When rock garden was introduced to the United States from England in the Iwo’s, it soon became a craze. Those early rock gardens were developed in the English tradition, with emphasis on the beauty of composition of both rocks and plants, but after a while many of these man-made gardens became an ugly con-glomeration of rocks and stones. Soon, they were overdone, followed by a decline, but after World War II, a fresh approach was introduced in the Japanese style, with stress on openness and simplicity.
A rock garden may be defined as an out-cropping of rocks—natural or devised—where alpine plants from the mountainous regions of the world are grown. Usually it is on a slope, and although the plants chosen generally come from rocky places, usually at high elevations, many are simply low-growing perennials, annuals, bulbs, and shrubs that fit into the category.
Many gardeners are fortunate in having natural rock gardens, where their choice treasures are brought in and arranged in an artistic manner. In other instances, they are constructed with rocks—and boulders—that have been hauled in. This requires great skill, and the best are the result of the skillful execution of outstanding landscape architects and plantsmen. Unless well done, a rock garden can be an eyesore, nothing more than a mere pile of rocks among which plants are set and often allowed to grow rampant.
The natural rock garden is characterized by light, poor, gravelly, well-drained soil. In the constructed garden, this kind of soil is essential. It provides the kind of medium in which most of these plants survive. A heavy soil in winter becomes water logged. By remaining too damp, plants tend to rot, especially where winter rains are heavy. A too-rich soil promotes lush, soft growth that likewise is inclined to become winter-killed.
The artificial rock garden should be constructed with the proper growing conditions in order to display plants that generally cannot be grown elsewhere. It is intended for alpine plants, which are found growing wild on mountains between the tree line and the lower limits of snow. The rocks not only show off the plants to best advantage, but perform other important functions. They help to keep the soil cool and to conduct moisture to the plant roots. Excessive moisture through evaporation is prevented, and the soil is held by them in place. Even when all these conditions are provided, the rock garden may not fare well, but for another reason. It has to do with the length of the growing season, usually varying with that of the natural habitat of the plants.
The well-designed rock garden, especially if large, will be represented by many different kinds of topographical areas. It may have a rocky hillside and a steep slope. It may display a low plain, a hidden valley, a bog, a brook or stream, and a quiet pool, as commonly found in nature. At some point, it may even possess a high and windy mountain peak where few plants grow.
In extensive stretches, larger, bolder plants maybe used. The smaller the rock garden, the smaller the plants should be. Most rock plants are under a foot in height when full grown, but dwarf shrubs, may be up to 3 feet. Although alpine and rock plants are usually selected, others qualify if their size and height are right. They may be mat-forming or spreading and may come from meadows, woods, prairies or bogs. Many that are typical rock plants are often grown in borders with other flowers, among them Arabis, Aubrieta, Gold-dust, Cerastium, Hardy randy-tuft, Dwarf Iris, Ground Phlox, and Epimedium. These can be added to a wide variety of small bulbs and low annuals, like Sweet Alyssum, Lobelia and dwarf French marigolds.
The classical rock garden, with its need for hand and knee labor by skilled gardeners has become a thing of the past. It was intended to copy nature and to display many interesting and unusual plants, some of them rare. Today’s rock gardens have changed to meet the needs of the times. Simplicity and ease of maintenance is the keynote. Yet there are many lovely compositions that have resulted from this new concept which have combined the best and most practical elements of the British and the Japanese, the styles that have helped to mold the contemporary rock garden of today.
As with other forms of gardening, certain basic principles apply—scale, proportion, balance and good design, which includes a pleasing arrangement of the various parts into a harmonious whole. Most of all, it is originality and imagination that count.
Rock Garden Location
The site of the rock garden is of prime importance. If there is a natural outcropping of rocks, such as found in New England, the Appalachians, the Rockies and other mountainous areas of the country, and then select it, since there is nothing more beautiful than an arrangement of rocks placed in position by the forces of nature.
In any case, allow for full sun for at least part of the day. Yet charming rock gardens can be established on natural outcroppings where large trees, too precious to cut down, exist on the property. In such instances, the rock garden will not be gay and colorful in spring and early summer, but it can impart simple charm and a feeling of coolness. In summer, bits of color can be added with Coleus, Patient Plant, tuberous begonias, Madagascar periwinkle, fancy-leaved caladiums and Thunbergii. In early spring, before trees drop their leaves, miniature bulbs and species daffodils and tulips will unfold their pretty flowers.
