Chapter 8. The Wall Garden

When laying out a garden on a piece of sloping ground, one will almost invariably be confronted with the problem of what to do with the banks which of necessity will be formed. They are usually made into grass slopes, which are difficult to keep well trimmed, and are consequently apt to look untidy if labour is at a premium. Alternatively, they may be planted with shrubs or may be covered with some trailing plant.

There is, however, a far better solution to the problem; the unwanted bank may be converted into a wall garden bright with colour and full of interest during the greater part of the year. The wall, too, will economize space permit of a larger lawn, perhaps for it may be nearly vertical, while the bank could not safely be built at a steeper angle than forty-five degrees. The wall, too, will enable the owner of a small garden, where there is no room for a larger rock garden on orthodox lines, to grow the ever-popular alpines and rock plants. There is yet another use to which the "dry" wall can be placed, and that is to form a boundary for the formal sunk garden; in this case the wall will be quite low, rarely more than three feet in height usually less, and should be made of stones thinner and flatter than those used for the ordinary retaining wall.

The "Dry" Wall

The "dry" wall, as it is called, is constructed of stones usually sandstone or limestone from two to eight inches in thickness. They may be of any size within reason, and untrimmed. Stones are better than bricks, for they provide cooler and moister root-beds for the wall plants. They should be bonded, that is, laid in layers so that the lateral extremities of a stone lie over the centres of the two stones in the row immediately below it. If the wall is to be over three feet in height, some means of strengthening the structure will be found necessary, unless very large stones are being used. This may be done by the use of ties, i.e., long stones built endways into the wall with their ends penetrating the bank. These should be in layers; the individual ties being some six feet apart in the layer, and one layer eighteen inches above the other, care being taken to have the centres of the stones above between those of the ones below, or, in other words, the stones should be staggered. This structure serves to keep the wall secure and firm. The stones should be long in proportion to their height, and are best when their upper surfaces are flat or even cupped. When placed in position they should be inclined slightly backwards, so that they are lower at the back than at the front, the larger and more weighty stones being kept at the base of the wall; the rain will then be collected and drained into the soil at the back of the wall to furnish moisture for the roots. No cement is used, but earth is rammed firmly into the crevices between the stones, sufficient mould being used to keep the stones about one inch apart, vertically; the greater the slope of the wall, the greater the amount of soil that may be used between the stones. This soil must be rammed well in to the back of the wall so that there is mould from the very front right though to the earth supporting the wall at the back. In addition, the earth should be well firmed after each row of stones has been laid, and no "air-pockets" must be left in the crevices.

The Compost

A mixture of good loam, cow-manure, and leaf-mould makes the best compost for the wall garden. The first essential is to provide a good foundation for the wall; this should be about ten inches deep and a shade wider than the base of the wall. Here the earth is rammed well down until a solid footing is provided. Now lay the first layer of stones, using the largest available, and place them so that their upper sides form one straight, horizontal line. If the stones are of moderate size, the gaps left laterally between them may be about two inches; the smaller the stones, the smaller the gaps between them. Next pack the vertical crevices between the stones very tightly with the compost of good loam and cow-dung. On this is placed the next row of stones, properly bonded as explained above.

The wall should not be built exactly vertical, but the top should incline slightly backward at an angle of about one in six, that is to say, in a wall three feet high the base will project six inches farther forward than the top. It is thus firmer and more easy to plant. The top of the wall is best left flat so that the rain may soak through.

The "Double" Wall

Fig. 5. Constructing the "Dry" Wall.

The wall should incline slightly back at an angle of about 1 in 6 from bottom to top; that is, in a wall 3 feet high the base will project 6 inches beyond the top.

The above remarks, of course, apply to the single wall, but when the wall is not to be used as a "retainer" (to support a bank), and where space will permit, there is no reason why the wall should not be "double" and have a face on both sides, each sloping inward towards the top. The space between the two walls would be filled with earth to form the rooting medium for the larger plants; each wall would thus support the other. There is no need to go into details of the construction of the "double" wall, for they are self-evident from the foregoing instructions.

