Answer: The roots of most trees are concentrated in the top 18-24" of soil and radiate out nearly twice the diameter of the canopy. While there is no guarantee the roots will not travel down eight feet to a water source, it is unlikely that they will. However, just to be on the safe side, you can build a barrier beneath the tree so the roots cannot penetrate down past three or four feet. If this doesn't appeal to you, you could plant a large shrub rather than a tree and then train that shrub into a small patio tree. For example, common Lilac makes a nice tree form.
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