There are few features in the garden that provide such a variety of interests in so little space as a well-planned and carefully planted rock garden. The smallest plot may contain a rock garden which will house a representative and charming collection of alpine plants; but, on the other hand, there are few features in the ordinary garden that are so neglected and so ill-understood.
It must be remembered that the chief function of the rock garden is to provide the plants grown in it with conditions, so far as possible, similar to those existing in their natural haunts.
Most of the alpines have to withstand considerable drought during the short, but parching, summer months. For this reason, and because there is often but scanty surface soil, the majority of them have long, running roots that can penetrate deeply into crevices among the rocks and thus draw moisture and nourishment from below.
Deep crevices, packed with a rich, well-drained mixture of soil suitable to the plants, are, therefore, of primary importance in the rock garden. The horrible mass of shiny, glazed lumps of brickwork in the cracks of which half-starved ferns and plants struggle for existence is nothing but a disfigurement.
The chief uses of the rocks and stones in a rock garden are the provision of coolness for the roots and the provision of moisture in crevices for the use of the plants when required, and this is not in winter when water would rot the plants and not grow them. But the idea that rock plants grow best in practically nothing but rock is a mistaken one. A generous allowance of good soil between, amongst, and beneath the stones is essential for the healthy growth of the plants.
As the function of the rocks is to provide shelter for roots, it is clearly useless to plant slabs of rock or stone perpendicularly in the soil unless by so doing the stones are very close together and tightly packed where possible with the mixture. The stones should not be planted like monuments in a cemetery. The roots must get down beneath them, or otherwise they do not preserve any moisture.
For the beginner, the easiest way is to use large masses of stones, two or more feet in length and six to twenty-four inches in depth, where possible, and they should be sunk well and firmly in the earth in a slightly slanting direction tilted backwards, not forwards, so that the rain may trickle down to the roots of the plants and quickly get away.
If the rocks lean forward, over the plants, the roots will be sheltered from the rain and probably parched. Although the visible portions of the rocks in the garden should be as pleasing as may be to the eye, and should all slant in the same direction to represent a natural outcrop or stratum of stone, it should never be forgotten that they are not there for the sake of picturesque effect only, but to protect the roots of the plants growing among them.
The slopes of the mounds in which the boulders are set must be as natural in appearance as possible; there should be miniature ranges and mountain peaks and, dividing them, valleys into which spurs from the hills project. Winding paths, eighteen inches to two feet in width, with stepping-stones, should be cut through these gorges so that every part of the rock garden may be easily accessible.
The pockets in which the alpines are to be planted should be irregular in shape and may vary from a few inches in diameter to as many feet across. Their surfaces must not be flat, but sloping to afford drainage. They must provide ample root-run and should be from a foot to eighteen inches in depth, and so constructed that the soil will not wash out of them. If there is any chance of the soil in the pockets becoming sodden, six to twelve inches of clinker and rubble drainage must be provided.
Much more can be said about building a rock garden, but the above pointers are a good start. Happy gardening!
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