Anyone who can live and garden in a place like our part of Connecticut without going overboard for rocks and rock-loving plants has more self-control than I. My bewitchment began one Sunday afternoon when, the weekend’s weeding done, I strolled up toward the small wild-flower plantings in “the point.” Just beyond the garage, before the trees began to cut off the sun, I stopped to glare at an ugly, erupting mound of soil and stones that had frustrated our every attempt to make this area more attractive.
I was pondering the monstrosity, when I noticed an intriguing detail. A partly submerged rock poking its head out of one side of the mound had the same color and patina as a snub-nosed piece sticking out the other side; and the lines of stratification were on the same slant. Could these be opposite ends of one continuous rock formation?
Five wheelbarrow-loads of rocky debris later, we had uncovered a beautiful boulder, perfectly placed so that its lines led gracefully into twin trunks of a wind-blown birch. It was a boulder with ancient age and character, artistically chiseled by the elements. And it said “rock garden” as plainly as any landscape plan.
This five-foot-long discovery may not be miniature; and the naturalistic kind of planting it inspired is neither practical nor suitable for many home grounds. But it was my introduction to rock plants, and to how effectively rocks and plants combine; and it led me to create and watch for rock gardens that are miniature, practical, and suitable for all kinds of grounds. Sometimes I think these are the most enchanting little gardens of all.
If they are to look like anything better than a pile of stones, miniature rock and wall gardens are not composed of miniature rocks. They simply contain fewer, perhaps somewhat smaller, rocks than average gardens; and they occupy far less space. These small plant-and-rock compositions brighten nooks and corners where other plantings would seem out of place.
By their nature, most rockery plants are miniature in height, foliage, and flower. Alone or combined with miniature bulbs, the smaller perennials and annuals, and the tiniest shrubs and trees, they invite close-up enjoyment of their daintiness in these small plantings. Rocks and rock-loving plants have a perfectly natural way of blending and contrasting, each bringing out the best qualities in the other.
Although they’re of different design and construction, rock gardens and dry walls are usually planted with the same or similar plants, and given similar care. Paths, pavements, and garden steps are close kin. But pools and water plants, even though they are often included in rock gardens, are a separate subject, and are covered in Chapter 12.
“Cemetery,” “penitentiary rock pile,” “collection of geological specimens”—these and other unflattering phrases have been used to describe what a rock garden should not be. This eliminates the artificial rock-studded circle in the center of a lawn, rocky slopes with nothing to back them up, isolated paths and steps that have nowhere to go, stone-stuffed piles of loam with no conceivable excuse for being where they are.
What a rock garden should be, is an integral part of the landscape, completely at home and in harmony with its surroundings, and having a sound reason for its existence. Except for a few functional affairs such as walls that retain terraces and steps that climb banks, their purpose is mainly ornamental. Neither rocks nor plants are featured at the expense of the other. Both work together to create the picture—the plants to add brightness, vitality, and life; the rocks to help provide favorable growing conditions and a naturally suitable background for their display.
Designs for these gardens can be, as for others in the landscape, formal or informal. The fixed geometry of walks, walls, and raised beds in formal pattern are seen less frequently. But this is a type of rock garden that might be placed advantageously in even the flattest, barest suburban lot where naturalistic plantings and rocks look out of place. If you long for a rock garden, try something like this in either classic or contemporary style.
Informal rock gardens must, above all else, look convincingly natural, as if the glaciers might have placed them. This is equally important to a cluster of bulbs at the base of a single, half-sunken boulder; the arrangement of rocks and plants on a small slope; or the series of rocks that line a winding path. Builders of ambitious rock gardens are often advised to study the nearby countryside and use it as a guide.
Designs for Naturally Uneven, Rocky Land
This type of terrain makes a natural setting for rock and wall gardens of all sizes, shapes, and contours. Generally, the design is informal and naturalistic. I don’t know whether it’s really easier to work out such designs in miniature, or whether I merely think it’s easier because that’s the kind of land I have to work with. So far, we’ve never disagreed with Nature about the placement of a rock outcropping, for example. The few we’ve uncovered and cleaned up have almost told us what and how to plant.
A small existing slope or bank is a perfect site for a composition of rocks and rockery plants, or, if you prefer, a retaining wall between the two levels with plants grouped at the bottom or top, or planted horizontally in the cracks. Or consider a series of ledges that create small, irregular terraces.
A slope or grade may be the perfect place for informal stone steps, with, of course, a path leading up to them and away. Colonies of small plants look well on either side, with, perhaps, very miniature evergreens at both sides at the top. Cracks in the steps and a walk can be planted with small carpeters.
