Traditionally, a rock garden is a place in which to grow alpine plants. Way back in Victorian days and before, such a place was usually referred to as a ‘rockery’, a word which, in these more enlightened times is very much out of favour with devotees of alpine gardening, who even eschew the word ‘rock’ and more often than not describe it as an alpine garden.
Even the word ‘alpine’ is suspect in this context, since many of the plants most fitted to be grown on a rock garden are far from having alpine origins. One must, however, have a brief collective description for plants which serve a specific purpose and it is convenient, and sufficiently accurate, to refer to all plants which one would associate with rocks and stones in the open air, as alpines.
There are more amateur gardeners today than there have ever been and the general interest in gardening is very evidently increasing, but it is gardening on a different scale from that to which our fathers and grandfathers were accustomed. It is much more a ‘do-it-yourself’ affair and this is good as it encourages a closer and more personal interest in plants.
Gardens are small and labour either unobtainable or too expensive. On the other hand, alpine plants are yearly increasing in popularity. So many of them can be grown in a limited space, and they respond so eagerly to individual attention that they offer an ideal form of gardening to those who come to love their jewel-like beauty and wish to grow as many of them as possible in surroundings which are often very restricted indeed.
That alpines are temperamental and difficult plants, needing very special care and attention is a myth going back to the bad old days, when ‘rockeries’ were made of ugly piles of stone heaped in unsuitable places-under the drip of overhanging trees, or in shaded and draughty corners. These were death traps for most plants and it is small wonder that alpines died in them and created the illusion that they were difficult to grow.
A select minority of alpine plants, mostly the true ‘alpines’ which come from great elevations and dwell in the rarified air and austere conditions of high mountains, demand skilled and knowledgeable care. It is to the cultivation of these that the novice progresses in gradual stages.
Although it is tempting to the beginner to attempt such plants as the delicious and exciting domes of alpine androsaces, or to ‘have a go’ at the exasperating but supremely beautiful Eritrichium nanum, wisdom should prevail and these delights postponed until the necessary skill has been acquired after experience with the many easy and lovely plants which are available.
One of the reasons for the avoidance of alpine plants by a few gardeners may well be the result of starting at the wrong end and trying to grow the rare and difficult minority. Failure with these will lead to discouragement and a disinclination to take further steps along what can be one of the most delightful paths into specialized gardening.
Those who are new to alpine gardening should start by growing the easier plants; those which will provide colour and interest over as long a period as possible. Another criticism which has been aimed at rock gardens is that they are supremely beautiful and colourful in the spring and early summer, but deadly dull thereafter.
This is untrue; by choosing carefully from the immense variety of plants available a rock garden can be colourful from earliest spring until winter. There is, and always has been, a tendency to plant too many aubrietas, alyssums and arabis. These all blossom in the spring and are invaluable for providing masses of colour, but if they are too generously used, there are wide blank spaces for the rest of the year.
Any such vigorous and space devouring plants must be employed in moderation and ample space left for later flowering specimens. The beginner would do well to seek among such genera as Dianthus, Campanula, Phlox, Gentiana, Primula, Helianthemum, Achillea, Saxifraga, Geranium, Iris, Lewisia, Ranunculus, Polygonum, Sedum and Sempervivum in order to secure a succession of flowers throughout the summer and autumn.
A few really dwarf evergreen shrubs should also be grown. For this purpose none is better than the genuinely pygmy conifers. There is a trap here for the unwary however. Too often conifers are sold as dwarfs which will ultimately grow much too large for any rock garden.
It is fatally easy to be misled by the dainty and miniature appearance of such conifers as Chamaecyparis lamsontanafletchers when seen as young, immature plants and only guaranteed dwarfs should be planted. Ideal examples are the little Noah’s Ark juniper, juniperus communis `C…ompressa’, any of the several pygmy forms of Chamaecyparis obtusa, and some of the tight, cushion-forming varieties of Picea abies.
One of the functions of the tiny trees is to create a sense of scale and proportion in the miniature mountainscape. The shape of the trees should be borne in mind when they are placed. Those of upright, columnar habit, such as the little juniper quoted above, should never be planted on high points. Place them lower down, against a small ‘cliff’ and reserve the prostrate kinds for the heights.
