The high end of the southwest-facing garden faces north, which gives it a microclimate that is cool and moist. Yellow Polyantha Primroses (Primula x polyantha) spill out of a crevice between two rocks. To its right is a large clump of blue & white Rocky Mountain Columbine (Aquilegia saximontana). Other crevice plants are the pink Alpine Catchfly (Lychnis alpina), yellow Alpine Alyssum (Alyssum wulfenianum) and red-violet Pansies ( Viola x wittrockiana).
When constructing crevices in a rock wall such as this one, it's important to be sure that the back end of the crevice opens into the soil behind the rock wall. If the crevice is closed up in the back, the result will be the same as growing a perennial in a clay pot. After a few years, there will be no more room for the plant's roots, and the plant will either decline or begin to push itself out of the soil.
Another important design consideration is the overhang. An overhang that extends beyond the planting niche beneath it will divert water away from the roots of the plant in the crevice. The plant will become dependent on you for all its moisture needs.