Designing the Rock Garden

‘What is a rock garden?’ People certainly have different ideas – some of them rather odd. Again and again I find that house owners have made a heap of soil say, in a shady part of the garden; have covered it with brickbats, old stones, pieces of marble and the like; and then have planted up this peculiar ‘excrescence’ with ferns, perrywinkle, an odd primrose or two, a couple of aubretia, and perhaps some Chinese lanterns and then have taken me down the garden to show me this ‘atrocity’ with pride.

A rock garden should be made naturally. It should be constructed with care and with the right environment. As invariably the root system of the rock or alpine plant is eight or nine times greater than the part above ground there must be a good body of soil below. Other plants will exist on the ledge of a rock in almost no soil at all; they are the kinds of plants that absorb the bulk of their nourishment from the atmosphere. Yet other plants grow in pockets that are made up of well-rotted vegetable refuse mixed with particles of flakes of stone or rock. Others grow in a position where they are completely sheltered from winds. The good rock garden will provide all kinds of positions and conditions in which the different types of plants will grow.

No rock garden should be made up of root stumps, bricks, and clinkers. It should be constructed of good weathered rock, preferably indigenous to the district. These rocks should be laid in position so that they look natural. A rock garden can give a sense of height, depth and distance, if the rocks are carefully placed. Its original cost may appear to be high but if this capital cost is spread over a period of years, it will be found to be cheap as compared with other forms of gardening.

Remember that the rock garden is being made to accom-modate plants that like growing under particular conditions. The very word ‘alpine plants’ give the secret of the situation. Here are plants that come from the Swiss Alps or from the mountains of China or Peru which grow where they are covered with snow throughout the winter and then in the summer the sun beats down on them. They are used to having ice-cold water at their roots, and though they are covered with snow in winter the cold is dry and so they live through the frosty weather. So often they hate our moist winters and have to be protected.

The good rock garden enables the gardener to have a large number of different kinds of dwarf plants in a comparatively small area, and when a rock garden is carefully planned it not only looks bright in the spring, but it can be very colourful at other periods of the year. If it is planned in such a way that it is slightly raised, or at any rate certain parts of it, the hoeing and titivation can be done without continual bending. Very often, too, because Alpines like growing in barren soil, weeds do not flourish.

The Position

Have your rock garden in a sunny place, for even if there are some Alpines that prefer shade, the big rocks you will use will certainly provide such spots. Never have the rock garden under trees where there will be a perpetual drip of moisture from the branches. Choose a high level if possible, but if a low spot has to be chosen see that the whole area is perfectly drained. When making a rock garden in a town where the area is small, make a mound of soil, with varying contours, on the south side with a path skirting its irregularly shaped outline. It is always a good thing to make the rock garden a separate feature where possible and to have a narrow bed of flowering shrubs for instance used as a division.

The Stone

As has already been suggested it is a very good plan to use the natural stone of the district. Those in Kent can use the Kentish Rag; those in Surrey and Sussex, the typical sandstone; those in Devon and Cornwall and parts of Wales, the harder stones like granite (though these are never so good); those in Somerset the weather-worn limestone or those in Derbyshire or Cumberland, that very attractive limestone that is both water- and weather-worn. Where there is no particular ‘county stone’, imported rocks are used with great effect. There are many rock gardens of Derbyshire limestone, for instance, all over the country.

If you can use local stone, the work is considerably cheaper. Remember when buying stones that three-quarters of the bulk of each one of them will never be seen and they should have quite a massive appearance before being laid into position. I have known, in big gardens, for there to be two or three stones that weigh 500 kg (J ton) or more each and all the remainder to weigh 150 kg (3 cwts) each; even for quite small rock gardens the two or three main stones should weigh 250 kg (5 cwts), and the others, say, 50 kg (1 cwt) each. I mention these facts because I want to emphasize the need for using good rocks, and not little stones.

Using the Rocks Intelligently

It is not necessary to start with a mound or sloping soil because the lower stones, when placed in position, may give some hint of a need for further height, so soil may have to be brought to that point. You must start bedding in the rocks at the lowest point and build upwards. They must be laid in position so that they look like a natural outcrop. They should be given a slight backward slope so that the rain gets down to the roots of the plant; then as further rocks are put in position, they will remain at the same ‘tilt’.

The general slope of the strata should then be followed right through so that the whole thing looks natural. Look at the face of the stone to be exposed and see if it is covered with any particular markings. Look for what we gardeners call graining and the weather markings. Watch for colours and try to blend all these into one whole. Sometimes there are vertical marks produced by rain and frost which may soon determine whether the stone is in its right position or not. In limestone rocks there are invariably horizontal lines which show the strata, and how much better it is to use rocks that have been weathered rather than those that have been quarried straight out of the bowels of the earth.

Ram soil tightly at the back of the rocks to keep them in position. See that pockets are provided and in some cases quite large areas for the plants you are going to grow. When rocks are to be placed one above the other, separate them by using hard stones as if they were ‘pit props’. By keeping one large boulder 100 mm or so (a few inches) off the other it prevents the soil that should be between them from getting squashed. See that the upper rock stands from the lower one and does not completely overlap or overhang it. Above all, make the rock garden look natural. Don’t have the stone sticking out higgledy-piggledy anywhere.

Do not make the mistake of trying to get tremendous elevations and deep depressions into a space of 17 m2 (20 sq yds). You only need slight undulations and a gentle slope. Don’t make the mistake either of not being able to get at your plants. Make it possible when building, to gain easy access to every part of the rock garden for purposes of weeding or giving other attention to plants. Always remember the root run. This must be the first consideration. There must not be stagnant moisture below. Rank animal manure must not be used.

A spot must be provided for the lime haters where a special compost can be used consisting of two parts of acid soil, one part of peat and one part of silver sand, to which can be added a good helping of granite chippings. When other pockets have to be made up, these may consist of six parts of good soil, one part good horticultural peat, one part of rough silver sand, one part of limestone chippings and one eighth part mortar rubble if available and if not, ground limestone. In the case of primulas and gentian it is better to use six of peat and one of soil and no lime.