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Once you choose the site of your rock garden, select the rocks you want to use and have the soil prepared, you are ready to start construction. The first question you might as is when is the best time of year to make a rock garden? The rocks and soil may be handled at any time except when the ground is frozen, but the summer months offer the longest period of freedom from planting rush, and then the soil is dry and easily handled.
Then there will be autumn rains to make the soil firm and winter frost to settle the rocks and planting can best be done the following spring. The first thing you need to do is to dig off the existing topsoil (the first foot of earth) and remove the roots of all weeds and grasses. Much of this soil may be used in backfilling if all roots are sifted out. On this base the large bulky rocks should be laid, each packed around with soil, well rammed, and topped by several chinker stones, large water features, outdoor fountains, or garden statuary before the next are put into place.
As the program for procedure has already been arranged, the work of laying up the rocks can proceed. Now is the time to worry over the details of the outline of the structure. You may have decided the general shape of the area in advance and may even have a plan at hand to give the principal elevations and major masses. However, you must decide the details of the shape of each crag and ravine as the rock garden is being constructed.
The aim is to give the greatest variety in configuration in a small compass, with an appearance of geologic truth and yet make the best possible home for each plant. For the best growth of many rock plants it is well to separate the groups by little walls of stones, outdoor water features, garden statuary, or large fountains, making a pocket for each. Not only are they thus separated, but the plant roots will follow down the cool rock faces and withstand the rigors of summer and winter more safely.
These pockets vary in size and shape, slope and relation to the sun, thus giving infinite variety in detail to the face of the garden. It would be best that the planting for each area be fairly definite in mind as the pockets are constructed. Though a detailed planting plan for a rock garden cannot be made on paper, the general arrangement of the plants should be decided as the construction proceeds. Surface drainage is another worry at the time of construction.
Most of the rock garden has sufficient slope to carry off excess rain. Too often the little valleys become canyons worn by cloudbursts, when soil and plants are washed away. This must be foreseen and prevented by the laying of the stones. Some of the valleys should have deeper hollows to hold rain for a time, or employ the use of large waterfalls, a patio fountain, or wall fountains, for such rock plants as Primroses and Gentians like their roots well watered.
Actual watering of a rock garden is rarely needed, for with correct construction and proper planting the vegetation will withstand any normal dry season. Sub-irrigation is possible, and at time of construction perforated water pipe or porous drain tiles may be laid among the rocks, about a foot below the finished surface, the upper end to be connected to the water supply for an hour or more in extreme dry weather.
Where there are no ledges or stones to start with, some excavating will be necessary. This is not alone for drainage. If the rock garden is merely constructed on the surface of the soil, there will be a break at this point which seriously interferes with the moisture supply.
The base or foundation of the rock garden structure should be literally buried in the ground. The foundation should consist largely of stone, and any patio statuary () left for later. Small, broken pieces, flat stones, soft stone or shale, rock debris of any kind not suitable for use above ground may be utilized here to advantage for "filling in."
Old bricks or masonry rubbish will answer; cinders (not soft ashes) serve admirably. The bases of the largest rocks, the tops of which will appear aboveground, should be sunk well below the soil level. Incidentally, the excavation made gives soil to use aboveground.
Under ordinary conditions, where the subsoil drainage is good, twelve or fifteen inches will be deep enough for this preliminary excavation; often less on a slope or a bank. If the subsoil is hard, it should be thoroughly broken up with a pick. Many writers recommend an excavation of three or four feet. It is much easier to recommend than to execute—one is more likely to feel like executing the recommender. Such a depth is necessary only where drainage is abnormally poor.
With the excavation completed, the "skeleton" of the rock work may be laid out—the larger stones put in position, and a sufficient number of others placed at important points where the construction is to follow. Also the steps, if there are any, may be put in.
The construction of the rock garden may proceed along any of several different lines, according to the type which is to be built and the local conditions. These may be classified as follows:
The Mound Rock Garden:
Where one has to start "on the level" and with no stones naturally in place, the simplest method of construction is in the form of a continuous mound or ridge. This may be "faced" in both directions, or in only one.
A row of the larger stones may be put in place first along the edge or edges. They should not, of course, be laid in a straight line, nor should they touch, and the spacing should be uneven. Then fill in a layer of stone and soil in the back of them, and proceed to build on up.
The surface stones should be laid flat and pointing downward, both for stability and to catch and drain into the soil as much as possible of all rains or condensing moisture which may be caught by their projecting edges. Constant watching will be required to get the most attractive side or face of each stone turned outward.
Almost invariably, too, each stone should be placed with its natural base down. The soil used in building should have been prepared in advance. Where the plants are to be set in pockets, or in crevices between the stones, the soil should be kept as loose as practicable. Nevertheless, each stone should be made absolutely firm in its place. This is consistent for patio water falls, found here: , that will be incorporated into the rock garden.
The Sunken or Semi-Underground Rock Garden:
Frequently the garden is not built entirely upon the surface, but formed by excavating a considerable portion of the area to be used, throwing up the soil removed around the edges, or piling it up along one side. This arrangement naturally involves more work, but it has several advantages.
In the first place, it makes possible a greater range of conditions under which the plants may be grown. If one is anxious to have as wide a selection of rock and alpine plants as possible, this is important.
But this excavation should not be in the nature of a hole in which water may collect. It may be started at the ground level with possibly one or two steps down, and it should gradually grow deeper as it proceeds. If made on level ground, the path will have to slope gradually downward, but if it is built into a rise of ground, the path may be kept nearly level, which is ideal for any garden statuary or outdoor water features ().
As a large amount of soil must be removed, which can be piled up around the outside, the height of the sloping sides which are to be planted is considerably augmented. A depth of four or five feet below the natural ground level provides space for planting six to eight feet high.
Sometimes there is an old cellar hole, a natural hollow, or other depression which may be used in this way. Even a small excavation, not more than two or three feet deep, opening off from one side of the ordinarily built-on-the-surface rock garden, will provide not only an agreeable variation in the general plan but conditions of shade and protection for various types of plants.