Answer: The thing about perennials is they don't put on the spectacular bloom each season that annuals do. Perennials figure, "Hey, I've got my whole life to bloom" whereas annuals know this is their only shot to really put on a show. The firstthing that came to my mind for your planter is an annual, petunia, the "Waves" series to be exact. They simply can't be beat for a long show and cascading feature. They reach a height of approx. 6". Geraniums are also a good pick for long bloom and a cascading feature. Nasturtium, Portulaca, and Verbena are also good choices. All of the above are annuals. For perennials (as mentioned please bear in mind the bloom of perennials is not as spectacular - I have tried to include the longest blooming ones) consider: Moonbeam Coreopsis (may exceed your height requirement, can reach 12-18"), Delosperma Pink (Ice Plant - 16" high), Walker's Low Nepeta (N. faassenii, 10" high), and Phlox pilosa 'Ozarkana' (15-20" high). Perhaps a good idea would be to have a mix of annuals and perennials for the best look possible.
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