If you possess a small stream, gulch, or ravine, by all means consider a little rock garden along either side. If you’re fond of a gnarled old stump, see what a few rocks and plants might do at its foot. Where your driveway curves, try a small rock garden. Before you fill in and flatten out a small depression, or level off a small knoll, find out how it would look as a small “island” rock garden.
Corners like those formed by an old-fashioned “stoop” of a country cottage sometimes make effective frames for miniature rock gardens. But in general, informal designs usually fit best out in the grounds, away from angular architectural lines.
Raised flower bed copied from an old English garden
Designs for Level, Rockless Grounds
Here there are fewer existing, desirable locations for rock and wall gardens. But for the sake of enjoying the dainty plants, and of marking your garden with originality, you may want to create a site. It is possible to dig out a “natural” depression or build up a slope, if it’s skillfully designed—and if you can get rocks that don’t look foreign and far away from home. Or dig down a foot or two to make a formal or semiformal sunken garden, the beds raised and held in place by a dry wall with crevices to plant in, and flagstone walks.
Corners sometimes offer possibilities—a far corner, where there is a background of trees, fence, or wall; or a front corner where the lawn slopes slightly down to the sidewalk. Or there may be a place near the patio for a tiny pool in a rocky setting.
A birdbath, sundial, or shrine that’s a focal point in a small garden can sometimes be embellished with a small rock garden at the base. So can the edge or corner of a raised patio. Terrace pavements with cushions of thyme rooted in the crevices are enriched with color, fragrance, and texture. The lines of stark steps are softened and the appearance mellowed by crowded clumps of fat sempervivums. Wherever you can find reason for a rock wall, or even a row of rocks laid horizontally and only a few inches high, you can plant it with rockery plants at top, bottom, or in between. In the artificial settings of many suburban lots, semiformal walls separating two levels of lawn or supporting the edge of the family living area are very suitable substitutes for rock gardens.
One of the main virtues of rock and wall gardens is their individuality—they don’t look like other gardens, or even like each other. But commonplace plants such as bedding petunias can cancel this distinctiveness in a minute. If you use annuals at all, get less ordinary varieties and use them sparingly, for temporary color in a bare spot or over the dying foliage of spring-flowering bulbs.
The plants that look best with rocks are those that grow naturally among them. Hundreds of rock-loving plants are available, and more hundreds of alpines from rugged mountain heights. Only the easier alpines are included here, the most adaptable to more luxuriant climates and soil, the least likely to pine for their rigorous, high-altitude homes. Tricky types from above the timber line are left to the dedicated collector.
Saxifraga seedlings—a natural rock-loving plant
Nurseries and catalogues of rock-garden specialists are so full of distinctive and delightful miniature plants that my first reaction was blissful delirium. And after I acquired as many enticing items as I could, my second thought was: These gems are too little known and grown. So I began to sort out some that might flourish in our small perennial borders. With favorable conditions of drainage and air circulation, a number of rockery plants have already made themselves at home in other, rockless gardens.
Most miniature bulbs are effective in rock gardens, and some miniature perennials—not ubiquitous types, and not those that spread voraciously by runners. Miniature shrubs and trees are indispensable in rock gardens of all sizes—to give variety, contrast, and substance; to act as accents; to create boundaries or backgrounds. Use them with restraint. Place spreading, wind-swept types at the top, bushy shapes down lower, upright exclamation points at the very bottom. Make sure they are in scale with the rocks, the plants, and the garden as a whole.
Select all kinds of rock and wall plants for their effectiveness of form, texture, and color in the complete design. And this repetition is important and unavoidable. Select varieties that naturally like, or will adapt to, the cultural conditions like sun and soil type you have or can provide.
For setting a few rocks into the side of a handkerchief-sized section of slope, or building many more into a garden in a good-sized corner, there are a few guiding principles that will help make sure you are happy with the result. Most important, have your design as complete as possible before you start to build. A sketch drawn to scale is a valuable visualizing aid. Then, make your construction solid and substantial, and favorable to plant growth and health.
If you have a choice of exposure, face a sloping rock garden toward the north or northeast, so it will get enough summer sun but not so much that it dries and cooks. Shrubs and trees in the background should not create dense shade in the garden, nor drip moisture on the plants and steal nourishment from their roots. Drainage is extremely important in selecting a site. If the subsoil is packed and clay-like, you can either replace it, make it more porous, or select another spot.
