Rock and Alpine Gardens
Gardens in which rocks and plants appropriate to them are the chief landscape elements are called rock gardens or sometimes, if the plants are entirely or mainly sorts to grow naturally at higher altitudes or under subarctic or arctic conditions, alpine gardens. Well planned and well executed rock gardens are aesthetically agreeable as well as horticulturally stimulating.
Joy of Rock Gardening
The satisfactions of rock gardening lie not alone in creating and maintaining pleasing landscapes, but also in developing intimate acquaintance with and caring for the plants accommodated. At its best, rock gardening is a splendid hobby, not excessively demanding, yet sufficiently challenging to reward reasonable dedication and attention. Because the plants used are chiefly small, many sorts can be accommodated in quite limited areas. This appeals to gardeners with a well developed instinct for collecting, a commendable expression of horticultural interest displayed by many amateurs.
Another attraction of rock gardening is that, apart from initial construction perhaps, the tasks connected with it are generally light and agreeable. Most can be accomplished while puttering around the garden at longer periods on more fixed time schedules as some other types of gardening demand.
English Rock Gardens
Historically, rock gardening began in the British Isles, its development an outcome of the greatly increased numbers of travelers from there who from early in the nineteenth century on visited Switzerland and other mountainous parts in Europe. Enamored by the great wealth of beautiful alpine plants they saw, unknown in their own countries, they were inspired to bring some back and attempt to grow them at home.
Because of a nearly complete lack of understanding of the needs of alpine and other mountain plants most early attempts at domesticating them were dismal failures. A few of the toughest and more adaptable sorts survived in the generally atrocious "rockeries" built by Victorians, but in the main those horticultural conceits, which sometimes included grottos, arches, bridges, and other elaborate architectural features, became graveyards for the choicer alpines enthusiasts had plucked from their mountain homes.
But gradually improvement came. As early as 1870, William Robinson, in his book Alpine Flowers for English Gardens, attempted to give some guidance, and by the early years of the twentieth century, an altogether better appreciation of the needs of mountain plants had developed and skills in cultivating them improved. Nevertheless, for a long time, rock gardens continued to be poorly made and many esthetically unsatisfactory ones were established, as, sadly, are some modern ones. The least attractive belong in the groups the inspired English authority Reginald Farrer characterized as the almond pudding, dog's grave, and devil's lapful styles and that later in America became known as peanut brittle rock gardens.
Before the end of the first decade of the twentieth century, capable Europeans were advocating sound principles for constructing and planting rock gardens and for caring for plants appropriate to them. Among the books in that decade are My Rock Garden, by Reginald Farrer, whose famous garden was in Yorkshire, England, and 'Rock Gardens' by Lewis Meredith, who gardened in County Wicklow, Ireland. Completed in 1913, but not published until six years later, Farrer's book The English Rock Garden became the bible of rock gardeners everywhere. A master of English prose, the author stimulated thousands to attempt the cultivation of the plants he so beautifully, entrancingly, and sometimes extravagantly described.
Another benchmark was the publication, in English in 1930, of Rock Garden and Alpine Plants by Henri Correvon, the distinguished Swiss pioneer in the cultivation of alpine plants. As early as 1877, Correvon exhibited at a horticultural flower show in Geneva a small collection of alpines he had grown from seeds, for which pains he was accounted a "young enthusiast who does not realize the needs of the gardening world." Nevertheless, at the urging of one of the judges who thought the Société d' Horticulture de Genève should "give him something as evidence that the Société is interested in encouraging young beginners," Correvon was awarded a prize, four little silver teaspoons.
American Rock Gardens
In North America, interest in rock gardens began later than in Europe, yet in 1890 an example patterned after that at the Royal Botanic Gardens, Kew, England, but much smaller, was constructed at Smith College Botanic Garden, Northampton, Massachusetts. In the 1920s, another was installed at the Brooklyn Botanic Garden in New York City, and in 1932, construction began on the Thompson Memorial Rock Garden (now the T. H. Everett Rock Garden) at The New York Botanical Garden in York City.
Meanwhile, keen amateurs were furthering the cause of rock gardening on both coasts of America. The publication in 1923 of Louise Beebe Wilder's delightful book The Rock Garden and the many other writings of this competent cultivator and talented author stimulated many Americans to engage in the new hobby.
