At the top you can leave some pockets for plants, or plant them just behind the wall. Cover cracks at the top with flat stones, so water won’t wash out soil.
A rock, a rose, and a miniature rose.
Planting
If you can remember not to damage plants at the bottom as you’re working above them, set the plants in each layer as you build. It’s easier then to make sure the roots are firmly embedded well back into the soil. Plant sparsely; leave plenty of room for growth and spread, and plenty of uncovered wall surface to contribute pattern and texture to the picture. Space the plants at irregular intervals, never in straight lines either horizontal or vertical. Water thoroughly, making sure the layer of soil behind the wall is completely moist; and keep it moist at least throughout the first growing season.
PATHS AND PAVING
Many little rock plants will make soft cushions and carpets, with their roots in crevices between pieces of paving, and also in stone steps. They’re very low-growing and very hardy, and usually spring back resiliently when stepped on. It’s important that acid-loving varieties not be planted near cement, which sometimes neutralizes or alkalinizes the soil. And, of course, the situation should provide the amount of sunshine and moisture the plants need.
The simplest kind of informal path or paving for an informal patio consists of flagstones or slates set in the ground, the top level flush with the surface of the soil. Plants are spaced irregularly in cracks between the stones. I’ve seen one path of old millstones laid this way, the plants rooted in the center hole.
Bricks create a more formal pattern, either laid on a bed of sand or set into a foundation of concrete over gravel. The popular “crazy pavings” can be odd pieces of either brick or stone arranged in any conceivable pattern and laid in mortar. In any such solid constructions, planting pockets should be scooped out before the concrete sets hard; or a chunk of wood can be inserted to hold the space open.
Give the carpeting plants as much good soil as you can, and keep them moist until they are growing well. Occasional trimming will stop them from growing straggly, and keep them neat, compact, and in a pretty pattern.