Pruning differs for different types. However, generally they don’t tolerate hard pruning very well. Instead, prune once flowering to remove any dead, damaged, or diseased wood.
Hard pruning of deciduous magnolias can encourage long shoots called watershoots to grow. However, for evergreen magnolias that are well established, you can hard prune if necessary – such as after a storm to remove any dead or damaged wood.
For wall-trained magnolias, a light prune can be done in summer, or if free standing then a light prune in spring.