Other circumstances that in the period between the two world wars encouraged the rapid expansion of enthusiasm for rock gardening were the organization of the American Rock Garden Society and the truly marvelous examples of planted rock gardens staged as exhibits at the great spring flower shows in Boston, New York and Philadelphia, by the superb artists of rock garden design and construction, Marcel Le Piniac, Ralph Hancock, and Zenon Schrieber. The American Rock Garden Society continues to flourish and to attract to its membership people interested in its special field. (Ed. note: ARGS membership in the United States and Canada exceeded 4,000 at end of 1991 and includes many members in other parts of the world. H. Lincoln Foster and Laura Foster and their 1968 book Rock Gardening were vital influences in America.)
Partly because of climate, which in many parts of North America precludes or makes extremely difficult the cultivation of many true alpines that are the glories of European rock gardens, and partly because of the availability of numerous charming small plants native to the continent that are not alpines, most American rock gardeners wisely do not limit their plantings to inhabitants of high mountains, but include other neat and choice kinds that look as if they properly belong. And this is as it should be.
Climate and Plants
Traditionally, and as generally interpreted, rock gardening involves the cultivation of mountain plants and other low sorts that withstand severe winter cold with impunity and is thought of as belonging only in temperate climates.
But viewed as an art form based on the agreeable use of rocks in the landscape, the development of rock gardens is as appropriate in warm temperate, subtropical, and tropical climates as in temperate ones. Certainly there are many places in such regions where cliffs, outcropping rocks, and similar formations are as inspiring as those of colder regions, and the principles of adapting or constructing such features as garden landscapes are not different.
The kinds of plants to employ in warm climates quite obviously differ from those useful in colder ones, but plenty are available. Fit choices to local conditions. In desert and semidesert areas, cactuses and other succulents in nearly endless array are obvious possibilities. They look especially well in association with rocks. For humid warm climate regions, there are available just as many sorts of plants appropriate for displaying in rock environments. They include ferns, as well as many kinds of begonias, gesneriads, peperomias, and other plants, many of which as wildlings inhabit cliffs and other rock features.
There are two chief types of rock gardens, natural and artificial. The first represents the development of sites on which native rocks are prominent as outcrops, cliffs, or perhaps strewn boulders. The others are made in areas in which all or most of the rocks used must be imported.
Existence of a site of the first description is reason enough for adapting it as a rock garden, but constructed gardens are generally only justified by a genuine desire to grow and display small plants the majority of which are not well suited for flower beds and borders. There are sometimes rockless sites, such as banks and steep slopes, where the development of a rock garden presents less problems than other treatments.
To begin a natural rock garden, first make a careful survey of the site and identify the plants growing there. Some, especially well located, deep-rooted trees such as oaks and hickories that can be relied upon to provide light shade for part or all of each day in summer, besides adding to the charm and perhaps majesty of the area, should be preserved, but remove overcrowded, spindly specimens and weedy sorts of little garden merit along with tangles of brushwood and similar undesirable growth. There may be too, evergreen or deciduous shrubs or herbaceous perennials, such as ferns, bulbs, and other wildlings, that should be retained where they are or transplanted elsewhere.
Clearing the area of unwanted vegetation may then be done by digging out completely all roots as well as tops. Then give attention to any pruning retained trees and shrubs need. Cut out all dead and seriously diseased wood and, if desirable, thin out branches from dense specimens. It is often advantageous to provide for more side light by removing some lower branches to "raise the heads" of trees that cast too dense shade.
Rearrangement of a few rocks, or even supplementing those on the site with others brought in, is permissible, but it must be done so skillfully that even persons knowledgeable about natural formations cannot easily detect the artifice. Transported rocks must match precisely those of the site and be positioned as though placed by nature.
Improving the soil is the next order of business. Unless you are dedicated to growing plants that need quite different types of soil than what you have, do not attempt drastic changes in its basic character. For example, if it is naturally acid or alkaline accept the condition and select plants adapted to it. Concentrate on bettering soil texture where needed by mixing in such additives as chips of crushed rock, coarse sand, perlite, or, for alkaline soil plants, crushed limestone or crushed clam or oyster shells. For woodland or moorland plants, add generous amounts of leaf mold, peat moss, compost, or similar organic material.
Make certain there is adequate depth of soil, especially in the crevices and crannies you intend to plant. It is usually desirable to rake out existing soil and, if necessary after deepening or enlarging the clefts or crevices, to replace it with a better mix, firmly packed, so no voids are left.
