Magnolias generally dont need a lot of pruning, but if you understand their growth habit they may be modified to fit a smaller growing space than they would normally require. With careful selecting of varieties, pruning will also increase the production of pickable flowers.
All magnolias go through an establishment growth phase which is characterised by open upright growth with few flowers followed by a maturing twiggy phase as the growth slows down to produce a more horizontal, tiered framework with many more flowers. From time to time they will also send up strong growths from the base or the main branches. Some varieties do this more than others, and with grafted plants it is important to know whether these shoots are from the rootstock or the grafted portion. Leaf form is a good guide, but if there is any doubt, these shoots should be removed. These shoots can produce a secondary leader with a very tight fork, which if left to develop can create a weakness that will sometimes split out in strong winds and damage the tree severely. Newly planted magnolias will often produce this type of strong growth in the second or third year from planting, showing they have settled in and are ready to build their branch framework. When this happens you can either leave the strong growth to develop a vertical trunk with a more even framework than the original plant, or if you are happy with the original structure, you should remove them as soon as they appear.
Redeveloping the framework takes about three years or more but it is well worth the effort to enhance the elegant stature of the tree as it matures into a formal framework.
In the first year the older part of the tree should be pruned by about a third to encourage more vigour into the new shoot, and each year after that the old framework should be pruned to allow the new structure to develop evenly. In the second or third year, the old portion may be removed completely, once the new frame is about half the total framework of the tree. Be sure to seal the cut with pruning paste to prevent rot entering.
When older trees produce these vertical shoots they will grow upwards till they reach the canopy then branch out and settle down to flower. If the tree needs to be reduced in size then it can be pruned back to these shoots and pruning paste applied to seal the cut. These shoots will mature faster when the tree is pruned to give them more light.
The overall size of a magnolia can be limited by annual pruning immediately after blooming. As long as the twiggy flowering wood is left and the long shoots and any dead wood is removed, the flower display is maintained on a compact tree. As the illustration shows, cut the longest branches back to a branch to reduce the overall size without getting the haircut look.
On some varieties this annual prune encourages the tree to produce stems which have up to six or eight flowers spaced along them, Apollo, Iolanthe, Denudata and SanJose being among these types.
Alternately magnolias may be limbed up to make shade trees or even timber trees. It is a little more difficult to limit the bushier types to a single stem, but as long as you remove undesired shoots as soon as they appear, they will respond to training.