[header] How to prune a magnolia tree [title] Prioritize dead and diseased branches over healthy branches. [step] 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. 
[substeps] When in doubt, be conservative with what you trim. Magnolias are very sensitive to pruning.