Planting is best done in early fall or early spring, but not until disturbed ground has had time to settle or before it is reasonably certain that it is essentially free of pestiferous perennial weeds. Whenever practicable, it is advantageous to allow an entire growing season to elapse between the preparation of the site and actual planting. This permits clearing the soil of weeds and ensuring clean planting areas by pulling up or hoeing off every one as soon as it shows aboveground.
Plants and Placement
The sorts of plants appropriate for natural rock gardens are likely to include many native to the region as well as others that thrive under similar conditions. In selected spots and corners, avid rock gardeners are likely to try a few more challenging sorts.
To be convincing, placement of the plants calls for an appreciation of how vegetation is disposed on natural rocky sites. Seek inspiration from such places, noting the unstrained informality that prevails. Here, irregular drifts of low plants may carpet the soil surface or occupy ledges, shelves, or miniature plateaus, with very likely outlying smaller groups or individuals, often at lower levels or to the lee of the main groups, the outcome of seeds that have fallen and been washed away or have drifted down from the main colonies. Note how plants run along narrow crevices or congregate at the bases of miniature cliffs. Without slavishly copying such native features, let your natural rock garden epitomize them and represent a distillation of what is good about what you find in the wild, miniaturized and tailored to accommodate the plants you want to grow.
Rocks and Garden Construction
Rock gardens constructed on sites devoid of native rock or where little is present clearly offer opportunities for imaginative development, yet in such places the most inappropriate examples amusingly characterized by Reginald Farrer, are often perpetrated.
Following Farrer's castigation, the better examples of British rock gardens were made in what their builders fondly imagined was a natural fashion, but because many of those who made such gardens failed to study rock formations as they occur in the wild, they were usually unconvincing.
At first, great emphasis was placed on creating "pockets" to be planted with individual kinds of plants and the structure was likely to consist of a series of such little flat or nearly flat terraces backed by and supported by more or less vertical walls of stone. Such was the rock garden at the Royal Botanic Garden, Kew, England, until the 1930s, and many others constructed in Great Britain and elsewhere followed the same general plan. The pockets, frequently referred to in garden writings of the time, were well drained and bottomless so the soil with which they were filled connected directly with the main body of earth beneath and made it entirely practicable for plants to grow and flourish, but the overall aesthetic effect was rarely satisfactory. But gradually improvements came, and between the two world wars, gardens more suggestive of native rock formations were developed both in Europe and America.
Choosing the Site
The choice of a site for a rock garden may be wide or limited depending upon the extent and character of the property. The advice so often given in older writings to locate the garden well out of sight of buildings and other formal features, is, on small grounds, often not tenable, and certainly is not essential to success in cultivating alpines and other rock garden plants.
It is by no means necessary to duplicate or even approximate an alpine scene to achieve a satisfying and beautiful rock garden. Such styles may be admirable in suitable surroundings, but so are rock gardens of other types.
It is even possible to install a garden adjacent to a building or cropping out of a lawn without being incongruous, possibilities earlier advocates of rock gardens and some contemporaries completely reject. Furthermore, garden features suitable for embellishment with rock plants that make no pretense of naturalness, and yet are congruous and beautiful, can be developed. To this category belong what are known as dry walls, of which more will be discussed later.
A secret of success of rock gardens that aspire to naturalness, be they near or remote from manmade structures, be they large or small, is the placement of the rocks. To be convincing the effect must be that they were positioned by nature without aid from man. Here, if ever, true art is to conceal art.
The surest ways of obtaining such effects are (1) to use the same type of rock throughout the garden or at least throughout major parts of it, (2) to position each piece so that it appears stable and, except for minor crevices, connected with neighboring pieces aboveground such that the whole apparently represents the exposed part of a massive underground formation, and (3) if the rock be stratified, to lay the pieces with the strata lines all in one direction. Granted, because of geological or other disturbances the disposition of rocks in the wild does not always conform to these principles, the departure from them, unless carried out very skillfully by one who has carefully studied natural deviations from them, is very likely to produce uneasy unconvincing effects.
Especially appropriate sites for rock gardens are slopes, banks, and small valleys or dells, natural or created, but flat areas can also be utilized. A first necessity is to evaluate the area, particularly with reference to any contouring that may be desirable. If a pool, stream, or waterfall is contemplated, and these can add greatly to the charms of rock gardens. Their locations and courses must be planned, and so, especially if the garden is sizable, must be paths needed to enjoy and service the area.
