incorporating other natural features, such as a waterfall and pool, requires plenty of room; moreover, a complicated design will not be effective on too small a scale.
In the design itself, proportions are very important, so do not use rocks that are too large. Rocks are expensive to buy, and a rock garden of modest proportions is often satisfying both aesthetically and economically.
Try to obtain stone or rocks from a local supplier, not only to obtain the type of stone indigenous to the district and so help with the natural effect, but to reduce transport costs.
TYPES OF ROCK
Select rocks that blend with the immediate surroundings. Quarried sandstone or Westmorland stone are usually suitable. Sandstone retains moisture and has an attractive finish, but is rather soft and is affected by frost. Limestone is an excellent rock to use, as it weathers well and has a very good colour. There are several different types, such as Derbyshire, Westmorland and Cotswold.
If bold effects are planned, order large pieces of rock, weighing perhaps several cwt. Each. On average, a ton of stone provides 10 or 12 pieces, depending on the type. Smaller stones will also be required to mix in with the larger rocks where necessary to give variety. A piece of sand-
Stone weighing 3 cwt. would measure about 2 ft. by l ft. by 1 ft.; a piece of Westmorland stone of the same weight would be rather smaller.
The rock garden must have goodand very good drainage. If the site is heavy, break up the subsoil thoroughly and work in plenty of old rubble. Then put a good mound of small rubble on top, adding a few large stones so that they will protrude after several loads of better soil, preferably loam, have been added.
The best soil mixture is good turf soil, mixed liberally with sharp sand. Incorporate peat or leaf mould at the rate of 4 bucketfuls per barrow-load (approximately l cwt.) of loam.
More than any other garden feature, the rock garden depends for success on the constructor’s ingenuity. In a natural rock, outcrop, the stones lie in strata, and most of their bulk is buried below soil level. When placing stones in a rock garden, try to imitate this effect by laying the pieces in strata with their broadest and most weathered faces showing. Both for safety and for effect, seat them firmly in the soil, packing where necessary with soil and small stones, and tilt them slightly downward into the bank so that rain will drain inward to keep the roots moist. As the work proceeds, view it frequently from a distance, to get an over-all impression.
Large, bold outcrops can be produced if two or more large pieces are placed on top of each other; but be sure that the pieces fit and do not overhang each other too much, or they may not be stable when in position.
If the rock garden is to be large, plan paths through it so that the plants can be admired and tended. Let the paths twist and turn as they would naturally.
Do not let the rock garden finish abruptly, but allow it to trail off gradually, with occasional rocks here and there. One very good setting for a rock garden is at the end of the lawn, where an effective finish can be made by setting small outcrops in the turf itself.
Where water is incorporated in a large rock garden it is usually allowed to run down into a pool or pools below. The water feature should, where possible, be situated conveniently close to a power supply and water source. Alkathene tubing is ideal piping to use, as it is easy to install and join together where necessary. The same water can be used over and over again if a waterproof electric pump is fitted.
At the bottom of the lowest pool, an overflow or return pipe will be needed to carry the water back to the top of the , rock garden. A specialist pump firm will supply the correct size and type of apparatus if a plan of the feature is provided, showing how high the water has to be pumped and how much will be circulating. For the sake of safety, a competent electrician should install the water pump.
Mouse the pump near the highest part of the rock garden, and arrange the outlet pipe so that it is concealed by overhanging rocks. The water from this pipe can fall into the highest basin and then overflow into the others, creating a series of small waterfalls.
Construct the basins so that they tilt slightly into the rock garden face and will thus retain a proportion of the running water. If the front edge of each basin is perfectly horizontal, the water will flow over the entire edge to give a bolder effect.
The water must not overflow into the rock garden itself. Hide the basins’ sides with carefully-placed overhanging rocks, and give each basin a rough face so that it has a natural, weathered look.
It is best to make the basins of actual rocks, using carefully selected, flat-faced pieces that can be cemented together. Keep the surface of the cement in the joints well below the top of the rocks so that it is not seen. It does not matter if open cracks or joints are obvious, as these would occur in a natural formation. If suitable rocks are not available, basins may be made of waterproof cement.
Make the pool at the bottom of the waterfall an irregular shape, so that it will look more natural.
There are many variations possible in the design of a water feature: for instance, instead of waterfalls, a winding stream of water can be arranged to flow round the rock garden, starting from the highest point and running down towards a pool at the bottom. But the size of the stream should be kept in proportion to the rock garden itself.
MORAINE AND SCREE
There can be other settings for alpine plants, smaller and easier to produce than a full-scale rock garden, and often more suitable for a confined garden. Interesting and colourful effects can be achieved with a moraine — a bed of small stones and grit which is naturally sup-plied with water from below; or a scree — a similar but unwatered bed. Both features are found in Nature among mountains.
To form the base of either feature, use stone chippings. These should be at least 1 in. in size and have a few larger pieces of rock among them. Lay the clippings to a depth of about l to 1-½ ft. on a slight incline to give good drainage and a natural effect. Cover this with a thin layer of soil mixture for the bed, made of approximately I part by bulk good quality loam to 10 parts stone chippings; plus l part peat and l part very coarse, sharp sand. To provide water below the bed for a moraine, it will be necessary to place perforated Alkathene piping in position first of all and then cover it with the chippings, etc. Make a few holes in the piping. These should be small, as an excess of water should be avoided; the correct amount is enough to keep the soil just moist. Connect the perforated piping to a piece of hose-pipe so that it can be linked with a tap as required.
Note: In many districts, Water Boards will not allow such an arrangement to be permanently connected to the mains, owing to the danger of ‘suck back’ to the mains if a fault in supply or pressure should occur.
TROUGHS AND SINKS
Another way to grow alpines is inor sinks. Disused natural stone troughs can sometimes be bought, but make sure that there is a drainage hole in each one — preferably at the lower end. Very old troughs may require a thin rendering of cement to a depth of 1/4 or 1/2 in. to make them waterproof.
Slabs of walling stone can be built into very attractive vessels. Make the base of solid concrete to a depth of 2 in., and secure slabs of stone, which need be only 4 to 6 in. high, into 1/2 in. of concrete to make the sides. Insert a piece of round wood in the base concrete while it is still wet, to leave a drainage hole when it is removed.
Make the soil for troughs of equal parts by bulk of loam, leaf mould and sand. (This is a good mixture for plants which require plenty of drainage.) If necessary, add one part of peat to retain moisture.
At the bottom of the trough place a 2-in. layer of broken crocks. Over this place a thin layer of rough leaf mould, and then add the soil mixture, making it reasonably firm. Bury suitably sized stones in the surface, leaving about one-third of their bulk showing. Select and plant a few very small-growing alpines, and top off the soil round them with a layer of chippings of either limestone or granite.