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How to prune a magnolia tree
Prioritize dead and diseased branches over healthy branches.
With magnolia trees, removing healthy, if unattractive, branches may cause more harm than good. You should never prune more than 1/3 of the tree at once, so always start with branches that are either dying or already dead.
When in doubt, be conservative with what you trim. Magnolias are very sensitive to pruning.