This section is from the book "How To Make A Flower Garden", by Wilhelm Miller. Also available from Amazon: The Well-Tended Perennial Garden: Planting and Pruning Techniques.
SOME of the most exquisite gems in the vegetable kingdom grow above the tree-line on mountains. These alpine plants are of low and compact growth, herbaceous or succulent in character, and produce flowers of exquisite beauty and colouring. They grow in crevices or in pockets, often overhanging and completely carpeting projecting ledges of rocks. The attempt to cultivate these plants has led to the making of rock gardens, where they can be provided with all the conditions under which they grow naturally, except, of course, altitude, which is of the least importance. Many of the rarer alpine plants cannot be grown successfully unless these conditions are given. But a rock garden may be made to provide a home not only for true alpinc plants, but also for a large number of interesting plants of small stature from much lower altitudes. A well-constructed and tastefully arranged rock garden can be made one of the most interesting features of a country home. Meaningless mounds of stones too often seen in gardens and in public parks are by no means the best conception of a rock garden. The rock garden should be a close imitation of a rocky mountain, though, of course, on a smaller scale.
It should have crevices, pockets, and overhanging ledges, and these should be filled with soils to suit the requirements of the different plants; some need loamy or peaty soils, while others require a large proportion of crushed rock. As few gardens contain a natural rocky bank or hillside, most rock gardens are, therefore, "artificial," in the sense that they are made by man, but there is no form of gardening in which one has more opportunity to give expression to his natural taste than in the construction and planting of a rock garden.
There are two kinds of construction, the "open" and the "underground. " The open rock garden is made from a natural bank or hillside, and very attractive it can be made, especially if the bank or hillside skirts a lawn and is entirely free from the roots of large trees.
The "underground" rock garden implies a cutting made below the natural level of the ground. This method is adopted when a garden does not contain a natural bank or hillside. The site chosen for an underground rock garden must, of course, be governed by circumstances, but, if possible, it should be in a secluded portion of the grounds, and near the boundary rather than in the center of an open place, if it can be avoided. The size should then be determined and the top soil all removed for replacing when the cutting is complete. There should be a central path of not less than five feet in width, and this should wind in such a manner that the cutting shall produce a variety of aspects to suit the requirements of the different plants. The cutting should begin at one end, being shallow at first, but gradually deepening until it is six or eight feet below the ground-level. All the soil taken out should be placed above the sides of the cutting, to still further increase the height from the path through the center.
The cutting should not be made regular, or smooth, but should present an uneven surface, with occasional mounds and depressions of various sizes. After the rough outline is formed, the top soil should all be placed evenly over the whole surface. The rocks may be then placed in position; the kinds used do not greatly matter. Sandstone is perhaps best, though very beautiful effects may be made with common boulders. Cut stones or stones with flat surfaces should not be used, as they detract from the natural appearance a rock garden should possess. The rocks should be of various sizes, and arranged so that their most rugged sides are seen. Like the rocks on the mountainside, they should occasionally stand out boldly, almost perpendicularly with the edge of the path, then withdraw into hollow recesses; but they should always provide crevices, pockets, and ledges for the reception of the plants. If a rock garden is extensive enough, a cascade will greatly add to its attractiveness. Rugged stone steps leading up to a sinuous path among the rocks on the upper part of the rockwork, with seats placed at intervals, may be introduced, and will add greatly to one's enjoyment of it.
Spring is perhaps the best time for planting the rock garden, though early fall is also good. The pockets and crevices should be so arranged that the water does not run off too readily; at the same time it is necessary that they should have good drainage, if the soil below the rocks is of a clayey nature. They should be well filled with soils to suit the requirements of the different plants. For members of the heath and orchid family a peaty soil is best; for the mossy and starry saxifrages, low-growing sedums, and sempervivums, and plants of a more or less succulent character, a soil largely composed of crushed rock or gravel is best. Plants belonging to the primrose and lily families will do best in a soil consisting largely of leaf mould, while for the cruciferĉ, composite, and most other families, a good loam is all that is necessary.
An "open" rock garden on the Stokes estate at Lenox (see page 161).
It is a mistake to plant a rock garden too thickly. Each plant should have ample room to develop without encroaching on its neighbour, and those plants which have a tendency to spread unduly, or cannot easily be kept in check, should be rigidly excluded from the rock garden, no matter what other good qualities they may have, for it is difficult to eradicate them from a rock garden when once they are established.
The matter of exposure requires careful study. The true alpines are better confined to the northern or northeastern aspects, where they would be protected from the midday sun. Plants of a succulent nature, such as the low-growing sedums, may occupy the sunniest positions. The more delicate alpines, such as the alpine primulas or androsaces, should be planted in sheltered nooks. Cerastiums, aubrietras, dwarf phlox, and plants of a similar habit, should be planted to overhang ledges of rock, while the starry saxifrages and sempervivums may occupy holes or crevices in the rocks. For the steepest places, or where it may be difficult for some plants to obtain a foothold, the wild ginger, Asarum Canadense, Arenaria Balearica, or plants of a like habit, are excellent. On the top of the rocks, at the most conspicuous points, or at the turn of the path, may be located such stately plants as Acanthus mollis, Spiraea Aruncus, or a small group of Aquilegia Canadensis. A small border, varying in width, and edged with small rough stones, looks well at the foot of the rocky slopes, and not only serves as an edging for the path, but will accommodate many plants, such as the dwarf composites, ajugas, or Iceland poppies.