Contouring is usually best achieved by stripping and stockpiling the topsoil, fashioning the undersoil to the convolutions and grades deemed appropriate (this may involve bringing in additional material), then after modifying it in any way that seems desirable, and if necessary supplying additional soil to achieve a depth of at least 1 foot, replacing the topsoil. Modification, if the soil is not sufficiently porous, will involve mixing in generous amounts of coarse sand, grit, or small chips of stone, and if woodland plants are to be grown probably the admixture of leaf mold, peat moss, or other suitable partially decayed organic material. If a section of the garden is to be devoted to plants that need alkaline soil, crushed limestone or limestone chips may be included in the topsoil mix.
Finding the Best Rocks
The kind of rock used is usually determined by availability. Where choice may be had, one that is porous, rather than such hard, impervious types as granite and schist, is to be preferred. Hard sandstone, and not excessively soluble limestone are very satisfactory. But remember, limestone, and water worn limestone is one of the most beautiful rocks, are distasteful or unacceptable to such acid soil plants as heaths, heathers, and rhododendrons. Tufa, a soft, lightweight, porous limestone type rock formed by calcium carbonate deposited in springs and streams, is easy to handle and congenial to plants, but of undistinguished appearance. Harder rocks can be used, but take longer to weather because they are less encouraging to the growth of mosses, lichens, and other primitive vegetation that soon conceals freshly exposed portions of softer rocks.
Unless no other is available do not use newly quarried rock. Its raw surfaces are likely to take a long time to weather and, even worse, may display marks of drilling. Weathered pieces collected from the surface of the ground and of a character and color that suggest age are likely to be ideal. In some parts of the country suitable material can be obtained from old stone walls. The pieces must be of manageable sizes and of acceptable relation to the size of the garden, although here some "cheating" can be done for, by careful placement, it is possible to arrange several comparatively small rocks so skillfully that they appear to be a creviced bigger one. When the chinks between them are filled with plants that effect is greatly enhanced.
Boulders are generally considered unsatisfactory for rock gardens, and certainly they should not be mixed with angular rocks, but if boulders are all that is available it is not impossible to fashion a convincing garden from them as was done at the Brooklyn Botanic Garden.
The secret is to use boulders of different sizes and to position them, some partly buried, some exposed, as they would be in the bed of a dry stream or wash. Let the areas between the stones slope gently except for some accumulations of what represent washed down, stony, gravely, sandy soil piled on the "upstream" side of boulders. These accumulations may be level topped or even tilted slightly against the prevailing slope.
Take care not to scar rocks when collecting and handling them. To minimize the danger it may be worthwhile wrapping choice pieces in burlap. Use crowbars, often necessary for levering large pieces, although sometimes staves of wood or pieces of two-by-four can be used for this purpose, with care to avoid bruising the rock. If possible have rocks delivered to the tops of slopes. It is easier to move them downhill than up.
Spacing the Rocks
When constructing the garden, do not distribute the rock evenly throughout; instead, make massive use of it in some parts, employ it sparingly or not at all in others. Take a cue from natural rocky places where accumulations of detritus and washed or blown soil form slopes and terraces about and between bold protrusions of rock. In gardens, rockless areas afford relief to the eye, splendid opportunities for planting attractively, and by contrast give seemingly greater massiveness and importance to the rocky portions.
Areas that lend themselves to rocklessness or to not more than suspicions of rock poking through the surface are gentle slopes downward and backward from the tops of cliffs, moraine type slopes forward and downward from the fronts of cliffs, valley like depressions between outcrops, and little flats bordering streams and pools.
Installing Special Features
Install the most massive features of the garden first. These may include bold outcrops, cliffs, and perhaps a waterfall. Give special attention to the location of the last as well as to other water features. Water spouting from the top of a hill or cliff is all wrong; it lacks the appearance of naturalness. To seem plausible, there must be, or by skillful construction or planting the viewer must be led to believe there is, a catchment area of considerable size above the point of emergence to account for the volume of water.
Pools and watercourses call for special thought. The supply may be natural, piped in, or recycled by a pump. If artificial, be sure its source is concealed. With careful planning, a comparatively small flow can be managed so that it is seen more than once to give the impression that the garden is much better supplied with water than it really is.
Placing the Rocks
When placing the rocks begin at the low parts of slopes and work upward, carefully setting each piece with its most attractive weathered side exposed and, so far as consistent with naturalness, with its top sloped slightly backward to direct rain or water from sprinklers to the roots. With this same thought in mind avoid overhangs that keep water from reaching rock faces below them.
