The pick of the low shrubs are the charming Daphne cneorum, which flourishes better for being lifted above the ordinary garden level, and Azalea amœna. The latter, however, should be so placed that its trying solferino does not make a bad color clash. Rhododendrons and mountain laurel fringe a rock garden well, and with one trailing juniper (Juniperus procumbens) will provide a great deal of the refreshing winter green.
Single roses, the species, fit in well where there is room for them. Good ones are R. setigera, R. rubiginosa, R. Wichuraiana, all rampant, and the low R. blanda. The roses would better be at or near the entrance or exit, or far enough above the rock work not to ramble over small plants.
The plants in this list cover all seasons and vary somewhat in their soil and moisture requirements. But the variation is nothing beyond the ordinary garden knowledge. Most will do better if their preferences are considered, but none is apt to perish with average care.
Alpines, as a class, would better be left to the amateur with the time, money, and disposition to specialize. Most of them take kindly to being transferred from a mile or more up in the air to sea level; the edelweiss, for one, grows here readily from seed, and the exquisitely beautiful Gentiana acaulis thrives in American rock gardens. But, on the whole, alpines do not do as well here as in England, where the summer climate is not so hard on them. When they flourish here, it is at the cost of a great amount of professional care.
THE WALL GARDEN
A wall garden is a perpendicular rock garden. But whereas a rock garden is of all things irregular, a wall garden has regularity. The wall need not be a straight line; it is better that one end should describe a curve, and rocks at the base may give it further irregularity. Yet it can never quite lose the air of man's handiwork. The prime object of the gardening on it is to reduce this air to a minimum.
The way to make a wall garden is to build a dry wall of rough stones—that is, a wall without mortar. Instead use soil and pack it tight in every crevice as well as behind the stones, which should be tilted back a little to carry water into the soil. This tilting may be accomplished with small stone wedges. The best kind is a five-foot retaining wall, as there is then a good body of soil behind to which the roots can reach out through the crevices. But a double-faced wall may be made, if the situation demands it, by constructing parallel lines of stones and filling in solidly with soil.
Although the face of the wall in either case may be strictly perpendicular, it is better that each layer should recede a bit. Construct it after the manner of the rock garden, laying the stones so that the top will be level, or approximately so.
In planting also, follow the same rules. It is better to plant as the work progresses. Either plants or seed may be used. If it is seed, press carefully into the soil in the front of the crevices. Small seed may be mixed in thin mud and this plastered on the soil. For a tiny crevice make a pill of the mixture.
The range of reliable plants that do not call for special care is not great so far as the crevices are concerned. All the stonecrops, the house leeks, Arabis albida, red valerian (Centranthus ruber), aubrietia, Alyssum saxatile, snapdragon, wallflower (Cheiranthus Cheiri), Kenilworth ivy, Viola tricolor, Dianthus plumarius, and Dianthus deltoides are all very serviceable. Behind the wall, at the top, a strip of earth should be left and there a wider variety of plants can be grown. Single Marguerite carnations and grass pinks will form a sort of cascade of foliage and bloom there if planted close to the wall or in the crevices of the top, and a similar effect, but much bolder, can be created with the perennial pea (Lathyrus latifolius).
If the dry wall is already made, the crevices can be plugged with soil if care and patience are used. Even a cemented wall is not hopeless; here and there the mortar can be chiseled out and an occasional small stone should be removed.
A wall garden has these advantages over a rock garden; it is more easily constructed, it is of practical use, and it is sometimes a possibility where the other is not.
WATER AND BOG GARDENS
Neither the water nor the bog garden is dependent on rocks. Either or both, however, may just as well be an adjunct of the rock garden. They solve the wet spot problem admirably, permit the culture of native water lilies, orchids, and numerous other beautiful plants, and certainly contribute their share of picturesqueness. If water is lacking, it may often be introduced at little expense.
In most cases it will be found that some cement construction is necessary, but not a bit of it should show. This is easily managed by building a cement shoulder on the sides of the pool or stream a little below what will be the level of the water, and then setting rough stones on that. A cement bottom for shallow water may be disguised by imbedding pebbles and small stones in the cement before it sets.
Dispose the rocks very irregularly, but they may be so few as to be mere notes. Avoid stagnant water, and if mosquitoes are feared introduce some goldfish. They like mosquito larvæ.
Water lilies and sagittaria—one plant will do if the pool is small—in the water and near it, but not in standing water, Japanese iris, yellow flag, globe flower, and Lythrum roseum are good selections. Forget-me-not is one of the finest plants for the banks. Use the perennial kind (Myosotis palustris semperflorens).
The bog garden simply reproduces bog conditions. As a rock garden adjunct it may be a small spot with the perpetually moist and moss-covered soil in which the native cypripediums and pitcher plants flourish. Eighteen or twenty inches of suitable soil, a mixture of leaf mold, peat, and loam, in which has been stirred some sand and gravel, must be provided. If an artificial bog, the bottom may be made of cement or puddled clay.