There is no special definition of a rock plant except that it must be a plant suitable for growing in rocky places. They come from all parts of the world and do equally well on dry walls built without mortar and with a substantial core or backing of soil, or in raised beds retained by walls, or even in flat beds well supplied with grit and stone to give the sharp drainage that most rock plants like.
A few of the choicest, most difficult alpine plants often take so unkindly to garden conditions that the enthusiasts who grow them do so in pots or pans and keep their treasures in frames or unheated greenhouses much of the time. However, there are plenty of rock plants which are as easy to grow as any other garden plants and some of them are capable of making a very fine display and bringing to the garden a special miniature beauty of their ow
Rules for Rock Garden Construction
If you decide to build a rock garden there are a few rules it is wise to observe. The site should be open and, in particular, should not be overhung by trees. Not many rock plants really like shade and those that do can be accommodated on the north-facing side of the rock garden. Trees are bad because the falling leaves in autumn tend to smother the small rock plants.
Good drainage is essential for most rock plants. If the rock garden is built above normal ground level there should be no difficulty in getting rid of surplus water, but if any excavation has to be done it may well be necessary to dig a soakaway, that is a large hole filled with rubble and topped up with soil, to catch drainage water and allow it to find its way harmlessly into the subsoil.
The Compost Some rock plants will grow well in ordinary garden soil but many of the choicer kinds need a more gritty compost. Coarse sand and small stone chippings. Up to about ^-in (05-cm) size, can be added to the existing soil, as can peat. As a general guide one part of sand, one of stone chippings and two of peat to six or eight parts of soil will make a satisfactory mixture for a great many plants, but this can be varied to meet special needs.
Choosing and Positioning Rocks Limestone and sandstone are the best rocks because they are moderately porous and plant roots take kindly to them, spreading themselves over any buried surface in search of moisture. Limestone and sandstone also weather pleasantly and very beautiful weatherworn limestone can be purchased, though it is usually rather expensive.
The rock garden will look more attractive if it can be irregular in contour, with a miniature hill or two separated by valleys. Rather than a plain mound. For the same reason the rocks should be well embedded in the soil and not just laid on top. They should give the impression of being natural – part of an outcrop of rock, most of which lies hidden beneath the surface. Much natural rock is stratified, which means that it lies in layers of various thickness one on top of another.
These layers are by no means always horizontal but may be tilted at all kinds of angles as a result of ancient upheavals as the crust of the earth was being formed. Sometimes different angles of strata are to be found close together but more usually all the strata in a particular area will have the same tilt. This is certainly the best pattern to follow in the garden and, cleverly done, will help more than anything else to give an appearance of rightness and inevitability to the finished construction. Any angle can be chosen but usually a moderate tilt is most effective and easier to manage than a very acute one.
Artistic Expression There are no rules for the actual design of the rock garden, which can express the artistic ideas and imagination of its creator. A fairly simple method which can give excellent results is to start from the bottom and build upwards in a series of irregular steps following the contours of the ground and forming pockets and shelves of soil in which plants can be established. As building proceeds see that soil is firmly packed beneath and under the rocks so that there are no hollow places nor any likelihood of subsidence. Do not place each rock hard against its neighbour, but leave soil-packed crevices, for in these many plants will thrive.
Alternatives to the Rock Garden
The rock bed
An alternative to the mounded rock garden is the flat, or nearly fiat, rock bed. Unless the situation is naturally very well drained it will be wise to excavate 2 ft (60cm) of soil and put 6 to Sin (15 to 20 cm) of brickbats or hard rubble in the bottom for drainage. Then return the soil, mixed with sand, stone chippings and peat, and sink some rather wide but not very deep stones into it so that they just protrude above the surface. They need not be very close together nor need they follow any particular pattern but the soil between them should be completely covered with stone chippings of the same character as the rock itself.
The finished bed will look like one of those areas of moraine one can see in the mountains, where finely ground rock has slid down and nearly covered larger pieces of rock. Such a bed, well planted with small, tufted and creeping plants, can look most attractive.
The Dry Wall
Yet another alternative to a rock garden is a dry wall, which is a wall built without mortar. Such walls are used in many parts of the country by farmers and are usually well clothed with small plants and moss. The farm walls, however, only have the soil that has been packed between the stones or has lodged there with the passage of time. More soil is required if a wide variety of plants is to be grown and this can be done either by using the wall to support a terrace of soil or by making a double wall with a core of soil in the space between.
A rock garden can be constructed on a flat site by making a mound of rubble. The mound is well firmed, covered with finer rubble and an 18-in (45-cm) layer of soil is then added. Rocks are sunk into the soil at a slight backward angle, care being taken to keep the strata lines running in the same direction
Dry walls may be constructed from dressed or random stone or even broken paving slabs.
Construction: A 3-in (8-cm) trench should be taken out at the base of the soil terrace and the first course of stone bedded into cement on the floor of the trench. Walls above 2 ft (60 cm) in height should be given a slight backward tilt for added strength. The wall is built up, inserting those plants required to grow in it as building progresses. The stones are bonded as are bricks and a thin layer of soil is used instead of cement
Either way, soil should be well packed between and behind the stones to give them stability and to enable plants to grow un-impeded. If dry walls are more than a couple of feet high it is usually wise to give them a slight backwards or inwards slope, known as a batter, to give them greater stability and reduce the likelihood of their being pushed out by the weight of soil behind.
The Raised Rock Bed A development of the double dry wall with its core of soil is the raised rock bed. It can be of any size but as a rule it is most convenient to have a bed between 2 ft and 3 ft (60 cm to 1 m) high and not more than 6 ft (2 m) wide as the whole surface of this will then be in easy reach from either side without scrambling. The details of soil and building are exactly the same as for dry walls.
Some very low-growing rock plants can be planted in crevices between paving slabs provided their roots can penetrate freely into good soil beneath.