Rock Garden Design
Before starting to build, whether you will plant around existing rocks or start from the beginning, make sketches on paper. A rock garden, like any other type of garden, is based on principles of design. If it is large, it will need paths and walks, or at least stepping stones and the paths should be of a winding, informal nature. Straight, rigid lines are not appropriate. Paths not only make delightful wandering, but make it possible to reach the plants in order to care for them. Unless comprised of stones, they should be covered with natural material, like pine needles, tanbark, shredded tree bark, or stone chips or pebbles. Be certain that these paths blend in with the surrounding plants.
If working with a steep slope, it will be necessary to make several terraces to hold back the soil. Areas can be leveled off every 2 feet before rocks are arranged on them. In many cases, this can add to the appeal of the rock garden, adding interest because of the level variations.
It is also a good idea to jot down on paper the positions of several plants. At this point, it is advisable to get to know their growth habits.
A rock garden can be built on level ground, although it takes far greater ingenuity to make it look as if it has always been there. Some of the great rock gardens of the world, often found in botanic gardens, are made, and are so artfully executed that they have every feeling of being natural.
When choosing the location, look for a spot that receives abundant sunshine, away from the shade of large trees which cut out the sunlight and rob plants of precious nourishment and needed moisture. When dealing with a slope, this is not always possible, but sometimes, there is a choice. Keep away from artificial surroundings, since a rock garden is essentially a casual, informal type of garden expression that should harmonize with its immediate surroundings. Avoid as backgrounds high, austere wails, porches or the facades of houses, driveways and sidewalks, and a strictly formal garden, with clipped hedges and plants arranged in geometric patterns.
Exposure should also be taken into consideration. Rock garden and alpine plants are sun loving, although this does not mean full exposure to the all-day sun in some instances, especially if the shone faces south, this can be harmful in the case of winter sun and winds. One that faces east is considered ideal, but northeast, west and northwest are also excellent. When dealing with alpines from high mountaintops, north exposure, open to the sky, without any interference from trees, is recommended. This is because these small plants are covered, in their native haunts, by a thick blanket of snow all winter, and are not exposed to the sun or biting winds.
Southern exposures, particularly in the case of more rampant plants such as Ground Phlox, Aubrieta, Arabis, Gold-dust, and Dwarf Iris, are not to be neglected altogether. Many out-standing rock garden specialists have thriving sides of reeks where they present a glorious sight when in full flower. Less vigorous kinds, like small alpines, should be placed in narrow crevices where they will not be overpowered.
When designing the rock garden, avoid pockets where water collects, since good drainage is essential for success. Secure rocks well by placing them deeply. Any that are loose can cause damage when accidentally walked on. Look for rocks that are native to the region. Weathered rocks of any kind are good, but obtain stones that are irregular and asymmetrical and dark in coloring. Rounded stones are bad because they do not look natural.
Select rocks of different sizes, but avoid the use of too many. A rock garden is not a collection of rocks, but a collection of plants arranged around carefully selected and placed rocks and stones. Few types of gardening are easier to overdo than this. A mountain of rocks presents a jarring note that not even a healthy grouping of flowering plants can ameliorate.
Soiling Rock Garden
In a way, soil and construction go hand in hand. If soil is not the right kind, it can be especially prepared to meet the needs of the plants. In the case of existing rocks, poor soil will have to be scooped out and replaced with the proper mixture.
Most rock garden plants are not fussy about soil, and will grow in almost any kind, provided there is good drainage. Some plants require an acid soil; others prefer one that is alkaline. Yet most thrive in soil that ranges between pH6 and pH8. A thin, porous one is best, more so in sections of the country where rainfall is heavy.
Where droughts prevail during the growing season, the soil should be heavier and more moisture retentive to meet the needs of plants. In this case, it should be prepared beforehand with humus. Other aids consist of using mulches of fine gravel or stone chips to hold in the moisture. These will also help to prevent weeds from taking over.