Strive to achieve a feeling of stability. Leave no impression that the rocks are separate pieces susceptible to being easily loosened and removed. This is most surely done, so far as practicable, by setting each with its broadest side as its base, a positioning that suggests the most common aspect of exposed portions of outcropping rocks and the usual attitude of surface rocks in the wild. To achieve stability, with the rocks in their best possible positions, it is sometimes desirable to prop a large rock on several smaller ones and then to fill the voids with firmly packed soil.
A variation of this procedure that, if skillfully done, carries conviction and is highly satisfactory, is to stand flat rocks, much thinner than long or wide, on edge with their most attractive broad sides facing outward to form miniature cliffs. In this way height is achieved with much less bulk of stone than is required if one or two or more superimposed pieces are set widest side down to produce similar effects. When using such rocks take special care to set them in positions of repose that allow of no easy disturbance. This is particularly important in regions where strong outward thrusting comes from the soil freezing deeply.
No matter how individual pieces are positioned, they must relate to each other as though representing bedrock exposed by natural weathering or as a result of gulleying by water or wind. This means the major rock faces will have the same general direction throughout the garden, and if the stone shows lines of stratification they will be at the same angle throughout. Minor exceptions are when rocks represent pieces broken away from the main body and that angle downward from the cliff like margins of a gullied stream or have tumbled from a cliff to repose on a slope or plateau below.
Fairly small rocks may be effectively employed to give the impression of being a bold outcrop.
Although it is true that natural outcrops occur in which, as a result of geological upheavals, their lines of stratification run vertically or nearly so, and there are others in which they are approximately horizontal, it is much easier and generally makes for the most satisfactory accommodation of a considerable variety of plants if in constructed rock gardens they are established at an angle of from ten to forty degrees from the horizontal. This means of course that the joints between the long sides of adjacent stones will run similarly, which emphasizes the stratification.
In natural formations of stratified rock, fractures, called primary joints, spaced from 1 foot or so to up to about 5 feet apart, commonly occur along the sides of up-tilted masses, but not along their faces. These are at right angles to the lines of stratification and cleave the rock into approximately rectangular blocks. They may be simulated in constructed rock gardens by positioning the ends of individual stones to produce chinks and crevices that cross the lines of stratification at right angles and where exposed surfaces, which consist of superimposed pieces of rock, extend through more than one layer. To accomplish this, take care not to place the rocks like bricks in a wall with their vertical separations staggered, but have them above one another, with the chinks thus formed angling downward from the tilted top of the exposed rock.
The ends of uplifted masses of stratified rock show no regular system of primary joints, such as just described, but they may be creviced vertically by frost action or as a result of water running down them.
Whether the garden be big or whether it occupies no more space than an average living room, the principles discussed are applicable. Only scale differs. In large gardens, bolder features necessitating the use of larger rocks are needed, and by the same token, rockless or sparingly rocked portions can be more expansive.
The final effect must be one of rightness, of belonging. If the development adjoins a house or other building, make sure it seems that the rock is native and the structure was built upon it, rather than rocks have been brought in and piled or positioned against or in front of the building. And if your rock garden is to outcrop from a lawn or meadow, perhaps rising no more than a couple of feet or so above ground level, perhaps higher, let it, by the way it slopes into the ground, suggest firm ties with imaginary underlying bedrock.
Scree and Moraine
A scree or moraine is often included as part of a rock garden. Such developments are patterned after natural features of the same names that occur in mountain regions. Their special characteristic depends upon the material of which they are formed and into which the plants root. This mostly consists of fragmented rock, in natural screes detritus collected in rock slides and at the bases of cliffs, and in moraines along the fronts and sides of glaciers. Natural moraines are further commonly characterized by having flowing through them some distance below the surface cold melt water from the ice. It is less natural for screes to have any constant flow beneath the surface.
Rock garden screes and moraines simulate to a degree natural ones. Their purpose is to provide plants with extremely well aerated rooting mixes of low fertility. Surface water should drain through them rapidly, leaving a film around each particle to meet the needs of the plants.
To make a scree, which may well slope away from the base of a cliff like rock or occupy a sloping gully, install over a base of crushed stone or other very adequate drainage a foot or more of a mix consisting very largely of crushed stone or gravel, grit, and coarse sand with a small admixture of topsoil. Approximate proportions may well be one half by bulk crushed stone or gravel, and one quarter part each grit or sand and soil, but these proportions may be varied somewhat depending upon the character of ingredients.