Fig. 6. How to Plant the "Dry" Wall.

It is, of course, easiest to plant the wall as it is constructed; the roots may then be spread out as they should be, and can be well covered with soil. Larger plants may thus be employed than when the planting is done when the wall has been completed, at which time it is very difficult to insert the roots deeply into the crevices. It is never wise, however, to use very big plants.

If the roots are dipped into mud just before planting, they will establish themselves all the more easily.

March and April, when root-growth is very vigorous, are the best months in which to construct and plant the wall garden. The plants have then ample time to become fully established before the winter sets in. Wall gardens may also be planted in September, but the early spring is the better time, except in the case of dry, arid situations.

As an alternative to planting the roots in April or September it is, of course, possible to sow seeds in the crevices in spring. The best way to do this is to mix the seeds with a little well-sieved moist, sandy loam, and to press it into the chink in the wall. A small piece of moss inserted into the crevice will keep the seed moist and will prevent it from being pulled out by birds or by some other agency.

The plants, above all, must not be overcrowded. When planting, the gardener should bear in mind the size to which the plants will eventually grow, and he should be careful not to set a vigorous grower directly above some smaller-growing subject, for, if care is not taken in this direction, a slow-grower will soon be smothered by a more vigorous neighbour.

Selecting The Plants

In selecting the plants the locality, climate, and aspect of the wall must all be carefully considered, so that suitable subjects may be chosen. The list that follows should make this an easy matter. It will be noted that the great majority of the plants given in our list as suitable for the wall garden thrive best in the sun. A wall garden situated in the shade, however, can be made anything but drab and uninteresting. For the shady wall in a cool position such hardy ferns as the Aspleniums, Polypodiums or the Phyllitis must not be overlooked.

When planting the wall garden, it is just as necessary to have a scheme of colour grouping in mind as it is when planning the herbaceous border. It will, of course, be more difficult to carry this out systematically when planting as the wall is being built, than when inserting the plants in an existing wall; considerably more care is, therefore, needed in the former case. In planting when the wall is already standing, every effort should be made to set the plants with their roots well spread out, and so that they may penetrate well into the soil at the back of the wall. A little thoroughly-rotted cow-manure inserted with the roots will prove beneficial. A plant should never be placed at the top (a) (Fig. 6, page 91), of a vertical joint between the stones; its roots would then be likely to become dried up. Rather should it be planted in the crevice (b) just above the centre of a stone: that is, at the bottom of the vertical joint.

A good six inches of compost should be firmly rammed down on to the top of the wall, and in this soil, at intervals, should be placed large stones, for these will help to keep the soil in place and will furnish moist, cool sites for the roots of such plants as Rock Roses, Zauschnerias, Wallflowers, Geraniums, Antirrhinums or Snapdragons, Sedums, and Saxifrages all ideal subjects with which to crown the wall garden.

Using An Existing Wall

We have rather taken it for granted that the wall garden has had to be constructed. It must, however, be borne in mind that there is many an old wall, either of brick or of stone, that, although rather dilapidated, can with a little ingenuity be made into a delightful wall garden. Some mortar may have to be scraped out here and there, and it may be necessary to dislodge an occasional brick now and again and replace it with compost; but, provided sufficient soil can be inserted, almost any wall plant may be grown in it successfully. It must be remembered, too, that the smaller plants require very little soil indeed, especially those loving drier sites.

Care Of The Wall

In spring after the frosts of the winter, the wall garden should be, carefully looked over, for it will probably be found that in places the frost has raised the soil and has loosened the roots of the plants. These will need "firming-down" again, and fresh soil should be pressed firmly into the crevices to replace that which has fallen out. At the same time all vacancies caused by the death of plants should be filled with fresh specimens from the nursery or reserve garden.

Plants For The Wall Garden

NOTE. For Fuller Cultural Details, see Chapter 12, Alphabetical List of Plants for the Rock, Wall, and Paved Gardens.

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