Wall garden

From Gardening Wiki

A wall garden is not to be confused with a walled garden. It is any wall in the crevices of which are grown appropriate plants. The mortar may be knocked out here and there in an old wall, but it is better to make a dry-one that is to say, one in which the crevices are filled with earth for planting. It is a large or small undertaking, according to desire and circumstance; nothing could be simpler than a simple one.