Answer: Generally a sunny retaining wall area will be hot as well, due to reflected heat. For annuals with all season color you might consider marigolds, petunias, callibrachoa, angelonia, dusty miller, or salvia. For perennials you might try assorted sedums, purple coneflower (Echinacea purpurea), black eyed Susan (Rudbeckia hirta), or creeping thyme. I hope this gives you some ideas.
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