In most cases a rock garden should be segregated from the rest of the garden by a screening of trees and shrubs unless the contour of the land makes this unnecessary; but tiny, pocket-handkerchief-size "rock gardens" consisting of a few rocks let into the soil of a bank or along a path margin can be altogether charming and afford homes for small bulbs.
Details of rock garden construction are discussed in treatises on the subject. Consult a good text before planning a rock garden of any size.
It is of major importance that rock garden plants have an adequate depth of suitable soil—and for bulbs this means at least a foot of good topsoil underlaid by a foot of porous subsoil. Natural pockets or hollows in rock formations are unfit places in which to set bulbs unless they extend downward to suitable depths or are connected with crevices so that the roots have an opportunity to stretch into good deep earth and are assured adequate drainage.
In rock gardens the kind of soil provided is commonly varied to meet the special needs of particular plants. Its character changes from place to place so that specimens growing a few feet apart may be in entirely different soils.
Most plants suitable for rock gardens are low in stature and are natural species and varieties that grow wild in some part of the world and that have not been "improved" by plant breeders. Here belong many bulbs, including Hyacinthus azureus (or Muscari azureum) and other wild hyacinths, crocus species from Mediterranean lands, "botanical" tulips ( so called because they are usually known by the Latin names used by botanists rather than by common names ), and tiny narcissuses exquisite in form and sometimes delightfully fragrant.
Despite this preference for natural species and varieties of plants, a few man-developed kinds are accepted for rock garden planting and are included even in the gardens of connoisseurs. The narcissus varieties "Queen of Spain" and "Raindrop" and "W. P. Milner" come to mind as examples. Provided they do not look out of place in naturalistic surroundings, few will quarrel about the admittance of such plants.
Stalwart bedding hyacinths, obese garden crocuses, bedding and border tulips, and the large-flowered developments of narcissus hybridisers are generally not for the rock garden. Just which bulbs other than natural species and varieties may properly be planted is largely a matter of taste, but one thing is certain: they should be reminiscent of wild types and should not suggest their garden origins.
In the rock garden more than anywhere else, bulbs must be cleverly related to their immediate surroundings, to neighbour plants, and to rocks close by. A rock garden should present a series of intimate pictures—a succession of plant close-ups that can be enjoyed and studied at near range. The larger, over-all picture of the garden is important, but always in the finest examples this will be found to consist of numerous lesser views worked out in fine detail. The designer of rock gardens does not use as bold a brush to paint his landscapes as does the creator of perennial borders.
Set rock garden bulbs in naturalistic, informal groups. Let the individuals within each be spaced irregularly so that they rise from the ground as casually as if they originated from chance-sown seedlings. In one spot they may be close together, in another more scattered, and occasional outlying individuals may suggest offspring that have grown from seed transported by wind or running water. The number of individuals comprising a group may vary from as few as two or three to hundreds.
Because bulbs are without foliage for part of the garden season, it is important to set them among plants that cover the ground when the bulbs are leafless. There are numerous rock garden plants that serve this purpose—creeping thymes and veronicas, Mazus repens and acaenas, trailing hypericums and Lotus corniculatus, for example. The list is long and well worth exploring for variety.
See that the groups of bulbs vary in size in different parts of the garden and, when more than one patch of the same kind is used, that these patches vary in size also. Small groups of choice bulbs such as dwarf narcissuses, rare brodiaeas, and species tulips may be used effectively near walks where their individual beauty may be enjoyed easily, while larger drifts of commoner kinds such as grape-hyacinths, scillas, snowdrops, and crocuses may be located farther back.
For sunny places, select tulip species ( those that the catalogs term "botanical") :_for instance, Tulipa Clusiana, striped red and white like peppermint candy; the waterlily tulip, T. Kaufmanniana, which comes in many distinct and beautiful varieties; T. tarda, with golden-centered pale yellow blooms, usually several from each bulb; and such magnificent species as brilliant red T. Greigii and T. Eichleri.
Crocus species—including fragrant, golden-flowered Crocus chrysanthus, early Crocus susianus, and Crocus tomasinianus, with rosy lavender flowers—are also good for sunny locations. The last named self-sows freely and if you are careful not to weed out the grass like seedlings, in a few years you will have a blow of hundreds where you planted perhaps a dozen. Not all crocuses bloom in spring. The flowers of Crocus speciosus and its many varieties expand in October and there are other fall-blooming kinds.
In sunny places, too, set gorgeous fall-blooming Sternbergia lutea, as well as Hyacinthus azureus, Iris reticulata and its varieties including the clear blue gardens.
"Cantab," brodiaeas, and the smaller alliums such as senescens glaucum, pulchellum, flavum, and the blue-flowered Beesianum, caeruleum, and cyaneum. For a particularly happy combination, plant Allium cyaneum close to or among dwarf Achillea tomentosa and in June delight in a display of blue and gold.
Scilla chinensis ( sometimes called S. /aponica) thrives in sun and increases from self-sown seeds. Like Scilla autumnalis, it flowers at a season when squills are least expected ( in this case in August) and displays pink flowers clustered along six- to twelve-inch stems. There are many other squills, the majority being spring-bloomers. Most, if allowed, self-sow as freely as Scilla chinensis. Glory-of-the-snows are excellent under conditions that suit these scillas.
