Planting a Rock Garden

There are rock garden styles to suit every taste, but great care must be taken that the style chosen suits the site. For example, a huge mound of rocks rising out of nowhere will look very much out of place in a grassy lawn. Flat, craggy limestone, while attractive in its own right, will not suit a yard dominated by a fieldstone house. Remember, a rock garden is essentially a re-creation of a mountain slope. Picture this in your mind and try to create it on a scale that suits your growing space.

The easiest rock garden to plan is always a natural one. If your garden has a natural stone outcropping, you can easily bring out its beauty by cutting back invasive roots, removing a few shrubs and trees to increase sunlight, and possibly digging away some soil to better reveal the natural rock. Even a small rock outcropping can be used to advantage by adding similar rocks to repeat and accentuate the original pattern.

Slopes are ideally suited to rock gardens. Not only are they hard to maintain otherwise (just ask anyone who has tried to mow a hillside lawn) but it is also easy to integrate rocks into a slope and make it look as though they were put there by Mother Nature. Flat surfaces are not obvious choices for a rock garden, but don't rule them out entirely.

This rock garden looks nice on a slope.
©2006 Publications International, Ltd.
This rock garden looks nice on a slope.

Generally speaking, rock gardens should be placed in full sun; most plants you'll use in a rock garden love sunlight. Although you can create an attractive rock garden in a shady spot, your plant choices will be more limited.

Perhaps no other step is as important in planning a rock garden as choosing the right rocks. All too often, a "rock garden" consists of a pile of rounded river stones of various sizes and colors randomly strewn on the ground: Nothing could look more artificial! Instead, use rocks that are uniform in color and texture; ideally, they should be angular in shape with distinct lines or strata. If these similar rocks are placed at roughly the same angle, it will look as though Mother Nature deposited them.

Rounded rocks, however, need not be banished from the rock garden, but they should be similar in color and texture. For a natural look, set the first ones quite deeply in the ground. As more rocks are added, make sure that about half of each rock is hidden from sight.

Make sure some of the rocks are very large ones: true boulders. These larger rocks are the keystones of the rock garden. One rule of thumb: If it can be moved by one person, it's too small. Once the boulders are in place, medium-size rocks can be added. Smaller rocks will be needed to fill in any gaps.

Rock gardens are also ideal sites for waterfalls. Even a steady stream of water droplets landing in a tiny pond at the garden's base will do. In fact, smaller waterfalls are often the best choice for the home rockery: large cascades of frothy, foaming water are for very massive rock gardens.

Building a Rock Garden

Once the rocks have been chosen, prepare the site by excavating to the proper depth. Make sure you remove any weeds or lawn grasses now: You don't want them reappearing later between two heavy rocks where you can't get to them.

Most alpine plants require perfect drainage. If your soil is naturally heavy, put down a drainage layer of six inches of gravel or crushed rock. Cover this layer with landscape fabric or two inches of sand so the soil you add later won't percolate through. Unless the soil taken from the excavation already drains perfectly, mix it with an equal quantity of sand. If you don't intend to grow alpine plants, simply add about one-quarter compost or peat moss to increase the soil's organic content. If you intend to grow mostly alpine plants, check the soil's pH and amend it with ground limestone if necessary; alpine plants tend to prefer neutral to alkaline soils. Only a few rock garden plants (heathers and dwarf rhododendrons are among them) need acid soil.

The bellflower, a popular rock garden plant, doesn't need acidic soil.
©2006 Publications International, Ltd.
The bellflower, a popular rock garden plant, doesn't need acidic soil.

To "anchor" a rock garden to its landscape, consider adding a few minor rock outcrops in peripheral areas. Also, add to the base of the rock garden a flat area of gravel or crushed rock in the same shade as the dominate rock. This is known as a "scree garden." This will help prevent lawn grasses from invading the garden, and the effect will appear quite natural, as if bits and pieces of broken rock had fallen off the rock outcropping over the years.

In regions where droughts are frequent, consider adding an irrigation system from the outset. The simplest method is burying a perforated garden hose just below the surface of the soil: It can then be attached to a supply hose whenever watering is necessary.

Maintaining a Rock Garden

Rock gardens are not hard to maintain. In fact, most work simply involves removing weeds on a regular basis. Even this task will diminish as the rockery plants establish themselves and fill in any gaps where weeds might grow. By covering any exposed soil with a layer of crushed rock, weed seeds will have a difficult time getting started. In a rock garden, weeds must be removed by hand, preferably as soon as they appear. Herbicides, even when carefully sprayed, tend to drip down rock surfaces and harm desirable plants.

Less hardy rock garden plants can be protected during the winter with spruce or pine branches or some other light mulch. Fallen leaves and other moisture-retentive debris should be removed as soon as it accumulates; most alpine plants rot when in contact with damp materials.
Prune as needed to control any plants that spread beyond their limits. Many low-growing, matting alpine plants can also be cut back hard after flowering to encourage the formation of new, healthy growth. Finally, don't be afraid to move plants that appear unhappy.

Most of the information given above describes the preparing and planting of rock gardens for sunny sites. Although this is the most traditional form of rock garden, there is no reason you cannot produce a beautiful rock garden in shady conditions. Use a richer soil mix with plenty of organic matter since most shade-loving plants prefer a moisture-retentive mixture.

Learn how to plant a rose garden in the next section.