What kind of rocks to use and where to get them depends on the part of the country you live in. Anything that’s indigenous to an area is first choice, always. Limestone, sandstone, fieldstone, porous tufa—native rocks “belong” as no artificial or imported rock ever can. If your land is not blessed (or cursed) by plenty of rocks, you can sometimes collect them near rivers, mountains, or abandoned quarries, or where a new road is being blasted through. Whether you have them, collect them, or buy them, all the rocks in one garden (or on the same property) should be of the same general type. Varicolored geological specimens are seldom attractive.
Consider each rock’s characteristic color, texture, size, and shape before you decide where to place it, or whether to place it at all. Even in the tiniest rock gardens, tiny rocks look ridiculously salt-and-peppery. Start with rocks of substantial size, in keeping with the size and contour of the garden; use smaller ones where they fill in effectively.
Naturally worn and weathered rocks look more natural than freshly chiseled surfaces; irregular, angular forms are better than uniformity. Rocks with stratified layers suggest ledges and cliffs. Glacial boulders or “hardheads” are not good in formal patterns. Use them naturalistically, in varying sizes, arranging them with studied carelessness as if they’d been scattered haphazardly and then left.
Avoid overcrowding with so many rocks that the plants are overshadowed by the debris. Place rocks as they might lie naturally, partly buried in the soil, the tops smoothed over by the elements, and so solid that they don’t wobble when you step on them.
Construction
Perfect drainage, so that water cannot stand around the roots and crowns, has been called “the most important need for all alpine plants.” The selected location (raised or sunken) and composition of subsoil and topsoil can either contribute to good drainage or deter it. In small gardens it’s not too formidable a job to install a system of drainage tiles, if necessary, or to add gravelly sand to subsoil to make it more porous. Easy insurance in any rock garden is to start with several inches of rather coarse gravel or rubble as a bottom drainage layer.
Any topsoil that is removed during construction should be saved carefully. If you can pile it in the center of a tarpaulin or piece of strong burlap, it can’t be scattered and lost; and you can also mix it with any additives with pleasurable ease. Simply pile on the peat, leaf mold, sand, or other ingredients; then, holding opposite corners (you may need a helper to hold one corner of a large piece), pull it back over the soil pile and fold it over the other end. Repeat this, from opposite ends, until the materials are thoroughly mixed.
The first rocks are usually set at the lowest part of the garden, partly sunk in soil or otherwise made extra-sturdy and secure. Build up from that point, arranging rocks with their broadest surfaces at the bottom, all lines of stratification horizontal or on the same slant. Each rock should slant slightly inward and downward, to catch moisture and encourage it to run into the soil in the garden.
Provide plenty of soil for spreading roots, and a minimum depth of eighteen inches or more. Plants in shallow pockets may be stunted and will certainly dry out too fast during droughts. Whether or not you plant as you go depends partly on your agility. Don’t crush the first plants under your feet or knees as you set rocks and plants above them. It’s often faster and easier to spread out roots and make soil firm around them before the next rock is set in place.
Planting
Set plants as deep as they were planted before; no deeper (danger of crown rot) and not too shallow (they may dry out). Tuck them firmly in cool, moist, rock-shaded soil. Keep the soil constantly moist until firm new growth shows that the roots are established and growing.
Rock-garden plants are seldom suitable for planting in neat rows or geometric patterns. Tiny, slow-growing types are arranged in groups of three or more; one or two spreaders per planting spot will do. Don’t crowd these plants. Give them room to develop and for air to circulate around them, and to display their charms at their best. Provide private quarters for the smallest gems, away from hearty growers that might overpower them. Keep color combinations and contrasting textures in mind. Set trailers where they can droop over stones. Place those that like moisture low in the garden; their roots keep cool in the shade of rocks.
As a finishing touch, after the rock garden is built, planted, and thoroughly watered, cover all soil surfaces with a mulch of fine gravel or stone chips. Work it up to each crown, under prostrate stems. Aesthetically, this gives a neat, well-groomed look and ties plants, rocks, and setting into one coherent picture. Functionally, the chips absorb heat and keep soil cool and moist, lift small, low leaves out of mud and water, discourage slugs, protect crowns against rot, and firmly discourage the intrusion of weeds.
Any small section of low wall—brick, cement, stone, or concrete—can be the background for a miniature garden. But it is in a dry wall, built of stones without cement, that many small rockery plants best display their jewel-like perfection. Planted horizontally, with their roots in pockets between the rocks, they make a vertical rock garden of glowing colors and fascinating foliage textures.
For small, low walls, and those in somewhat formal settings, thin-layered rock such as slate or shale is usually preferred. Irregularly shaped fieldstone or other rocks of native origin are used in more natural, informal designs. Any lines of stratification are, of course, laid horizontal and parallel to each other. Another good rule to go by is: The smaller the wall, the smaller the rocks.
