The first step in the actual construction of a rock garden is to lay it out, at least in sufficient detail to show its general contour, to indicate paths and so on. These points had best been marked by fairly stout stakes driven into the ground, as otherwise they are likely to be obliterated.
Where there are no ledges or stones to start with, some exciting 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 interposes with the moisture supply.
The base or foundation of the rock garden structure should be literally buried in the ground. The foundation should consist of a large piece 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 existground.
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 drain is abnormally poor.
With the excavation completed, the "skeleton" of the rock work may be laid out in 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 covered by their projection 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. Neverheless, each stone should have made absolutely firm in its place. This is consistent for patio water falls that will be incorporated into the rock garden.
The Sunken or Semi-Underground Rock Garden:
Frequently the garden is not built entirely up the surface, but formed by excavating a reasonable portion of the area to be used, throwing up the soil removed around the edges, or piling it up along one side. This arrangement typically 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 set near 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 sliding sides which are to be planted is significantly 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 cell 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 acceptable variation in the general plan but conditions of shade and protection for various types of plants.