Plants for Special Purposes
The Alpine House Yet another aspect of alpine gardening is the cultivation of plants in an alpine house. There is a general impression that only the rarest and most tricky plants may be grown in such a structure. It is true that skilled growers keep their choicest specimens in such a house, but there is no reason why an alpine house, or a cold greenhouse, should not form a valuable adjunct to the rock garden.
There are many weeks during the year when gardening out of doors is far from being a pleasurable occupation and a select collection of plants grown in an alpine house can provide pleasure and interest at a time when other gardening activities are impossible and plants can be admired and cared for in comparative comfort.
There are specially designed alpine houses which differ from conventional greenhouses in having a lower pitched roof, and more ventilators. This is the ideal but is not essential. An ordinary house can easily be adapted and the only major modification likely to be needed is the provision of extra ventilation. No artificial heating is necessary, or even desirable and this is an economy.
By growing a number of alpine plants in pots and pans a constant succession of interest can be obtained. When not in flower the containers can be stood out of doors, preferably plunged to the rims in ashes or sand. Very early flowering alpine bulbs make a splendid beginning to the year and for this one would choose from the tiny narcissi, crocuses, snowdrops, snowflakes, winter aconites (eranthis), fritillaries, erythroniums etc.
After these come the early flowering saxifragas, particularly such kinds as S. irvingii, S. kellereri, S. apiculata, S. `Cranbourne’ and S. burseriana. Then come all the spring-flowering alpines of dwarf, compact habit, with campanulas and dianthus to follow on and some of the choicer sedums and late-flowering gentians.
The staging on which the pots and pans stand is best made of a solid base, not spaced wooden slats. If a layer of sharp shingle or ashes is spread over the staging this will help to keep the soil in the containers moist and cool and avoid the constant drying out to which the plants object.
In the summer, with some pots and pans outside those in the house can be spaced more widely. In the winter the entire population can be brought inside and stood more closely together. Autumn potted bulbs can be stored on the floor beneath the staging until they begin to grow.
Pond and waterside plants
The addition of water, and especially moving water, to a rock garden has several advantages. Not only does it widen the variety of plants which can be grown, but it adds life to the scene. There are now available several inexpensive types of submersible pump which can be used to circulate water from a pond at the lowest level to another higher up, from which it can be made to flow in a small stream, or a succession of small pools, back to its origin.
Pools and streams can be constructed with modern fibre-glass materials and with plastic sheeting, or they can be made more conventionallv with concrete. Anyone with moderate abilities as a handyman can construct streams and pools without difficulty and at little expense.
By arranging for small overflows here and there moist areas can be contrived by the pool or streamside in which to accommodate some of the plants which relish having their roots in wet soil. Do not ever imagine that, without making deliberate moist areas, the plants which enjoy them can be made happy by merely planting them at the waterside. The very edge of a stream or pond creates the illusion of dampness but can be very dry.
Appropriate plants for such damp situations would be all kinds of mimulus, moisture-loving primulus, for example P. rosea, P. denticulata, P. frondosa and, of course, all the candelabra primulas such as P. japonica. The double kingcup, Caltha palustris ‘Plena’ hangs its golden flowers attractively over the water and such plants as tiarella, astilbe, Saxiliaga aizoides and the double ladies smock, Cardamine pratensis ‘Plena’ would help to create a pretty scene.
Whatever one does in a garden it is well to have an ideal at which to aim. It is not always possible to achieve perfection, but the very act of striving to attain it is rewarding and is certain to provide better results than aimless attempts.
The ideal site for a rock garden would be a gentle slope, on well-drained soil, falling to the south or west. Not many gardens will be able to provide such a perfect position, but there is no need to despair; many alternative situations will be quite satisfactory. Avoid if at all possible a due east aspect.
A sloping position is better than a flat one, but a rock garden built on the flat can be perfectly successful. The absolutely essential thing is good drainage. It is quite useless to construct a rock garden on soil which is likely to be flooded and waterlogged in the winter.