A moraine, in the horticultural sense, differs from a scree, although the words are often used interchangeably, in that a foot or two below its surface there is a constant slow flow of water. This may be arranged by a shallow basin of concrete or one formed of clay or of earth covered with heavy polyethylene film as a base, with faucet or other source of a trickle of water supplying one end and an outlet at the other. To be most harmonious, arrange for the surface of the scree or moraine to slope gently away from the base of a cliff or down a gully and have a few pieces of rock a little bigger than the average, of which the rooting mix largely consists, showing at the surface.
Planting the Garden
Planting a newly built rock garden is best, but not necessarily, delayed for a few weeks to allow for any settling of the soil or rocks that may occur. But if each rock is set on a firm base and the soil is packed well around it and between neighbor rocks there will be little, if any, movement and planting may begin as soon as convenient. Early spring and early fall are the most propitious seasons for this work.
When placing the plants keep two objectives in mind, any special needs of individual kinds and the overall effect you are creating. If the first is not respected, as for instance the need of dianthus for exposure to sun, of primulas for some shade, of ramondas for a vertical crevice, of sorts finicky about the pH of the soil for acid or alkaline areas, and of other kinds for drier or moister soils, the growth, the flowering, and even the permanence of the plants may be adversely affected.
Endeavor to achieve a relaxed landscape, a feeling of naturalness. If too many single plants of different kinds are spotted around, or if there are not some areas, fair-sized in relation to the extent of the garden, clothed with low creepers such as thymes, creeping phloxes, or Mazus to afford rest for the eye, the effect will be too busy. If groups of the same kind are too equal in size, are of too formal an outline, or are of individuals too evenly spaced, the effect will be unnatural.
Some single specimens advantageously located may serve as special points of interest. Especially appropriate for such use are selected varieties of dwarf conifers, among them arborvitaes, cedars, false cypresses, firs, hemlocks, junipers, pines, and spruces. Occasional individuals of other kinds of plants may be used similarly.
But for the most part, plant in informal groups and drifts that suggest a natural ecological association of kinds. This requires knowledge about how different sorts will grow after planting. You may have this information. If not, acquire as much as you can by observing other gardens and by reading.
Groups may drift down gentle slopes, with the plants closer together in the upper than the lower end of the group and with perhaps a few specimens irregularly placed some little distance from the low side of the main planting. Such outliers suggest the results of seeds dropped or washed from a higher place.
Other groups may occupy little plateaus, hang from the tops of cliffs, or below crevices. They need not be clearly defined. If adjacent groups mingle somewhat at their margins, and if an occasional plant of one crops up as it were as a seedling inside a group of another kind, the effect of naturalness is enhanced.
No matter how knowledgeable and careful you are, it is not improbable that some errors of judgment will creep into your selection of spots for some plants, but if these be comparatively few they can be corrected later by transplanting to sites that afford better growing conditions or more appropriate display.
Choose time for planting when the soil is pleasantly damp, neither wet nor dust dry. Make sure the roots of plants dug in readiness for planting are protected from exposure to sun and wind. Space individuals with some regard for the amount of top growth they are expected to make. Do not break the balls of soil in which roots are growing, but spread roots not encased in soil in their natural positions and work soil between them. Set the plants at the same depths or very slightly deeper than they were previously, firm the soil around them, and soak with a fine spray of water.
Depending upon the kind of plant and the part of the rock garden it is to occupy, the surface between individual plants may be mulched lightly with chips or fragments of stone or, about woodland plants in shaded areas, with screened leaf mold or peat moss mixed with grit or coarse sand. For the best effect, see that the stone chips consist of a mixture of sizes and are of the same or closely matching kind of rock to that of which the garden is constructed.
Care of the Garden
Routine care of the rock garden demands regular attention, but not arduous toil. Beginning in the late winter or early spring, the first task in regions of cold winters without an adequate blanket of snow is the removal of the winter covering. Do this before growth is well advanced and in two or three stages rather than all at one time, so that new shoots and foliage become gradually accustomed to full exposure.
Choose dull, humid, quiet days rather than sunny, windy ones for taking off the cover. Push back into place any plants that have been heaved up by frost action and replace any labels that have been disturbed.
Do not be in too great haste to cut back what appears to be the lifeless tops of woody stemmed plants. Some may surprise you by leafing later. But if you are certain they are dead, do not hesitate, and at the same time clear away dead foliage and any weeds overlooked from the previous year.
