The previous owner of our house built a long box along one side of the deck. We found that it is hard to keep annuals going. Sooner or later the box would drown or dry out, and that would be that.
We finally found the solution. We planted a dozen different hens and chicks, euphorbia and other low-growing succulents, all of which have spread out and covered every inch. Miniature daffodils add a lot of color in the spring.