A simple preparation consists of equal parts soil, coarse sand, and peat moss, leaf mold or compost. Another combines equal parts loam, leaf mold, peat moss, sand and fine gravel. Since most rock garden plants are lime-loving, add agricultural lime. Unless soil is very acid, a heavy sprinkling will do. Bone meal or superphosphate, slow-acting phosphoric fertilizers, can be added at the recommended amounts. Some rock plants do not need it, but others like Dianthus and campanulas appreciate it.
If scooping out soil in pockets and between crevices in natural rock outcroppings, dig to a depth of about a foot, where this is possible. Place a layer of stones, pebbles, or pieces of broken bricks at the bottom. Then add a layer of coarse sand or gravel before placing the soil on top. Wash each layer with the hose to make it settle firmly and eliminate air pockets.
Constructing Rock Garden
Constructing the rock garden is not the easiest task. It is advisable to do considerable reading beforehand and, where possible, employ the services of a qualified landscape architect. In either instance, observe and study rock formations in nature. The idea is not to copy them, but to receive inspiration and understand how they comprise a harmonious whole. Small rock can be lifted easily, but with larger ones you will need suitable tools. One or two crowbars will be among the handiest.
If proceeding on your own, first bring together the rocks to be used. Unless you have mastered your design so it is clearly in your mind, keep the plan sketched on a piece of paper close at hand.
Start to work at the lowest point. After placing a layer of drainage material at the bottom, add prepared soil in that particular spot, leaving the rest to spread around the rocks when in their final position. Generally speaking, keep the largest rocks for the base. In some instances, existing soil will have to be removed to make room for these boulders. Place them on their broadest bases, making certain they are secure. When completed, more than half of each rock should be under the surface of the ground. Arrange each so it leans toward the soil in order to catch rain water. Most of the rocks will have to be concentrated in steep places to hold back the soil. Use fewer where the grade is less abrupt, and allow for large levels where quantities of vigorous rock plants will be permitted to spill over the sides. Here and there small rocks can be used to give the impression that they have tumbled down. The key of the successful rock garden is to make it look as natural as possible, rather than man-made.
Before setting each rock in its permanent position, stand back to see how it looks. Turn it around a few times, and you will discover that, what was previously the bottom, may well be on the top. At this stage, it is easier to make changes.
When completed, and before you start to plant, let the rock garden rest for a few days. Up to this point, you have been too close to it and need to get away from it. You will have the opportunity to stand back and see the rock garden from several different angles at various times of the day, under divergent condition of sunlight and shadows. Strive for unity, harmony, with pot grown plants, as is often the case nowadays, you can do the work any time during the growing season, if water is available. Set out plants when soil is moist and crumbly. Avoid a very wet soil, which tends to cake and pack the roots, cutting down on the air supply.
When planting, firm the soil around the roots. You will have to take special precaution to get rid of air between rock crevices. Work slowly, ramming the soil as you proceed. Where space permits, use 3 or more specimens of the same kind in order to produce a broad splash of foliage and color. In small crevices and nooks use small alpines. They look more endearing, and are protected from vociferous neighbors by surrounding rocks. Dwarf types, as saxifrages, primulas, aubrietas, and small achilles, can be spaced 6-8 in. apart. More spreading thymes and Ground Phlox will need at least a foot.
Always strive for informality in the rock garden. A formal rock garden does not exist in nature. Plant singly or in clumps, but never in rows. Allow an occasional plant to stray here and there. Tuck one in a sheltered crevice, another in a narrow opening between stones. Always permit some to cascade, for they impart a special charm. Bring together beguiling foliage textures and patterns, not so difficult if you put your imagination to play.
Maintaining Rock Garden
On the whole, the rock garden requires little care, no hoeing or cultivating and very little weeding, once weeds are pulled up and thick mulches are applied. A minimum of feeding is needed, since a too-rich diet will promote lush growth that tends to rot or winterkill.
Even so, like any other form of gardening, general upkeep must be practiced if the rock garden is to look its best. It can quickly become an eyesore.
In the early spring, after winter covers are removed, gradually, according to the dictates of the weather, check plants to see if they need to be firmed back. Winter thawing and heaving will loosen them, but with the hands or feet this is easily done when soil is moist, but not wet. Some plants may require replanting if they have been pushed out of the soil too much.
