I am sorry for this delayed reply to your gardening question. The spring rush has brought a deluge of questions and we are working hard to catch up!
I wouldn't worry about the roots on the crape myrtles as long as you plant at a proper distance from the pool or any concrete structure for that matter. Distance from the pool depends on the size if the variety. Look at the varieties mature size and it can be planted about half that distance from the pool. If you need to really squeeze the plant in you can probably go with 1/3 the height. Keep in mind however that the spent blooms and the fall leaves will end up dropping into the pool more the closer you set those plants.
Thanks for the question. Best wishes for a wonderful gardening season. Please stop in again soon!
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