Construction
Low walls (two feet high or less) usually do not need a foundation layer sunk below the frost line. They can be started at, or just under, ground level. But they must be set solidly on a firm, wide base—two feet wide, for example, for a wall two and a half feet high. Set the largest stones and complete the lower layer first. Tilt each rock slightly backward and downward so soil won’t wash out of pockets, water will run back to plant roots, and the heaving pressure of frost won’t thrust the rocks out of place in winter. A guide line of string or wire moved up with each new layer will help you keep the construction level.
The wall should slant slightly in toward the bank behind it, and usually grows more narrow as it goes up. Each layer is made absolutely solid before the next is applied. Set each stone so that it is completely steady, and rests on parts of two stones beneath it. This distributes weight evenly, avoids crushing plant roots, and prevents unattractive and impractical long, vertical cracks created when joints coincide. Use small, flattish rocks to fill chinks under large stones and make them steady.
Behind the wall, as you build, firm in a goodly layer (one foot thick is usually recommended) of topsoil that has been enriched and aerated to promote drainage. Roots will reach back here for water and food. As you ram soil into pockets, make sure it reaches all the way back to this layer. Unless all the soil is made very firm, it may settle and the stones may slide out or into the bank.
At the top you can leave some pockets for plants, or plant them just behind the wall. Cover cracks at the top with flat stones, so water won’t wash out soil.
A rock, a rose, and a miniature rose.
Planting
If you can remember not to damage plants at the bottom as you’re working above them, set the plants in each layer as you build. It’s easier then to make sure the roots are firmly embedded well back into the soil. Plant sparsely; leave plenty of room for growth and spread, and plenty of uncovered wall surface to contribute pattern and texture to the picture. Space the plants at irregular intervals, never in straight lines either horizontal or vertical. Water thoroughly, making sure the layer of soil behind the wall is completely moist; and keep it moist at least throughout the first growing season.
Many little rock plants will make soft cushions and carpets, with their roots in crevices between pieces of paving, and also in stone steps. They’re very low-growing and very hardy, and usually spring back resiliently when stepped on. It’s important that acid-loving varieties not be planted near cement, which sometimes neutralizes or alkalinizes the soil. And, of course, the situation should provide the amount of sunshine and moisture the plants need.
The simplest kind of informal path or paving for an informal patio consists of flagstones or slates set in the ground, the top level flush with the surface of the soil. Plants are spaced irregularly in cracks between the stones. I’ve seen one path of old millstones laid this way, the plants rooted in the center hole.
Bricks create a more formal pattern, either laid on a bed of sand or set into a foundation of concrete over gravel. The popular “crazy pavings” can be odd pieces of either brick or stone arranged in any conceivable pattern and laid in mortar. In any such solid constructions, planting pockets should be scooped out before the concrete sets hard; or a chunk of wood can be inserted to hold the space open.
Give the carpeting plants as much good soil as you can, and keep them moist until they are growing well. Occasional trimming will stop them from growing straggly, and keep them neat, compact, and in a pretty pattern.
Planting times vary according to climate and season of dormancy. In warm areas it’s almost always late fall. Where winters are rugged, it can be during brief semidormancy in early September, but it’s safest in early spring when severe frosts are no longer likely and favorable weather blesses plants at the height of their annual growing power. In temperate sections spring-flowering varieties can be planted from mid-August through October, others in spring.
Lemon thyme and ivy showing results of being trimmed and kept neat
Mr. and Mrs. Alex O’Hare
Strong, healthy, pot-grown plants can be planted throughout the season in almost all areas. They transplant easily, with little disturbance and shock. In fact, whenever we acquire plants during the fall, we keep them in their pots or pot them up, and protect them in the cold frame during the winter. One of my favorite and most knowledgeable rock gardeners tells me that he always pots tender or difficult plants, and keeps them potted until their roots are fully developed, before he sets them out in the garden, no matter what the season.
Soil
Providing the proper soil is vitally important to rock-garden plants, and particularly to alpines. They’re naturally dwarfed by the rough conditions in their native mountain areas. But when they move down to our comparatively lush climates and longer growing seasons, they respond to a too-rich diet by growing too tall and rangy, with too few flowers, and they lose their charm. They do need some nourishment, of course. And they are extremely sensitive to soil textures and drainage conditions. Heavy, soggy soil causes them to rot quickly. Soil that lacks organic matter dries out too fast.
Because of climatic and other variations, I hesitate to set down any “ideal soil mix” for these plants. Instead, I suggest a basic formula that can and should be varied according to the aridity or rainfall in your area, the size and location of your rock or wall garden, the composition of your garden soil, the availability of some ingredients, and the personal preferences of the plant varieties you want to grow. Here it is:
Three parts fairly friable, fertile loam.