Alpine plants on the whole are extremely tolerant of widely varying conditions of soil, aspect and climate but they will not, under any circumstances, endure having their roots surrounded by sodden soil. Water they love, and need, and it does not matter how much there is, as long as it flows readily through and past their roots. The positions in which many of them grow naturally may appear to be dry, but water from melting snows flows constantly below the surface.
If the natural drainage is adequate little more preparation is necessary than deeply digging the area and removing all perennial weeds. It is a good idea to leave the area alone for a few weeks after it has been cleared and dug to allow weed seeds to germinate. These can then be destroyed by applying a post-emergence weedkiller. This will save a lot of later laborious weeding. Weedkillers which leave a residue in the soil and prohibit planting for some weeks or even months after application, should be avoided.
Site the rock garden right out in the open and never, if it can possibly be avoided beneath overhanging trees, or in narrow alleyways between adjacent buildings. The first will ensure a detested dripping of water from branches above and the latter will be draughty, and all plants hate draughts. Positions for plants which relish some shade can always be contrived in the construction of the rock garden. An outcrop of rocks running east and west will provide a warm south face and a cool north aspect on the other side.
A detailed, stone-by-stone description of the making of a rock garden is almost impossible to write and quite impossible to follow in practice. So much depends upon each individual rock, the site, and what sort of scene it is desirable to create. One can really only set down some basic rules, which amount to a series of `do’s’ and ‘clones’.
Do not set the stones in isolation over a mound of soil, This only creates a Victorian ‘rockery’ and there will be constant erosion of soil from higher to lower levels. Do not set stones up on edge so that they erupt from the soil like fangs. Instead lay them on their longest edge and make sure that they join in pleasant complexes or outcrops much as they would in nature on a hillside.
The sort of compost to use is described below in the section devoted to cultivation, but make sure before you begin to build that you have available enough mixed soil. It always takes more than you expect and nothing is more annoying than to run out of compost halfway through the construction. Very roughly, you may estimate that, on a flat site, one cubic yard of soil will be needed for every ton of stone. A sloping site may need rather less as you can cut into the hillside to help fill the pockets.
If, after reading the above you feel that there is no possible position for a rock garden on your property, and if you still wish to grow rock plants, do not throw up your hands in despair and abandon the project. It may be a heretical statement, but it must be said that the great majority of rock plants can be grown very successfully without the aid of a rock garden; a rock garden is the ideal setting for them, but it is not essential.
Possible situations for alpine plants are between the cracks of paving stones. In fact, such a position provides everything that they like, such as a cool root ‘run which does not dry out, their heads in the light and their collars protected from too much wet by the closely adjacent paving stones.
Retaining or free-standing walls provide excellent homes for alpines. Here too, they find perfect conditions. If all else fails, you have only to dig out a depth of soil in an open, sunny position, ensure good drainage and fill in with suitable compost and you have a bed which will grow a wide variety of them to perfection.
Many of the smaller plants will do well in old stone troughs and sinks. These little gardens are invaluable where space is really limited and it is astonishing how many different plants can be gathered into a small sink.
Finally, some people resort to a specially constructed raised bed. The sides can be built up in brick, stone or even planks or old railway sleepers, to a convenient height. Many who find the ground getting a long way away as the years advance discover in such raised beds a delightfully easy form of gardening. A good height to aim at is from 2z ft. Fill the bottom half with good roughage for drainage and top up with compost and a perfect home is provided for alpine plants. Cultivation There are two main divisions in the requirements of alpine plants. There are those which either like or will tolerate a soil containing chalk or lime and there is another, smaller but quite intolerant section, which cannot abide lime in any form.
The lime lovers will grow in limefree soil, but the lime-haters will not put up with alkaline soil. If, therefore, the natural soil is chalky, you must either refrain from growing the lime-haters, or make special provision for them. Unless you particularly desire to grow the lime-haters, many of which are very lovely plants, you need not be too sorrowful, for most alpines are tolerant. It is only the minority whose idiosyncrasies must be pandered to.
If you insist upon growing the lime-haters in spite of the lime in your soil, then you must make a special part of the rock garden for them. This should be on one of the higher places, so that lime-impregnated water does not soak into the soil from above. Many of the lime-haters are also plants which appreciate some shade so try to organize their special situation so that it is protected from full sun.
