This section is from the book "The Gardener V2", by William Thomson. Also available from Amazon: The New Organic Grower: A Master's Manual of Tools and Techniques for the Home and Market Gardener.
THE making of rockwork on which to cultivate rock and alpine plants, and also as a feature of taste and variety in the garden, is a labour often undertaken, but one which is very difficult to execute. Legions of examples of rockwork are to be found all over the country, - indeed, in most gardens, - in more or less good or bad taste, from the offensive heaps of vitrifactions from brick and glass works, to the admirable imitations of stratified rocks on a large scale executed by Pulham, with the assistance of the mason's art and plenty cement. Indeed, so good is the imitation that one might fancy the quarry brought bodily into the garden, as they move houses in America. The best specimens of artificial rockwork are generally to be found where the natural rock and rock scenery is most abundant - where Nature's materials are at hand, and where Nature is near to show the way. The worst specimens, on the other hand, are to be seen where the difficulties of site and materials are the greatest - where the products of the glassworks are pressed into the service, and where the site would be more suggestive of a brick-yard than of living rock. I think one grand flower-show at least can be remembered - an international one - where the rockwork was done in coke.
Between Pulham, on the one hand, and the coke rockery on the other, there is room for many varieties of the art and mystery, and the gap is well filled up. When a rockery is to be made, there are two primary objects in view. The first is, that the fabric to be constructed shall be pleasing to the eye, and harmonise with the surroundings, - that it should not look toy-like, too artificial and incongruous, but be an agreeable feature in the garden; and the second is, that it be so constructed that it shall be suitable for the growth of the plants desired to be cultivated. A rockery may be a first-rate imitation of a real rock or cliff, showing the dips, stratification, and other features, and yet offering but small facility for the introduction of plants, thus favouring one of the objects at the expense of the other - just as magnificent conservatories are sometimes built in which plants can only languish.
In order that a rockery may not be an offensive or incongruous object, the site must be well chosen. Pyramids of stones on a flat surface, an amphitheatre, symmetrically arranged, fronting a shrubbery or in the corner of a terrace, or in any geometrical position adjoining buildings, are instances of positions where rockwork would be out of place. But given an abruptly sloping natural declivity - if adjoining water all the better; and if the aspect can be varied by bending round an angle, better still - there a rockery may be made. The site may even be artificially improved in deepening the slope, by adding to its height with soil or planting. We do not think that it is at all necessary that a rockery should be an exact copy of nature in any of its forms - either of stratified rocks or the many forms in which the detritus of rock is found. A tasteful piece of rockwork may be made without any very close imitation of nature, - just as a landscape can be thrown on canvass by the painter which at once pleases the eye of taste, artificial though it be, by the truthful look there is about it.
A certain amount of Nature's guidance must be admitted, as a matter of course; for instance, irregularity and variety, as if by chance, an orderly disorder - like Hamlet's madness, "there must be method in't." The materials must be natural fragments of rock - not water-worn stones, or stones with every indication of the quarry about them: unstratified rock we like the best. These can be so arranged that an endless variety of little terraces and recesses, and pockets and ridges, from base to summit of the position, can be made to suit any variety of plants, from the shallow-rooting Saxifrages to the deep-rooting Gunnera. Overhanging ledges can be made, with recesses for shade-loving plants. In short, in the construction, our second object proposed - that of preparing suitable positions for the plants desired to be cultivated - must ever be kept in mind. The most of our European alpine plants require a permanently moist soil, as every one knows who has trudged the mountain slopes in search of plants; although many do prefer a scanty soil on cool rocks, - which means that no position exists which may not be selected by some plant where to live and thrive, as witness the stone-and-lime wall seen from our window, covered with Aspleniums, and Scolopendriums, and Cete-rach, and Drabas, and Linarias, and many more things equally green, if with names more homely.
Some require the cool moisture of running water always among their roots, as the Parnassia, Narthecium, or Pin-guicula; and if a trickle can be conveyed over some part of the rockwork, it will be the one source of success in many instances. Some will require a sharp shingly soil, as the Dianthus, which can be supplied in the shape of the smaller broken chips of the rock, giving an opportunity of varying the construction of the surface. Some plants require a stiff holding soil, such as the Primulas, which can be supplied in little flat terraces formed to catch all the rains. Others, as the dwarf Veronicas, such as taurica, will thrive on dry sloping ledges: the various Thymes, Aubrietias, Erodium, and some Geraniums are of this class. Many of the smaller shrubs are most appropriate as rockery plants. Ivies of the finer class, such as the variegated, may be made to creep over large blocks, or the pretty Ampelopsis Veitchii in the same way. Muhlenbeckia complexa, which has stood the late winter on the rockery, is a choice plant for fringing a ledge. The Kilmarnock Weeping Willow, in a dwarf form, will spread its long slender branches over a peak of rock, as well as the Savins and Periwinkles, Helianthemums and Genistas, and many more of a woody nature.
In writing the foregoing, we have supposed the formation of a rockery on a somewhat enlarged scale; of course, the extent and form of the rockery must be determined by the requirements of the owner, and the nature of the position to be occupied. It may be of a curved form either outwards or inwards, it may be of a long ridge shape and undulated, or it may be in the form of a group of hills and valleys, and need not after all cover an extravagant area, and need not require by any means an extravagant amount of stones. One of the best managed of this last kind we have seen is in the grounds of the Down House, Dorset - the scale not very large. Whatever be the shape, if the rockery has any pretension to be anything more than a toy by the side of the walk, provision must be made to give access to the different parts, in order to be able to clean it and attend to the plants, and particularly that the owner may conveniently at any time enjoy an examination of his plants. This can be managed by introducing a pathway, curving and undulating about, among the compartments of the rockery. These pathways can be made a feature of the rockery itself, and far from spoiling its appearance, can be made an improvement.
The pathway itself, of course made with pieces of rock, should be planted with Sedums, or Stellaria, or prostrate Pyrethrum, or any creeping plants not easily destroyed.
Sometimes a rockery can be made more imposing, if the scale is large, by having groups of the smaller Pines planted on the higher parts, such as Pinus Pinea, Abies orientalis, or any of the dwarfer pendulous forms of Abies excelsa, or plants of the common Sumach, Spiraea Lindleyana, or indeed any of the Spiraea, where they would not interfere with the wellbeing of the choicer plants below.
When a rockery abuts upon grass, it is always well to have a narrow margin of rough gravel following the undulations of the base, for the sake of tidiness.
The Squire's Gardener.