Why are you growing an Ume in a pond basket?
Like Adair, I think it's a mistake.
Prunus in general like to have "wet feet". If the roots grow too long, they can be trimmed, or cut before repotting, in Feb-March. Pond baskets I think are better for pines, or in another continent, ficus, but to me, no good for Prunus.
Houston can be very hot and dry in summer (I suppose) : Prunus resent that kind of environment, putting them in a pond basket will raise the risks of the roots to bake and die.