Set plants as deep as they were planted before; no deeper (danger of crown rot) and not too shallow (they may dry out). Tuck them firmly in cool, moist, rock-shaded soil. Keep the soil constantly moist until firm new growth shows that the roots are established and growing.
Rock-garden plants are seldom suitable for planting in neat rows or geometric patterns. Tiny, slow-growing types are arranged in groups of three or more; one or two spreaders per planting spot will do. Don’t crowd these plants. Give them room to develop and for air to circulate around them, and to display their charms at their best. Provide private quarters for the smallest gems, away from hearty growers that might overpower them. Keep color combinations and contrasting textures in mind. Set trailers where they can droop over stones. Place those that like moisture low in the garden; their roots keep cool in the shade of rocks.
As a finishing touch, after the rock garden is built, planted, and thoroughly watered, cover all soil surfaces with a mulch of fine gravel or stone chips. Work it up to each crown, under prostrate stems. Aesthetically, this gives a neat, well-groomed look and ties plants, rocks, and setting into one coherent picture. Functionally, the chips absorb heat and keep soil cool and moist, lift small, low leaves out of mud and water, discourage slugs, protect crowns against rot, and firmly discourage the intrusion of weeds.
MINIATURE WALL GARDENS
Any small section of low wall—brick, cement, stone, or concrete—can be the background for a miniature garden. But it is in a dry wall, built of stones without cement, that many small rockery plants best display their jewel-like perfection. Planted horizontally, with their roots in pockets between the rocks, they make a vertical rock garden of glowing colors and fascinating foliage textures.
For small, low walls, and those in somewhat formal settings, thin-layered rock such as slate or shale is usually preferred. Irregularly shaped fieldstone or other rocks of native origin are used in more natural, informal designs. Any lines of stratification are, of course, laid horizontal and parallel to each other. Another good rule to go by is: The smaller the wall, the smaller the rocks.
Construction
Low walls (two feet high or less) usually do not need a foundation layer sunk below the frost line. They can be started at, or just under, ground level. But they must be set solidly on a firm, wide base—two feet wide, for example, for a wall two and a half feet high. Set the largest stones and complete the lower layer first. Tilt each rock slightly backward and downward so soil won’t wash out of pockets, water will run back to plant roots, and the heaving pressure of frost won’t thrust the rocks out of place in winter. A guide line of string or wire moved up with each new layer will help you keep the construction level.
The wall should slant slightly in toward the bank behind it, and usually grows more narrow as it goes up. Each layer is made absolutely solid before the next is applied. Set each stone so that it is completely steady, and rests on parts of two stones beneath it. This distributes weight evenly, avoids crushing plant roots, and prevents unattractive and impractical long, vertical cracks created when joints coincide. Use small, flattish rocks to fill chinks under large stones and make them steady.
Behind the wall, as you build, firm in a goodly layer (one foot thick is usually recommended) of topsoil that has been enriched and aerated to promote drainage. Roots will reach back here for water and food. As you ram soil into pockets, make sure it reaches all the way back to this layer. Unless all the soil is made very firm, it may settle and the stones may slide out or into the bank.