Answer: Merrill magnolia is more of a shrub than a tree but it will still get 12' high and 15' wide at maturity. The roots of magnolias can be shallow and can grow outwards one and one-half times the diameter of the canopy. Because of this, I'm afraid planting it only a few feet from a retaining wall can cause some problems in the future. Why not plant a shrub with a smaller, deeper root system instead? Camellias and hollies have well behaved root systems and shouldn't compromise the integrity of your retaining wall.
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