What kind of rocks to use and where to get them depends on the part of the country you live in. Anything that’s indigenous to an area is first choice, always. Limestone, sandstone, fieldstone, porous tufa—native rocks “belong” as no artificial or imported rock ever can. If your land is not blessed (or cursed) by plenty of rocks, you can sometimes collect them near rivers, mountains, or abandoned quarries, or where a new road is being blasted through. Whether you have them, collect them, or buy them, all the rocks in one garden (or on the same property) should be of the same general type. Varicolored geological specimens are seldom attractive.
Consider each rock’s characteristic color, texture, size, and shape before you decide where to place it, or whether to place it at all. Even in the tiniest rock gardens, tiny rocks look ridiculously salt-and-peppery. Start with rocks of substantial size, in keeping with the size and contour of the garden; use smaller ones where they fill in effectively.
Naturally worn and weathered rocks look more natural than freshly chiseled surfaces; irregular, angular forms are better than uniformity. Rocks with stratified layers suggest ledges and cliffs. Glacial boulders or “hardheads” are not good in formal patterns. Use them naturalistically, in varying sizes, arranging them with studied carelessness as if they’d been scattered haphazardly and then left.
Avoid overcrowding with so many rocks that the plants are overshadowed by the debris. Place rocks as they might lie naturally, partly buried in the soil, the tops smoothed over by the elements, and so solid that they don’t wobble when you step on them.
Construction
Perfect drainage, so that water cannot stand around the roots and crowns, has been called “the most important need for all alpine plants.” The selected location (raised or sunken) and composition of subsoil and topsoil can either contribute to good drainage or deter it. In small gardens it’s not too formidable a job to install a system of drainage tiles, if necessary, or to add gravelly sand to subsoil to make it more porous. Easy insurance in any rock garden is to start with several inches of rather coarse gravel or rubble as a bottom drainage layer.
Any topsoil that is removed during construction should be saved carefully. If you can pile it in the center of a tarpaulin or piece of strong burlap, it can’t be scattered and lost; and you can also mix it with any additives with pleasurable ease. Simply pile on the peat, leaf mold, sand, or other ingredients; then, holding opposite corners (you may need a helper to hold one corner of a large piece), pull it back over the soil pile and fold it over the other end. Repeat this, from opposite ends, until the materials are thoroughly mixed.
The first rocks are usually set at the lowest part of the garden, partly sunk in soil or otherwise made extra-sturdy and secure. Build up from that point, arranging rocks with their broadest surfaces at the bottom, all lines of stratification horizontal or on the same slant. Each rock should slant slightly inward and downward, to catch moisture and encourage it to run into the soil in the garden.
Provide plenty of soil for spreading roots, and a minimum depth of eighteen inches or more. Plants in shallow pockets may be stunted and will certainly dry out too fast during droughts. Whether or not you plant as you go depends partly on your agility. Don’t crush the first plants under your feet or knees as you set rocks and plants above them. It’s often faster and easier to spread out roots and make soil firm around them before the next rock is set in place.
Planting