Top dressing is next. Prepare a porous mix of topsoil, peat moss and grit or coarse sand as a base and modify it as needed for particular areas of the garden devoted to plants with special needs by adding additional peat moss for acid soil plants, crushed limestone or agricultural lime for lovers of alkaline soils, bonemeal for plants likely to benefit from some extra nutrients, and for kinds known to appreciate richer diets, such as primulas, some old rotted or dried commercial cow manure. But beware of using too much fertilizer. The vast majority of rock garden plants thrive in rather lean soils and become too lush and gross in those too fertile. Before spreading the top-dressing, stir the soil shallowly with a hand cultivator so that the new layer will integrate with the old.
Summer care consists chiefly of weeding, watering (do this only when clearly needed and then soak the ground to a depth of several inches), and taking off faded flowers, plus a certain amount of propagation. Weeding calls for special knowledge. In a garden containing many species and varieties it is not a job for a novice or an odd-job man. Not infrequently, choice plants that perhaps have defied the gardener's best efforts to propagate reproduce voluntarily and one or more precious seedlings will appear in some unlikely spot, in a crevice, on a little plateau, or perhaps among some spreading plant of another kind. Only the keen eye of an experienced rock gardener is likely to detect such dividends, with the result that instead of being ruthlessly rooted out they are nurtured to add yet further glory to the garden. Besides, weeding in a rock garden can be a delightful task, one that gives opportunity to know one's plants more intimately, to observe their manners of growth, and to note their individual idiosyncrasies, that is, if weeding is done when it should be, at the very earliest evidence of the weed growth and before it has begun to take over from the rightful occupants.
In the fall a general cleanup is needed. Cut back the dead tops of plants that are not evergreen and make comfortable for the winter all that are perennial. This is the time, too, to plant hardy spring flowering bulbs.
Winter protection in regions where hard freezing is experienced, but a continuous snow cover cannot be relied upon, is necessary, but it is easy to overdo this. Do not install the cover until the ground is frozen to a depth of 2 or 3 inches, otherwise mice and other rodents may establish winter quarters and harm plants. A covering of branches of evergreens (discarded Christmas trees are fine for this purpose) such as pines, spruces, or hemlocks, is ideal. Or salt hay can be used. It is important that air circulates freely through the covering. Common errors are to put in place too early and too thickly.
Propagating the Plants
Propagation is an important phase of rock gardening. Many of the very finest rock plants are comparatively short-lived or are fickle in cultivation. This makes it necessary always to have at hand a stock of young plants to replace those that may succumb to the heat and humidity of the summer, to the extreme conditions of winter, or to other causes. Raising young plants is fascinating work and makes a particular appeal to the real plant lover.
Rock garden plants are increased in several ways, and the method followed in any particular case will depend upon the character of the plant, the availability of propagating material, and the percentage of increase desired.
Plants of a mat forming type, such as creeping thymes, Mazus, and Draba repens, are easily increased by simple division of old sods. This method also serves splendidly for many kinds that form clumps, as do most veronicas, primulas, and asters. If more rapid increase is desired, or if divisions are not obtainable, cuttings afford an alternative method of securing additional stock. Seed provides an excellent means of obtaining stock of many wild species of plants, but it is not reliable for garden varieties or for improved kinds that you may want to grow. Then again, the species of certain genera hybridize very freely if they are grown near to one another, thus seed collected from any such species growing in a garden where others of the same genus are grown will very likely result in hybrid progeny of unpredictable characteristics and desirability. Dianthus, aquilegias, saxifrages, and sempervivums are typical of this group.
Many rock garden plants can be propagated in the spring. September is also an excellent time to attend to this work, for at this season the trying conditions that have prevailed during July and August no longer have to be faced, and the young plants still have an opportunity to become established before the onset of winter. Stock of kinds known or suspected not to be reliably hardy must be established in pots and plunged to the rims of the pots in a bed of sand in a cold frame for the duration of the winter.
Division is, of course, the simplest means of propagation. All that is necessary is to lift the parent plant and carefully divide it into suitably sized portions, each with some roots attached. If the plant has a great deal of top growth, this is usually cut back somewhat to compensate for the unavoidable root disturbance caused by the operation. The divisions are then planted directly back into the rock garden or potted into the smallest size pot into which their roots can be comfortably fitted in a soil mixture similar to, but lighter than that in which established plants of the same kind are known to thrive. The addition to the soil mixture of a liberal amount of grit or coarse sand ensures lightness. Shade from strong sunlight must be provided, at least until new roots have thoroughly taken possession of the medium in which divisions are growing.
