Which kind of Magnolia, Grandchild4u? Deciduous ones can be pruned lightly in winter, always cutting back to a side branch, to preserve as many twig tips as possible, since that is where the flowers appear. Heavier pruning should be done right after bloom in late spring, since the flower buds form in late summer, early fall. Evergreen Magnolias (M. grandiflora varieties) can be pruned lightly right after the first flush of bloom in early summer, but heavypruning may scotch the bloom for a couple of years.