Answer: You might like a Rose of Sharon, the hardy Hibiscus syriacus. These can be trained to a single stem and bloom prolifically over a long period during the summer. There are numerous colors available. A crepe myrtle would also be a possibility if you opt for one of the smaller maturing varieties. Yet another option might be the butterfly bush or Buddleia.
All of these can be pruned as needed to control size in early spring. The butterfly bush does not lend itself to the tree form but they other two can be trained that way easily enough. They all bloom on new growth of the season, so pruning in spring will not diminish bloom performance. They all do best in a full sun location, meaning full sun all day long or for at least six hours including the hour of noon.
I am not certain what plant you saw, although I suspect it may have been one of the lovely non-hardy tropicals brought in for sale as patio plants. Your county extension as well as professionally trained nursery staff should also be able to help you identify plants that will thrive in the location you have in mind.
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