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In the planting of rock gardens, we must aim to secure by means of careful selection and grouping, an impression of breadth and boldness in the color masses. Nothing is more satisfactory when contemplating any form of garden art, than the feeling that the designer has from the beginning worked with the idea of achieving some definite purpose. This is especially so in the case of rock gardens.
Beautiful garden pictures are only possible when each small plant is made to play its part in the building up of a definite scheme.
There are hundreds of plants suitable for rock gardens, but only the most desirable should be included. Rarity should not influence selection. The commonest and easiest grown flowers are often the most beautiful. Nor should too many varieties be grown. It is better to plant two or three representatives of a family in bold groups, than to obtain perhaps four times the number of different species, with but a single plant of each.
For rapidly establishing themselves in cushion-like tufts covering the edges of the larger stones, the Rock Cresses (Aubrietia) are unequalled. They will grow in almost any soil, and may be increased by seeds or division. A.Purpurea and A.Deltoidea are the oldest of the group and have long been favorites.
A.Groeca, lilac-purple, and A.Campbelli, deep violet-blue, give lighter shades among the purples. Near these, a sheet of snowy Arabis would afford a brilliant contrast, and to complete the picture we would arrange a cluster of golden Alyssum. These three families of rock plants, all perfectly hardy, provide materials for creating one of those bold color effects, which are often more welcome than subtle harmonies.
Another good grouping would consist of Iberis sempervirens and the yellow Alpine Wallflower (Cheiranthus alpinus). This combination is particularly beautiful on an almost perpendicular rock face, the deep green and white curtain of Candytuft falling among the golden Wallflowers, which thrive well in the earth fissures of rock walling.
An exquisite picture is achieved by a few plants of Lithospermum prostratum (Gromwell), and a cluster of the Pyramidal Saxifrage (S.Cotyledon). Given a large boulder, rising from an earth ledge, we may plant the former so that it falls, a cascade of gentian blue, into the lap of the silverleaved Rockfoils. In addition to the beauty of blue and silver, tall pyramids of white Saxifrage flowers will rise in graceful clusters before a curtain of azure.
Between scattered clumps of the Blue Winter Windflower (A.Blanda), drifts of Snowdrops may wander freely, a carpet of the white and silver Saxifraga caesia giving an added beauty to the rosy tufts of the Glacier Pink (D.Neglectus).
Saponaria ocymoides forms with the Creeping Sandwort (Arenaria) a mist of pink and white over rock ledges and grey stones, the graceful flower sprays of Maiden's Wreath (Francoa) are seen to greatest advantage when backed by the rich green foliage of Alpine Rhododendrons and other shrubs.
It would be easy to suggest further combinations, either for form or color, but the value of garden books lies more in suggestion than in the laying down of fixed rules.
A garden heedlessly planted can never be really beautiful, good pictures require something more than the mere covering of a canvas with splashes of paint. The grouping of plants, so that each individual is of some benefit to its neighbor, is a pleasant task, and calls forth the best instincts of the lover of Nature.
The gardener knows that there is nothing like coarse, gritty sand for increasing the drainage properties of a soil. He uses it in his cutting bed, in the soil he mixes for potting. So sand will be one of the ingredients.
The roots of rock garden plants, as we have seen, like to cling around the moist surfaces of bits of stone buried in the soil, while the leaves rest upon those pieces which work their way to the surface, thus avoiding direct contact with the dirt.
An important ingredient will be stone chips. Ordinary crushed stone, such as is used for surfacing roads is suitable; this can be readily procured in most sections. If not, bank gravel, preferably not too fine or smooth, and not "washed," will serve as a substitute.
For our third ingredient, we add humus or decayed vegetable matter, which is found almost invariably in soils in which rock plants grow. This material holds an additional supply of moisture, besides furnishing some plant food. For supplying humus, granulated peat moss is best. It is so slightly acidic that only the extreme lime-loving plants object to it, and it absorbs and holds more moisture than any similar material.
Moreover, it is both pure and absolutely free from weed seeds, an advantage which cannot be overemphasized in rock garden planting. Peat moss is now readily obtainable anywhere; but if you do not happen to have it, finely sifted leaf mold will serve. Commercial humus has more of a tendency than either of the above to get wet or soggy.
Fourthly, and lastly, to give additional body to the plant food, we add good light garden loam. This, however, should be wholly free from clay, which is the last thing, in the way of soils, to be used where rock plants are to go.
All this has required some time in the telling, but if you boil it down it comes to this. To make a satisfactory all around rock garden soil, mix thoroughly together the following:
1 part clean, gritty sand, 1 part stone chips, or clean, gritty gravel, 1 part granulated peat moss, or sifted leaf mold, 1 part clean, light garden loam.
You will have a soil in which 90% of the rock plants you are likely to try at the start will grow satisfactorily.
Special Soils for Special Purposes
Occasionally, however, you will find plants that require something different from the above in the matter of soil; these will grow in the above mixture, but will grow better if their tastes are catered to.
Some insist upon having an extremely acid soil, or a lime soil, in order to survive at all. Such plants as these may either be grouped by themselves, or may be started in pockets filled with a soil supplying their own special dietary requirements.
These special soil mixtures may be made up according to the following prescriptions, the chief changes being an increase in one part or another of the several ingredients. These proportions are approximate; there is no necessity for weighing out the ingredients on a jeweler's scale.
Acid Soil
1 part sand, 1 part stone chips, 3 parts acid leaf mold (that is, leaf mold gathered from under evergreens, laurels, or the like), Lime or Sweet Soil 1 part sand or 3 parts old plaster, 1 part loan, 1 part peat moss
Dry Soil.
3 parts sand, 2 parts stone chips, 1 part loam, 1 part peat moss
Moist Soil.
1 part sand, 1 part chips, 1 part loam, 3 parts sphagnum moss or granulated peat moss, or both
Fertilizers
For plants other than the true rock plants and alpines—such as garden perennials and annuals, shrubs, evergreens, and most bulbs—and also for such of the rock plants or alpines as take readily to a somewhat stronger diet, very often decayed manure and bone meal may well be added to the general soil mixtures suggested above.
For evergreens, shrubs, or other plants which are to be set around the rock garden, by way of a background or setting, such conditions as are usually provided for them should, of course, be given. Particularly if any specialty garden features have been added, such as those in the Williamsburg Collection, .
Do not use any of the commercial fertilizers, except bone meal, and even with this great caution should be exercised.
Lime is not a fertilizer, but may occasionally be needed for the rock garden. If old lime rubbish, which is better for this purpose, is not to be had, ground limestone or gypsum may be used, to modify a soil otherwise too acidic.
Aluminum sulphate, now offered by most seed houses, works in the opposite direction. This may be utilized either to neutralize a lime soil, or, by applying it in larger quantities, to make the soil acid-reacting. Acidic leaf mold, which is to be found in most sections if trouble is taken to hunt for it.
Leaf mold gathered under oak trees is sufficiently acidic for most purposes, if decayed laurel leaves, conifer needles, or rhododendron leaves are not to be had.