Rock Gardens and Tufa Blocks
Plants that have learnt to exist naturally among rocks are often small, at least above ground, though their roots may extend a long way in search of food and water. Some of these plants live in the mountains and are sometimes referred to as ‘alpines’, but the broader term ‘rock plants’ embraces these and also those that dwell in rocky places at much lower levels, even beside the sea.
For gardeners they all have a special value because their smallness gives them a particular charm and makes it possible to grow a considerable variety of plants in a small space.
You can make an attractive setting for your rock plants by grouping them together in a — a miniature rocky landscape that you can build yourself, with stones inserted into a basic earthmound. Although a rock garden will look more ‘natural’ sited away from buildings in a wilder part of the garden, not everyone has the space for this and it is by no means essential. Many people like to combine a pool (see ‘Water Plants’) with their rock garden, and it is as well to plan for this from the outset.
When deciding which kind of stone to use, consider how it will look beside the plants you wish to grow. Provided it is likely to give a pleasing effect, there is much to be said for using a local stone. Not only will it look more natural in its new setting, you will also save on transport costs, which can be considerable.
Most rock plants are accustomed to growing naturally in well-drained conditions, though often with access to considerable reserves of water retained beneath the rocks. In a garden you can simulate these conditions by adding plenty of peat or leafmould plus sharp grit or sand to the naturalof the garden. A surfacing of stone chippings can increase the natural appearance of the rock garden and provide extra drainage around the collars of the plants, where it is most needed. Limestone chippings can be used for lime-loving plants and granite or sandstone chippings for those that dislike lime. Make sure the soil is at least 40cm (16in) deep in all parts of the rock garden.
In general, a few large rocks give a better appearance than a greater number of small ones. Almost invariably the rocks need to be well bedded in the soil so that they appear to be part of a much more extensive rock formation, the rest of which is hidden from sight. Often the easiest and most satisfactory method of producing this effect is to lay rocks almost side by side to form a series of low, irregularly shaped shelves in the way that naturally stratified rock so often appears when exposed by weather and erosion on a hillside. Rock dug out of the surface, rather than excavated from quarries, will probably have natural markings caused by frost and water on exposed surfaces, and if you keep these areas visible when re-laying in the garden, they will enhance the illusion of a natural outcrop.
Soil should be packed in behind and between rocks so that there are no loose or open places. Roots should be able to grow unchecked wherever there is soil to sustain them; left to their own devices they will find their way deep into crevices and flatten themselves against the cool, moist undersurface of the rocks.
DRY WALLS AND RAISED BEDS
These are both devices which can make it possible to grow rock plants very successfully in surroundings which may seem incongruously formal for ‘natural’ rock gardens. A dry wall in the garden is one built with soil in place of mortar and it may be used as a retaining wall for a terrace or as a free-standing wall to divide one part of the garden from another. If plants are to be grown in a free-standing wall it should be built double, with a core of soil between the two courses of stone, so that plants can root into this and down through it into the bedrock soil beneath.
A raised bed is a development of both these ideas. A wall, or walls, are built without mortar to enclose any convenient area and shape, which is then filled with soil, peat, sand and grit in whatever proportions are most suitable for the plants to be grown in it. Convenient dimensions for such beds are between 60-100cm (2 – 3-1/2ft) high and not more than 2m (6-1/2ft) wide at any point so that they can be tended conveniently without actually having to climb up on to them. Plants can be grown in the faces of the walls as well as on top of the beds, giving you a variety of sun and shade to suit a rather broader range of plants. All dry walls, for whatever purpose they are made, should be built of good-sized blocks of stone with plenty of soil rammed behind and between them so that they sit firmly one on another. Bed the first course firmly into the natural soil to provide a firm foundation, and stagger the stones in the upper courses so that the vertical crevices are broken and a bond is formed just as in a wall built with mortar. Since the dry wall will lack the adhesive qualities of mortar, it may be necessary to give each stone a slight downward slope and build the walls themselves sloping a little backwards or inwards so that their own weight helps to hold them in place. As soil settles and plant roots fill most of the crevices, these walls become increasingly stable and, if well built, can stand for years.
TROUGHS AND TUFA BLOCKS
There are yet other ways in which rock plants can be grown, making it possible to use them as ornamental features even in the most sophisticated surroundings of terrace, courtyard and patio gardens. Genuine old stone troughs and sinks are becoming a rarity but excellent reproductions are available. Tufa, which is a very porous type of limestone, can be obtained in blocks of any convenient size and many small rock plants can live on it, actually penetrating it with their roots. For new plants, gouge out a small hole and fill it with a core of soil.
All sinks and troughs must be provided with at least one good-sized hole in the base to allow surplus water to drain away. Place some pieces of broken pot or stones around and over the hole to prevent soil blocking it. Also for drainage, raise the trough or sink a little off the ground. Soil used for filling should be adequately porous as well as suited to the plants to be grown: a good average mixture is equal parts of good soil (loam, if available), sphagnum peat and coarse sand. The surface can be covered with stone chippings (as for rock gardens). A few small pieces of rock may be bedded into the surface both for appearance and to provide the roots with additional cool, moist surfaces to cling to.