Living up to its name, the star magnolia (Magnolia stellata) produces scores of fragrant, white starfish-like flowers in spring before its leaves emerge. A well-shaped multi-stemmed large shrub, this magnolia should not require pruning, but an occasional errant branch or twig should be earmarked for removal in early autumn to prevent copious bleeding of sap. Do not over-prune as wounds may not heal perfectly, and you will diminish the following spring's flower display.
Examine the branch structure of the star magnolia and note the areas that warrant shaping. Ideally, only small twigs, with diameters smaller than that of a No. 2 pencil, need removal to attain your desired shape. Remember that you don't want to remove too many branches since the star magnolia usually has a good branch structure.
Look for a lower branch junction where you could make the pruning cut on each branch you wish to trim.
Trim the branch tips back to a lower, living branch or bud, making the pruning cut 1/4-inch above the branch junction or bud.
Remove any diseased, rotten or dead branches during this shaping project.
Allow the plant to grow without further pruning for two to four growing seasons. Repeated or annual pruning on a star magnolia is a bad practice and likely will ruin the natural structure of the shrub and create uneven flowering in springtime.