I have been experimenting with a variety of low growing sedums and other plants tucked into a stone retaining wall. There is a lot of diversity within the Sedum genus, which has at least 250 species.
The name, sedum, comes from the Latin and means "to calm." It is actually very calming to create a living wall of easy growing sedums especially if one already has a wall into which they can be placed. Sedums enjoy well-drained soil and sun, and pockets within a stone wall are ideal for tucking their roots.
Sedums, sometimes referred to as stonecrops, have fleshy leaves and produce dense clusters of flowers with five petals that are shaped like tiny stars. Choose perennial varieties if you live in a climate with cold winters. The sempervivum species, commonly called hen and chickens, are good for walls, as are the tectorum rosettes.
Small ferns, corydalis, and small dianthus species can also be used in stone walls in the sun.
Plants, like sedum, with succulent leaves are the easiest as they thrive without much water and enjoy the heat.