A cutting is essentially a division without roots that, if placed in an appropriate environment, may be expected to develop a new root system. Until new roots are sent out, cuttings require special care, and every effort must be made to provide conditions favorable to root development. The medium in which cuttings are planted is usually clean, coarse sand or perlite kept constantly and evenly moist, but some kinds, for instance heaths and heathers, root more readily in a mixture of sand or perlite and peat moss. Protection from currents of moving air, shade from direct sunshine, and the maintenance of humid atmosphere check excessive transpiration and evaporation. This is important because if the cutting continues to lose from its tissues more moisture than it is able to replace, it quickly withers and dies. A well managed cold frame provides suitable conditions for rooting cuttings of a great many rock garden plants. If a considerable number are to be inserted, install a 3 to 4 inch deep bed of the rooting medium in the frame. For lesser quantities, a flat will suffice. Be sure that the medium is moist and packed down firmly by pounding it with a brick or an equivalent tool.
The cuttings will vary in length according to kind, the smallest perhaps not exceeding ½ inch, the largest up to 3 inches. Cut them cleanly across with a keen knife or razor blade at the base just below a joint or node, and trim off the lower leaves. Plant them so the base of each sits squarely on the bottom of the hole it occupies, and pack the sand firmly against it. After the cuttings are planted, water them thoroughly with a fine spray, and then cover the frame with the sash. In the beginning, ventilate not at all or at most sparingly and provide shade from direct sunshine. But when the cuttings commence to form roots, more ventilation and less shade are in order and finally the young plants should be exposed to the ordinary outdoor conditions that suit their kind.
As soon as good root systems have developed, transplant the new plants into small pots. Use a gritty or sandy soil mix and make sure of good drainage by putting into the bottom of each pot a few crocks. After potting, sink the plants to the rim of their pots in sand or peat moss in a cold frame.
Plants from Seed
Raising alpine or other rock garden plants from seed sometimes brings interesting problems, but it is impossible to generalize as to procedures except in the broadest way. Experience and observation suggest that the importance of compounding exact soil mixes to meet the requirements of individual species is frequently over stressed. In their early stages at least, the vast majority of plants can be successfully raised in one of three distinct types of soil. The first contains lime, preferably in the form of ground limestone, but ordinary builders lime will do. The second is free of lime, but contains an abundance of leaf mold or peat moss. The latter is particularly desirable for plants known to need an acid soil. The third is an ordinary, porous seed soil, such as you would use for the majority of garden annuals, but considerably more gritty. This type will be used most often, since the majority of plants thrive in it during their early stages. Lime loving plants such as encrusted saxifrages, need the first mixture. Woodland plants in general prefer the second mix. Of far more importance for most sorts than the exact chemical reaction of the soil is its physical condition. It must be porous and drain freely.
Pots, pans, or flats, according to the amount of seed to be sown, may be used. Most gardeners agree that it is desirable, after sowing, to expose the seeds of alpine, to or near freezing temperatures for a few weeks before putting them into a cool greenhouse or similar environment to germinate. But often the most practical plan is to sow seeds as soon as they are obtainable. Many will germinate in a few days to a few weeks, others may take several months, even a year or longer. Keep those that do not germinate quickly moist, and in fall sink their containers to their rims in a bed of sand or peat moss in a cold frame or outdoors. They may be left there until they germinate, or to hasten germination, they may be brought into a cool greenhouse in February. Alternatively, mix the seeds with slightly damp sand or peat moss and store them in a plastic bag in a refrigerator at 35 to 40°F for three or four months before sowing.
Detailed care following germination plays an important part in the degree of success attained. It is particularly important that the soil be kept uniformly moist. When the seedlings are of such size that they can be transplanted, a little more thought than when seed sowing should be given to the exact soil mix most suitable for each particular kind. Only by experience and experiment, and often a certain amount of error, can these facts be determined, for above all, it is unwise to be too dogmatic about a subject having such wide ramifications as this. Frequently, gardeners following widely different practices get equally good results provided fundamental principles are not violated.
Rock garden plants for temperate and cold temperate climates include a vast array of alpines as well as natives of lower elevations that, by custom and for convenience, are accepted as appropriate. Most are species and varieties that somewhere occur as wildlings, but practically all rock gardeners admit a selection of garden varieties and manmade hybrids. These are usually limited to sorts that look as if they could be natural species and varieties, although this is scarcely true of a few that have double flowers. Nevertheless, it is generally considered inappropriate to admit plants of distinctly gardenesque appearance, those that strongly suggest the hand of the plant breeder.