Dig out the natural soil to a depth of at least 1 ft. and fill in with prepared lime-free soil. This can be a mixture of lime-free loam, leaf-mould or peat, and sharp sand in the proportions, by bulk, of 3-2-1. Make sure if leaf- mould is used that it is really lime-free. If made from the leaves of trees which have been growing on chalk it is likely to have a lime-content. Peat is safer, and it should be fine grade moss peat. If there is any doubt about the lime-content of any ingredient test it with one of the inexpensive soil-testing outfits which are available.
Plants likely to demand this treatment are most heathers (except the lime-loving, winter-flowering forms of Erica carnea), cassiopes, andromedas, pultherias, pernettyas, autumn-flowering gentians such as G. sinoornata and its allies, and most members of the family Ericaceae (which includes all rhododendrons). Most good catalogues quote the family to which plants belong and this can be a guide to their needs.
The first and most important basic need of alpine plants is perfect drainage and no time or trouble expended,in ensuring this is wasted. Once you are certain that water will pass rapidly through the soil and will not linger around the plant roots you are more than halfway to success. On a naturally gravelly or sharply draining soil, little preparation will be necessary, but if you have to contend with a heavy, sticky clay, then some preliminary work is essential to provide the necessary drainage.
If the site on which the rock garden is to stand, or indeed, any position where alpines are to be grown, is flat, and if the soil is clay, or has a hard pan beneath, causing water to stand in puddles instead of draining away, then a deep sump must be excavated in the centre of the site and filled with old clinker, rough stones, ashes, gravel, or any good draining material. The sump cannot be too deep and a minimum depth of 5 or 6 ft. should be the objective. Its width will depend to some extent upon the area of the site. A large area may demand two or even three sumps, but a small space, say 200 squire feet, would be sufficiently served by one such excavation approximately 2 ft. in width. If it is decided to manage with only one sump on a larger site, it should be twice that width.
On a sloping site drainage is more simply provided. If necessary surplus water can be led away down the slope by a series of narrow trenches, in which land-drains can be laid, or the trenches filled with clinker or other rough material.
Simplicity should be the aim of the beginner. Experts who delight in accepting the challenge offered by really rare and difficult plants will have ‘cookery-books’ of complicated soil recipes with which to pander to the tastes of individual plants, but yo per cent of alpine plants (excluding the line-haters) can be successfully grown in one basic soil mixture.
The three main ingredients of a desirable compost are loam, a good organic material and sharp sand or grit. Loam is either rotted turves or good top-spit soil containing an appreciable amount of fibre. Good, friable, well-nourished garden soil will serve very well.
The organic material will consist of either moss peat, leafsoil or well-decayed compost: peat should be of the finer grade, and sedge peat should be avoided; leafsoil should be nicely rotted and broken down into a dark, friable material.
Sand is an important ingredient; its primary purpose is to ensure an open, freely draining compost. The soft yellow sand used for mixing mortar or cement is quite useless for alpines’. The sand must be sharp in texture. If such sand is not available locally it should be replaced with any form of fine but sharp grit.
Measuring by bulk, the proportions of the three ingredients should be 3 parts of loam, 2 parts of organic material and 1 parts of sand or grit. These must all be thoroughly mixed together, adding at the same time bonemeal in the proportion of 5 lb. to each cubic yard of compost.
Such a compost is ideal for the great majority of plants and no form of artificial fertilizer will be needed for at least the first two years. After that, a spring topdressing with similar compost is desirable and an additional light dressing of bonemeal in autumn and spring, scattered on the soil and gently forked into the top inch or so.
When filling the pockets and joints between the stones of the rock garden during its construction press the soil down firmly, using a blunt-ended wooden rammer to pack it well under and around the rocks. When building is completed leave the whole area unplanted for a week or two, to allow the soil to settle, which it will do after rain, or a thorough soaking. Some compost should be kept in reserve to top-up where necessary. It is better to do this first rather than have settlement after planting has been done.
Great care should be taken to ensure that all spaces and crevices between the rocks are well filled with soil. These are the crannies into which alpines delight in delving with their roots in search of the cool, moist conditions they need. Should they emerge into a soilless vacuum they may perish.