Answer: Based on your description, I suspect the planting pockets would dry out almost immediately during the summer -- hot sunny location, concrete and reflected heat are very inhospitable. Rather than plant in the pockets, maybe you could plant some sort of vine at the base of the fountain and allow it to grow up and soften the look of it. Then you could strategically place some large planters of annuals or possibly exotic tropicals to add color.
You might consider using the evergreen English ivy (Hedera helix) to cover the fountain, especially if you look at it often in winter. In addition to the classic plain green we are accustomed to seeing, there are an increasing number of new varieties that are smaller and less aggressive and could be planted in combination to add interest.
You could use the annual ornamental black and/or chartreuse sweet potato vines for a striking look, or you might also use a flowering annual vine such as morning glories to scramble up over the fountain but this would have a less formal look.
Annuals for a hot location would include the typical marigold, petunia, zinnia, verbena, salvia, nicotiana, cosmos, and dianthus.
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