A fully developed estate to-day is not complete without an interesting rock garden, not because it gives an interesting physical variety to the landscape, but because it provides an opportunity for the development of one of our most interesting groups of plants, those plants which grow their best and prove most interesting in a miniature landscape of this rocky character. These gardens have been developed to perfection on many English estates.
The group of plants valuable for the development of rock garden work is comparatively little known to the amateur, and yet there are used in rock gardens many interesting types frequently used for other purposes. It is true that many of the plants grown for rock gardens are very dwarf in their habit of growth and much more sensitive to changed conditions of soil and exposure, and that many of them therefore require expert labour for their normal development.
The most interesting group of plants, perhaps, for rock garden work, includes the plants known as "alpine" plants, which are low-growing, very dense, and compact in their habit of growth. Most of these plants have small leaves and the flowers are rather brilliant and marked in their colours. The term "alpine" plants to-day is applied in its general use to that dwarf and low-growing group of plants which have a tendency to compactness of habit, and which in their mature form of development seem to fit into the confined atmosphere of the average rock garden. The true rock garden plants may perhaps be the "alpine" types, but those plants which landscape architects use to-day for rock garden purposes include not only the "alpine" types but many small plants, even though they come from the lowlands, from the woods, or from the more arid desert sections. There are a few of the tall-growing types of plants, such as foxgloves and some of the single roses, which, though not dwarf in character, are admirably fitted to the scale of rock garden work.
To one who is in the beginning of this work of selecting plants for rock garden use the impression should not be conveyed that every plant which is dwarf in its habit of growth is desirable for the rock garden. Many of these plants are extremely undesirable, such as the creeping Jenny (lysimachia) and dead nettle (lamium maculatum), mostly because of their tendency to grow rampant and to crowd out and smother many of the more sensitive and more beautiful types of rock garden plants. These plants are also difficult to eradicate from the garden when once they become established. They should never be used except in a rock garden on an extensive scale where the tendency to spread will not eventually become offensive. In order to maintain the true rock garden character it is very essential that plants should be selected which are in harmony with the spirit of the garden. Many so-called rock gardens are filled with the more common annuals, with sweet williams, phlox, hollyhocks, and even large irises - plants which belong to an entirely different type of garden, or which, because of their size, are not in keeping with the scale of a minutely detailed rock garden.
It is not necessary, in the development of an interesting rock garden, to use a large quantity of different types of plants. The success of a rock garden is dependent largely upon the ability of the designer to select proper types of plants for a specific purpose, whether the rock garden be very small and occupying only a corner of the lawn, or whether it be an extensive area in some wooded portion of the property. Such plants as hydrangeas, spireas, petunias, and many plants of these types which the reader has often seen in rock garden work, give evidence immediately of the lack of knowledge of plants and of their proper usage.
It is true also that the plants which are used in rock gardens require an amount of care in their maintenance equal to that given plants in the more refined and formal types of garden work.
For the person who has progressed along the path of successful rock gardening it might be well to suggest that he should endeavour to become intimately acquainted with the plants which he is using, especially their source of origin and the conditions under which they grew in their native locations. Plants which will withstand extreme drought, hot suns, and extreme cold, if they are planted in the correct locations in a rock garden, will not be hardy to any extent when planted in the open border. In other words, such plants as the cheddar pink and the wild pink are considered to be true crevice plants, and they should be used only for that purpose in rock garden work. These plants have a type of environment equally as much as persons or animals and under which they thrive best. The beginner who is developing this type of garden should therefore only use the more common types of plants which have withstood the abuse of "amateurs " and should make use of the specialized plants only after a thorough knowledge is gained concerning them.
One writer has said concerning the development of a rock garden that the designer should "have an idea and stick to it." We see so many rock gardens which are so-called and which in reality are only a miscellaneous pile of stones. Rock gardens in their true sense are an imitation of some condition of nature, both from their physical makeup and from their planting. We should therefore make a double effort to strive toward the development of the idea.
One of the most successful ways for obtaining good rock garden plants is to grow them from seed. It is often easier to seed plants in rock garden groups than it is to plant nursery-grown stock.