A light scattering of a high phosphoric fertilizer, such as 5-to-5, can be spread on the surface of the soil and scratched in with a weeder where this is permissible, if it does not interfere with plant roots. Top dress the rock garden, using a mixture of 3 parts garden soil, 1 part leaf mold or peat moss, and 1 part coarse sand. To this add a 6-in. pot of bone meal to each wheelbarrow of prepared soil.
Planting in Rock Garden
Planting the rock garden requires a special kind of skill. First, become acquainted with the different kinds of plants. Some are shy, others are vigorous. Some are very hardy, others will need winter protection. It is important to know the forms and growth habits of each, as they vary to include the prostrate, rounded, spreading and upright forms.
As a beginner, start with some of the easier kinds, but this does not imply a limited variety. In fact, much of the interest in the rock garden stems from its varied number of plants. As you become familiar with these easy kinds, brine in the more difficult. They call for more specialized attention, but they offer keener pleasure.
A harmonious composition between rocks and plants is the aim of every rock garden, be it large or small. In a way, it is no different from other forms of gardening. Colors of many rock garden plants and alpines are bright and vivid—magentas, rose-pinks, golden yellows, orange-reds. Yet this does not mean they cannot be brought together into a harmonious unit. Where colors tend to clash if placed side by side, break them up through the use of white, the “peacemaker.” Also in the unobstructed sun-shine, where rock gardens are located, bright colors go together more easily, as is often seen in tropical gardens.
Early spring is a good time to plant, but better still is late summer or early fall when most rock plants are dormant. In spring, they are making rapid growth to come into bloom.
The wall garden is more difficult to construct than the rock garden, but the same principles of design are involved. In it are gown small plants that abound in crevices and on cliffs, some that are tufted, some that droop, some that cling. The early spring is the best time to build and plant the wall garden which allows enough time for roots to become firmly established before the ground freezes.
A wall garden is usually placed in front of a bank to hold back the soil behind it. To do this properly, it should be solidly built, able to with-stand the pressure exerted by freezing soil behind it. Properly made, it can be as much as 55 ft. high.
As with the rock garden, the largest rocks should be used at the bottom, and the smallest at the top. Since no mortar will be used, it is the weight of the stones, one on top of the other, that will keep the wall firm and make it last for years. It is preferable to use local stone, although exotic vines can be brought in. The kind of stone to be used will depend, in the end, on the desired effect and the overall surroundings.
The wall garden inclines backward, so that it is lower at the back than in the front. The individual stone also tilts the same way toward the bank. This way the wall is held more firmly in position and the sloping angle permits rain to seep through the crevices to reach the roots the plants as they stretch out to the soil beyond.
When gathering stones, avoid those that are rounded, and select those that are flat any narrow. The largest, that will form the foundation, need not be below the frost line, but they should be secured firmly. Place them in a sloping position that is toward the soil, to about an in. deep, which is sufficient to provide a firm hold and prevent them from moving after heavy rains or cold winter weather. The width of the base should be about one-third of the height of the wall.
The larger the wall, the larger should be the stones. First place a row of the heaviest at the base, each leaning backward. Then add a few inches of soil, and it is well to use the specially prepared mixture recommenced for rock gardens. Always place about 6 in. of this soil in back of each rock or stone. Pack is in firmly to avoid air pockets, which dry out quickly and usually result in poor growth.
The next layer of rocks will require careful placement. Do not rest a rock on top of another but between two, so that its weight is borne by the rocks and not the soil. Continue in this manner all the way to the top. Always place each rock in a horizontal position. When completed, the weight will be carried by the rocks, and there will be no vertical crevices in the dry wall. The ideal way to plant is as you go along. After the rocks arc laid and 2 or 3 in. of soil is placed over them, rest plants in position and spread out the roots, covering them with 2 or 3 in. of the soil preparation. Plant layer at a time, and be certain to tamp the soil carefully.
In many instances, it is not possible to plant as you build. When the construction is completed, scoop out 1 or 2 trowel full of soil from a crevice, insert the roots of the plant, and replace as much of the soil as possible, pressing it firmly. Use smaller plants than you would by the other method, but also be prepared to expect some losses. Water and keep moist until plants are established.
Seeds can be sown in the wail garden in the spring. Mix the seed with moist sandy loam and press into the openings and crevices. A small piece of moss placed on the soil will help to prevent excessive drying out.