The rock garden should usually be in full light and air, away from the shade of trees and structures, but full sun in summer (and winter) is very trying on many rock plants. It is wise to locate the garden so that the important sides face north or west, taking full advantage of the dew or any outdoor fountains and cooling of every night, and to make the slopes facing the sun short and abrupt unless the vegetation is to be mostly Sedums and desert plants.

To give further relief from the sun for a part of the day, hollows should be created in the level reaches, lowered at one side for drainage (unless bog plants are placed here), that the stones of the rim may cast shadows across them, whereas big boulders or ridges of rock may be erected boldly for the shade which they afford. A clever touch is to slope the whole area to the north at such an angle that the sun in winter will hardly hit the soil or at such a low angle that there will be no thawing.

The problem of winter killing is largely controlled by angle of slope and shade from protruding boulders. Again, the requirements of many rock plants make necessary further choosing in the kind of stones to be used. Some plants grow best in an alkaline or lime soil. For these a lime rock, as marble or coral, should be used if available, or else lime rubble is mixed with the soil. For those which hate lime and demand acidity in the soil, a hard rock, (as granite) is needed, with plenty of peat fiber in the soil.

Fortunately, many rock plants are neutral in these requirements. While the stones are being gathered and hauled to the site, it is well to get the soil ready also. Ordinary garden soil will not do at all, nor will the best of garden soil—far from it! A soil that will raise prize lettuce is a fatal dose for alpines. While it might be said that each kind of rock plant requires a special mixture for its best diet, the general foundation for the majority of them is one-third sharp sand, one-third vegetable fiber, and one-third garden loam.

These may be gathered separately and mixed just before being used. The ideal natural soil is the top sod of stony pastures, long rotted, and screened to take out roots of weeds. The sand should be sharp and gritty, even rather coarse, and never fine river sand or that containing clay. It is porosity that is required, and sifted hard coal ashes may be added, especially for lime lovers, like Dianthus. The vegetable fiber may be from any source so long as it is well decayed.

Examples are old sod, leaf-mold, or turf from the woods. The best is peat from the bog piled a year ahead, pulverized by the frost of one winter, and dried by the sun of one summer. Then it will break into small pieces and is surely free of weed seed or roots. For city gardens it is most convenient to use any of the prepared peat fibers, as these are well ground and free from weeds. In any except special spots the coarse grades are better than the fine.

The garden loam should be free from clay and not of very fertile nature. It gives body to the mixture, the sand being porous and the fiber capable of holding water. The great danger is the presence of roots and seeds of common weeds, which soon will cause great trouble. Screening through a sieve will take out the roots of weeds, but the seeds will have to grow to their next stage. No fertilizer is ever added, except perhaps a little bone meal. Barnyard manure, in any stage, is forbidden, for it is too rich a diet for rock plants and is the original home of all weeds.