Before outlining the actual procedure of rock garden construction, here is a word of advice to those who hand over this feature to experts. It is best to let the craftsman on the job choose a style; but it is also wise to have some idea, beforehand, as to what you want from the rock garden. If you can tell him that you want just a picture, or that you want a home for rare alpines, or that you want to be able to walk between the rocky ledges and valleys, his task will be easier.
Further, don’t expect an established, picturesque appearance immediately. Rock gardens, even when new, may look reasonably well, but such characteristic items as dwarf shrubs and drooping cascades of flower and foliage must be waited for, just as you must wait for thein the herbaceous border.
If you insist on an immediate picture, you will soon have, as in so many gardens of my acquaintance, a collection of overgrown shrubs, far too large for their positions, and possibly quite unsuitable in character. An immediate effect is made by planting young, quick-growing conifers in place of the dwarf varieties, or by planting small potted shrubs of the kind that very quickly become giants. Even if you could keep them to correct proportions by regular, you would find that the would penetrate through the rocks to such an extent that other plants would suffer.
Therefore be as explicit as possible with your instructions to the rock specialist, leave it then to him, and above all, be prepared to be patient over immediate results.
Now for the process of rockery construction. The first and essential step is to decide roughly on the general contour which will suit the garden. If water is to be used, it must be decided
where it is to run from and where it is to run to, and pipes must be laid accordingly. When company’s water is to be used, instructions should be given for a supply, with tap, to be taken to the intended highest point of the rock garden.
The next step will be rough excavation and building of the contours desired; but as this proceeds, the top layer of fertile soil must be kept on one side. It is wasted if buried under infertile subsoil. Large stones and other porous rubbish might well form the foundation of the rockery hills, as they help to form, and good drainage in the rock garden is very important indeed.
Paths through the rock garden will be constructed as the building-proceeds. Paths in this case are really beaten soil tracks, top-dressed with stone chippings, with perhaps an occasional flat stone or stairway of stones, to make for ease in walking up and down the steeper slopes. Granite or other chips make excellent surfacing material for these paths, not only because they are picturesque, but also because many of the rock plants grown in the soil pockets will tend to creep in through the stone chips of the paths, and soften the edges. If several steps are used together, the crevices between will, of course, be set withor ferns according to aspect.
Before beginning to set any rocks in, a quantity of good soil should be prepared. The ideal mixture for most parts of the rock garden is made by rubbing old -mould and old stable manure through a coarse sieve, and mixing it with coarse sand or gritty soil. Certain lime-loving plants do best if some crushed mortar rubble is added, and certain peat-loving plants like peat in the if it is obtainable, though generally the peat-lovers will do well in any mixture free of lime.
Do not be afraid that light sandy soil will wash away from the necks and roots of the plants. Perhaps it will, but the plants in their native haunts grow in just such soil, and rains do wash it away frequently. Rains also bring down from upper slopes enough new soil to take the place of that which is washed away. The rock gardener performs the same service when he top-dresses—as he must do every now and then—with fresh soil.
In the initial stage of rockery construction it is not necessary to use artificial fertilizers in the compost, though it must never be presumed that all rock plants will thrive in poor infertile soil. Where flowering plants are to be grown, bonemeal can, if desired, be used in the compost, as it is slow-acting, and encourages flower production.
In addition to a supply of mixed compost, you need to have by you a quantity of small stones or breeze or other porous material. You will find this useful to fill in at the bottom of very deep pockets, ,so as to keep the drainage free in all parts of the rock garden.
When an artificial stream is planned, or a small cascade of water runs from pool to pool down a steep slope, this part of the rock garden should be constructed first. It is almost impossible to give detailed instructions, since so much depends on the kind of stone, the kind of site, and various other points that differ widely in different gardens. I have in mind at the moment a small rock garden made by an amateur on a flat site.