Answer: There are a number of different plants you can try. How about a mixture? If your marigolds have done well, then I think you'll be safe choosing sun-loving plants. Portulaca is an attractive and durable plant that has a somewhat vining habit and would drape down over the retaining wall. Another vining plant is Helichrysum, which has dusty green foliage and no flowers. I like nasturtiums, but find they flower best in relatively poor soil, with little or no fertilizing. Petunias are always popular, though most require regular deadheading to produce their best. The 'Wave' petunias are low-growing and vining, and create a low ground cover. Finally, low-growing zinnias, such as the Profusion series, would be attractive.
You might try adding a few perennials in each year. That way you can build up a permanent planting that you can then supplement with annuals as you wish. This would keep the initial cost down, and you could see what thrives there.
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