An open, north- or east-facing slope is best for alpine plants; woodland and desert plants are less choosy. Or, you can create a mound or hill from a fast-draining soil mix. Flat ground can work, too, as long as the soil drains well, but it is less visually interesting, and the plants may be more difficult to see.
The most important visual element in a rock garden is the rocks. They should look natural. You can achieve a natural look by following two rules: Choose rocks all of the same type and position them so they look as if they were arranged by nature's hand, not yours. Use the largest rocks you or several people can handle for the most natural look. You might even wish to consider hiring a professional landscaper who can move boulders and large rocks with a small forklift or backhoe.
Porous rocks, such as limestone, sandstone, shale, and tufa, work best for alpine plants, because they absorb water, keeping the roots cool and moist. Choose nonporous rocks such as marble, basalt, and granite for desert rock gardens. Woodland rock gardens do well with either rock type, depending upon other growing conditions.
Position the largest rocks first to create the rock garden's structure, then add more rocks for balance. Partially bury the rocks so that at least a third of each rock is underground, so they look like a natural outcropping; don't place them directly on top of the soil. Tilt flat rocks slightly backward so they funnel water back into the soil. Align stratified rocks in the same plane, so they project from the side of a hill or out of flat ground as if they were exposed by erosion. Group rounded rocks in a flat area at the base of a slope to imitate a boulder field left behind by an ancient glacier.
Most rock garden plants are sensitive to poor drainage and perish in any soil that is not fast-draining, although this does not necessarily mean they prefer dry soil. They do best in moist but well-drained soil with a low to moderate nutrient content, and will rot in rich or damp soil. Amend all but gravelly soil before installing the rocks to make it more fast-draining, then fill in around the rocks with a soil mix made from one-third coarse sand, fine gravel, or stone chips; one-third peat or composted leaves; and one-third loamy soil.
Continued on page 3: Planting the Rock Garden