What is proper way of trimming a large shrub magnolia tree? It stands about...
Magnolias can and should be pruned to control growth and shape. Because this is a spring flowering plant, the time to prune is right after it is done blooming. If you trimmed it now, or in late winter you will lose many of the flowers that began to set last spring.
Here is a link that will help you if you decide to do the pruning yourselves. If this seems too big a task, or if you are afraid of damaging the plant please contact a certified arborist who has the equipment, and the skills to do a good job.
In any case, do not remove more than 1/4 of the plant's living branches at one time. I prune my 30' (and still growing) magnolia "bush" every year.
I hope this answers your concerns. Please contact AaE again if you have further questions.