Answer: The roots of most trees extend out about one- and one-half times the diameter of the canopy. Timeless Beauty has the potential to reach 25' in width and has an extensive root system which will extend out to the edge of the canopy. While the roots typically grow downward, Magnolia trees can have many surface roots so there's no guarantee that they won't compromise the structure of the wall. You can help it develop a deep root system by watering deeply and infrequently, but I think to be on the safe side I'd move the tree to a more open area and plant a smaller tree at the base of the wall.
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