Two parts humus—peat, leaf mold, compost—for lightness and fertility.
Two parts stone chips or crushed rock—perhaps the most important ingredient, to keep soil open and cool.
One part sharp builders’ sand, or the gravelly sand used in mixing concrete, to lighten heavy soil and promote drainage.
If your garden loam is sandy or gravelly, of course, you would increase the proportion of humus. If it’s rich in organic matter, you would decrease the quantity of humus. Very sticky, heavy clay soils are best replaced.
Rock and alpine plants are also particularly sensitive to the acidity or alkalinity of soil. This is measured by the pH scale, ranging from very acid pH 4.0, through 7.0 at neutral, to very alkaline 9.0. Rock-garden specialists usually indicate the preferred pH for the plants they sell.
By using a soil-test kit or, better still, sending a sample to your state agricultural service to be tested, you can easily find out whether the soil in your garden is acid or alkaline, and to what degree. Then, to make acid soil more alkaline for plants that need it, mix in small quantities of agricultural lime—the amount depends on the degree of acidity and on the plant itself. Don’t overdo it. Too much lime can do as much harm as good. Alkaline soils are not quite so readily converted to acid, but you can use oak-leaf compost (not completely rotted), acid peat, or pine “duff” (rotted pine needles). Or you can be satisfied with the tremendous number of rockery plants that really prefer alkaline soil. These preferences vary, not only between different plant families, but also between individual members of the same family.
For our small gardens, I’ve found it not too tiresome to provide soils of several types. For specially precious plants, I often mix special, small batches of soil. Then I clean out the planting pocket, make sure that it is tightly closed in with rocks so water won’t wash this soil out or other soil in, and refill it with the preferred mixture.
Watering
Except during rare droughts in spring, when the plants are in active growth, we seldom have to water our rock and wall garden. Our summer rains are usually sufficient for their needs. But whenever or wherever there are long weeks of extreme heat and dry winds in summer, water may be needed. Use a fine spray gently, so that the small plants won’t be washed loose. And let the water soak in long enough so the entire area is moist in depth.
Fertilizing
Some rock plants can take and use fertilizer, in spring; but most alpines are better off without it. For them, we mix a little leaf mold with the gravel mulch when we renew it. For the more ordinary types, we provide a very light feeding of whatever balanced fertilizer we have on hand.
Grooming
In rock gardens (and particularly in very small rock gardens) even slight untidiness or imperfection shows up all too clearly. But I consider it a pleasure to weed and care for the little plants a few minutes every weekend. Faded flowers are snipped off. Weeds are removed with determination, but with gentle protection for any small, weak plants. Spreading shoots that grow long and out of line are cut off, or sometimes uprooted, and removed. Louise Beebe Wilder says it’s an art to keep a rock garden in balance, with the proper proportion of plants against stones. And she is an artist with hers.
In spring, even before the last of the winter mulch is removed, we look for cracks in the soil and for plants that have been heaved out by frost, and press them firmly back into place so their roots can take hold again. We also fill any washed-out pockets with fresh soil mix, and top-dress any low areas. Any rotted leaves and stems and other debris are removed; and the entire garden gets a thin fresh layer of stone-chip mulch.
In fall there’s the general cleanup operation—weeding, pressing in loose roots and packing them with soil, trimming back rambunctious growers, replenishing the mulch of stone chips if needed.
Winter Protection
If you can count on a continuous cover of snow over your rock garden all winter, there’s little you need to do for these hardy little plants. But we can’t; we have the danger of alternate freezing and thawing that can do so much damage. So we provide special protection.
In winter, after the ground is frozen at least an inch deep, we cover the complete garden with salt-marsh hay. (Some types of Fiberglas, and wood excelsior from packing cases, are also suitable, reusable materials). This light, airy blanket keeps plants cold and dark, yet gives them air to breathe. If it threatens to blow away, we hold it in place with a few light evergreen branches.
These are, of course, perennials and with slight variations can be propagated like other perennials. (See pages 255–59.)
Seeds
Some need a pre-germination cold and some don’t; some germinate best at cool temperatures, some at warm; some need light, others need dark. Check seed packet, catalogue, or reliable reference book for specific information for each type of seed.
We start seeds of delicate varieties in the greenhouse, where we can hover over them, in late winter; others may be sown in the cold frame in March or April, so the plants will be as mature as possible before their first winter.
Stem Cuttings
For the smaller plants, these can be as short as three inches or less. Try to take snappy new growth, neither weak and watery nor hard and tough. Sharp sand is a suitable rooting medium for many types.