Grape-hyacinths thrive in sun and are well known. No rock garden should be without a few of the rarer ones. Be sure to include Muscari moschatum or one of its varieties such as ma/us or flavum. Although the flowers are not showy, they will enchant you with their fragrance, especially after rain. Other sun-loving bulbs for the rock garden are zephyranthes, cooperias, oxalises, and calochortuses, as well as bulbocodiums and leucocrinums.
In locations that are shaded for part of each day but are exposed to sun for at least a few hours, many bulbs recommended for sun will thrive. Here you may also plant bulbs that prefer part-day shade such as puschkinias, Ornithogalum nutans, snowdrops, colchicums, cyclamens, camassias, and choice narcissus species. Among the latter, Narcissus Bulbocodium and its varieties are "musts," for who is not charmed by the slightly upturned hoop petticoats they display in earliest spring?
Narcissus cyclamineus, a diminutive one that lays its petals back like the ears of an angry horse, is excellent; so, too, is that three- to four-inch daffodil—the earliest of all to flower—Narcissus minimus. Other good narcissus species for the rock garden are N. /uncifolius, N. triandrus and its varieties, and N. calcicola, the last named known to be native in only one locality in Portugal but easily raised from seeds and very adaptable in American gardens. These bulbs for part-day shade prosper also where a not too heavy canopy of trees creates a dappled shade throughout the entire day.
Bulbs for shaded spots in the rock garden should be chosen from among low-growing kinds that like woodland conditions. This brings us to a consideration of woodland gardens and of bulbs suitable for planting in them.
The woodland garden may be large or small, flat or hilly, heavily or lightly shaded. Its soil type and the kinds of trees that dominate it will affect its character and its possibilities. For the best results the woodland itself must be controlled. Unwanted trees and underbrush must be cleared, low-hanging limbs that serve no useful purpose removed ( for dappled shade or good side light is much preferable to dense shade ), the soil prepared by forking it deeply and by incorporating into it generous amounts of organic matter, compost, leafmold, or peatmoss. Each fall a mulch of leaves or compost should be spread to protect and nourish plants in woodlands.
In a patch of tended woodland many bulbs will thrive—fewer if the shade is dense than if a little sunshine filters in from top or sides, but a surprising number in any case. Many of the best are native Americans such as trilliums, bloodroots, jack-in-the-pulpits, Dutchman's-breeches, squirrel corn, spring beauties, wood anemones, and dainty rue-anemones. Here belong, in dampish soil, camassias and deadly zygadenuses. Among Americans, also, are certain erythroniums, fritillarias, and dainty hepaticas that welcome shade and woodsy earth.
Important as native bulbs are, they are not the only ones for woodland landscapes and shaded rock gardens. Spanish and English bluebells revel in such an environment and multiply from year to year if given congenial conditions.
Their blue- and white-flowered forms are lovely. Plant them generously wherever you have space. The pink-flowered forms are often disappointing because the pink is too bluish to suit most people's taste. Some of the European anemones are delightful and are suitable for shaded areas: Anemone blanda, A. nemorosa, and easy-to-grow and entirely charming A. ranunculoides belong here. Give them choice positions where they do not have to meet strong competition from the roots of vigorous neighbours.
Winter aconites and snowdrops go well together in well-thinned woodland where shade is not dense. Also under such conditions you may successfully grow summer snowflakes and hosts of narcissuses, and in a moistish spot the lesser celandine ( Ranunculus Ficaria) and its copper-coloured and double-flowered varieties.
Colchicums appreciate the very lightest shade and rich porous earth. The large white, lavender-pink, pink, and rosy violet flowers of kinds usually grown make brave displays in August and September. The flowers come up in groups, many from each old bulb, and quite naked of foliage. In spring large coarse leaves appear and are attractive until June, when they die down and for a period assume sear-brown hues that seem out of place in midsummer, but less out of place at the fringe of a woodland or in other "wild" landscapes than they would be in the tidier environment of more formal flower borders.
There are true lilies that either need or tolerate some shade, but none are plants of deep woodland and all need good light although not necessarily much direct sunshine. Lilium Hansonii, L. testaceum, L. elegans, L. superbum, L. canadense, L. philadelphicum, and the somewhat difficult L. Grayi are all worth considering.
The placement of bulbs in a woodland garden calls for much the same skill as planting a natural rock garden. It is based on an appreciation of nature's patterns, yet not a slavish imitation of them. In any patch of planted woodland there are likely to be many more different kinds of plants than there would be in a similar natural area, yet the effect should appear natural and convincing.
Avoid overplanting. Remember that under woodland conditions individual plants and groups of plants need more space between them than do those that grow in the open. The forest floor in temperate regions is rarely as crowded with vegetation as the meadows and open fields; a feeling of restraint characterizes woodlands. To achieve conviction, try to locate each kind of plant where it is likely to prosper. Thrifty plants that increase moderately and bloom freely are the best advertisement of a gardener's skill in planting. If most of the bulbs in your woodland garden are really doing well, it is more than likely that their arrangement will be pleasing.