Rock gardening

Sunset, Nov, 2000 by Jim McCausland

Grow gemlike plants in jewel-box gardens

* A classic rock garden has the visual quality of a jewel box, with tier upon tier of delicate plants sparkling like gems among the rocks.

Historically, rock gardening was the province of serious plant collectors (particularly English gardeners) who gathered rare plants from the world's alpine zones and grew them in greenhouses or rockeries. More recently, however, the definition of rock gardening as it is practiced by American gardeners has expanded to embrace not only the decorative use of natural stone in the garden but also "the cultivation of rock plants in any garden setting, with or without rocks," according to garden writer and landscape designer George Schenk.

The definition of rock plants has also evolved. Once the term referred to wild plants, usually less than a foot in height, which most often grow in rock crevices. Now it may refer to virtually any plant a gardener wants to grow in a rock garden. The size range of rock plants has changed, encompassing taller species as they have been discovered, notes Baldassare Mineo in Rock Garden Plants (Timber Press, Portland, 1999; $59.95; 800/327-5680).

On these pages, we present a gallery of contemporary Western rock gardens and a sampler of rock plants.

Elements of a rock garden

Site. In their natural habitats, many rock plants grow in exposed locations, so they will thrive in a site that gets full sun, especially near the coast. Most also grow well in a spot that gets filtered afternoon shade from high-branching trees.

Rock gardens don't need to be large to be effective. In fact, you can enjoy growing rock plants in a few square feet of ground or in containers as small as sinks, like the one shown on page 85.

Rock. Rock itself is the main element, tying everything together visually and contrasting perfectly with the living detail of plants. Some of the best rock gardens mimic natural outcroppings. In most rock gardens, stone makes up 10 to 40 percent of the visible landscape. Rocks should be of different sizes of just one type of stone.

When Kathy Allen of Central Point, Oregon, created the rock garden shown above right, she first prepared a 2 1/2-foot mound of soil, then dug rocks into place before planting. She buried about two-thirds of each rock and left the top showing. Her garden looks natural, and plants love it.

In so-called crevice gardens, where plants are tucked into crevices between adjacent rocks, stone makes up an even larger proportion of the landscape. When rocks or boulders are very large, as in Mineo's crevice garden in Medford, Oregon (top), soil is filled in as the rocks are placed, then plants are arranged.

When you set rocks in place, arrange their exposed surfaces so they form patterns. The faces of flat-sided rocks should lie in parallel planes; major crevices should be parallel so they resemble natural faults.

Plants. Evergreen plants, including dwarf conifers such as hemlocks, junipers, pines, and spruces, can form a backbone to anchor the design of a rock garden. For example, when Czech designer Josef Halda created Mineo's crevice garden, he planted several dwarf mugho pines (Pinus mugo mugo). Over time, as the pines encroached on rock plants, Mineo has removed about half of the trees.

As for rock plants themselves, there are thousands of perennials and small shrubs to choose from, including a host of dianthus, saxifrages, sedums, and sempervivums. Most of these hardy plants bloom in spring.

Soil. Horticulturists who hike above the timberline have noticed a remarkable phenomenon: Some of the most delicate and beautiful wildflowers grow in exposed scree. Such gems would quickly die if transplanted into normal garden soil, but they can flourish in a fast-draining mix formulated for rock gardens.

In Kathy Allen's garden, for example, the native soil is very heavy, so she covered it with an 8-inch layer of topsoil first, then topped that with a 2- to 3-foot layer of rock garden mix consisting of equal parts topsoil, crushed rock, sand, and compost. Many plants, especially alpines and Western natives like penstemons, need an even lighter mix (for these, Allen uses 1 part each sand, 1/2-inch bits of rock, and compost).

Such fist-draining mixes allow you to water rock plants during the hot summer months without causing constantly wet soil conditions, which lead to root rot and death. Drainage is further enhanced by mounded plantings.

Water. Water adds a natural complement to stone. That's why many rock gardeners include water features such as streams, waterfalls, ponds, or dish rocks--boulders with bowl-like depressions that collect rainwater. Keep in mind, though, that most rock plants can't stand too much water--make sure the water is well contained and doesn't soak the soil.

Resources

For more information, check out the North American Rock Garden Society website, www.nargs.org. For a $25 annual fee, members receive Rock Garden Quarterly access to the society's seed exchange and the Pennsylvania Horticultural Society's lending library, and connections to fellow enthusiasts. To join, send your check to Box 67, Millwood, NY 10546.