Answer: You're on the right track - a tree's root system can invade the soil adjacent to the retaining wall and can actually push it over or cause it to crack and crumble. Trees are not good candidates for planting near retaining walls. Their roots often travel nearly twice as far out as the longest branches. So a tree with a canopy of 30' could have roots extending out 50-60'. Instead of planting a tree atop your retaining wall, why not plant a shrub? Many can be trained into tree shapes with a single trunk and most will have shallow and narrow root systems so your retaining wall won't be compromised. Some shrubs to consider include viburnum, lilac, camellia, and crape myrtle. There are many more to choose from; visit your local garden center and you'll come away with ideas galore.
Best wishes with your landscape!
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