Rock Garden Basics
Since the original habitats of many species are mountainous and rocky regions, they are for the most part hardy and can tolerate thin, stony soils, and some can survive drying winds. Provided their natural habitat’s good drainage is imitated, many rock plants are easy to grow.
Their brilliant colours enhance any garden, and since their size allows them to grow well in troughs or containers, no garden, however small, need be without them. Once planted, they demand little attention, except to be kept clear of weeds.
Natural Rock Habitats: Alpine Houses and Outcroped Principle
In their natural habitats, nearly all mountain plants are covered by an insulating blanket of snow each year, have a short growing season, and do not suffer from the high temperatures and humid conditions they may have to endure under cultivation in many regions. Where there is little sustained snow cover, even if the winters are cold, plants will need the protection of a loose mulch. (Pine boughs are especially good.) This should be applied after the soil freezes.
Some high-altitude plants are best grown in pots in an “alpine house.” This is simply a greenhouse with little or no heat. Most rock plants, however, grow best in the open and are commonly used in specially prepared rock gardens. Ideally, a rock garden should be on a slope, but a flat site is suitable if adequately drained.
Rock gardens are most practically constructed by using the “outcrop” principle-that is, partially embedding a few large rocks in the soil to give the impression of more rocks beneath the surface. The outcrop system is possible with all types of rocks, although rounded stones require more effort and must be set deeper in the ground.
Other flat sites on which to grow rock plants are paved areas or “alpine lawns.” To grow plants between paving, lay the slabs on sand, with gaps between to accommodate aromatic species that give off fragrance when they are walked on. An alpine lawn consists of low-growing rock plants with bulbs growing among them and can be located in the lower parts of your rock garden.
Alpines and rock plants can also be grown in raised beds. In this manner plants with different requirements-lime lovers, lime haters, or sun lovers that require good drain-age-can all be grown in separate beds filled with different soil mixtures. A system of raised beds can be constructed from several types of material. The best material is rocks, built up like a retaining wall, with soil between them. Plants can then be positioned to trail down the sides of the beds.
Bricks can also be used to build raised beds. They must be cemented in place, with holes left for drainage. Landscape ties, logs, and even boards can all serve as building material, as can cement blocks. However, they are likely to be unattractive unless used with skill.
The height of a raised bed may vary from 6 inches to about 3 feet. The higher beds are best for those who find it difficult to bend or stoop.
Things to Know Beforehand
Before attempting to build a rock garden, the gardener should have in mind (if not on paper) a plan of the basic design he or she wishes to make. To get an idea of the possibilities, visit a rock garden in one of the many botanical gardens throughout the country. Smaller rock gardens that might be appropriate to a small plot can be seen in some private gardens that are open to the public on special occasions. Information regarding these can usually be obtained from local gardening clubs, horticultural societies, and garden sections of newspapers.
To build a successful, natural-looking rock garden, follow a few basic principles. Never attempt to copy an actual mountain in a garden; the scale would be quite out of proportion. Instead, try to simulate a natural outcrop of rock; this will be far more effective.
The relationship between height and width is also important. For every foot in height, a rock garden’s planting area should be 4-5 feet wide at the base. A 4-foot mound, for example, should measure about 20 feet across at its base.
Once the basic rock garden has been built, other features can be added. Water is always an attractive feature; but if you construct a waterfall, take care not to make it look too artificial.