We were taught, and I have pruned many star Magnolia's to take out the dead, diseased, damaged and crossing. Then walk away and look at it. Then you can prune to take the branches outwards and upwards.
They are tougher than you think; so you can be a bit ruthless with them. My mothers one died right back due to the dry summer and the water ban. So I cut it right back and it has grown back, well.
Beautiful plant, but take don't take any nonsense from them, as they do grow fast here.
Regards from England.
Neil.
