Answer: When deciding what to plant, keep in mind that woody plants such as shrubs and trees have large root systems. Typically the roots stretch out at least twice as far as the branches, sometimes even more. So the constrained space behind a retaining wall may not be suitable to large shrubs.
Also, depending on how narrow and/or shallow the beds are, there may not be enough soil volume to maintain a shrub; if too small, the plant's roots will be constricted and will pull moisture from the soil quickly as though in a container. This may mean you will need to water frequently.
Too, if the soil volume is small then it will not provide good insulation to the roots so they could be subjected to excess cold in winter and excess heat in summer, compared to how it would be planted in the ground normally. This is also true for plants growing close to the wall front.
For all these reasons, I suggest you use plants that are considered heat and drought tolerant and also with good winter hardiness. You may also discover that certain plants thrive there while others fail, thus "helping" you select appropriate plants.
As far as arranging the plants, to some extent it is just personal taste. You may find that a combination of trailers planted to the front, bushy or grassy textured plants to the center, and then some more upright forms slightly off center to one side works well. Have fun with your project!
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