Most Magnolia varieties grow to a regular shape with an open centre. They rarely need to be pruned for shade purposes. If planted in the right place, then Magnolias rarely if ever need any pruning.
Understanding the growth habit of your proposed Magnolia, will ensure that you plant it in a sittuation where it does not need any pruning whatever, and will reward you with years of blooms almost regardless of the weather. Almost for Magnolias sometimes fall foul of late or hard frosts. Such damage can rob you of flowers for a whole year, and no pruning of any kind will help other than to remove unsightly the dead blooms!
Some Magnolias grow into a wide bowl or chalice shape Magnolia x soulangeana, others grow into a large conical shrubs Magnolia soulangeana Heaven Scent, whilst some form a mass of branches and twigs but remain reasonably small and slower growing Magnolia stellata. A few are evergreen requiring different treatments if they are to be grown as wall shrubs!
Magnolias x soulangeana types in particular are relatively quick growing once established. Therein is a little problem in any hard pruning of these types. They tend to send out many shoots from the pruned cut leaving you with the choice then, of which ones to prune and which to keep.
All Magnolias are tip flowering, on growth branches or twigs that were formed in the early part of the previous year. They require time to develop the flowering bud, and often these are not visible until late in the winter. Leaf buds are somewhat similar, and are often mistaken as being flower buds.
If any pruning is to be done, then it will need to be carried out in the dormant period early to late winter. This invariably means that you will be cutting off the flower buds with the pruned branch, but to leave pruning until spring or summer, will mean sap bleeding, and subsequent die back of the pruned shoot or branch.
Pruning carried out in the dormant season should mean that growth will start early in spring following, and the development with some flower buds as well. Many Magnolias flower on the tips of lateral branches, so for a profusion of flower, little or no pruning will ensure a gradual build up if these lateral growths.
Sometimes Magnolias get damaged with heavy snow fall or in high winds when in full leaf. Whilst most are open crowned, which allows the wind through, they are also quite brittle in the early spring - sometimes leading to unexpected damage.
In the case of damaged branches that need to be pruned during the spring or summer, do the least possible as a first aid measure, and cut only to the first healthy shoot back from the damage or break. There will almost certainly be some dieback as well as vigorous water shoots, but disease is rarely a problem. Pruning proper, can take place later in the year - winter.
Any hard pruning will almost certainly result in the straggly or whipcord shoots known as water shoots. Most of these will need to be cut out, and the cycle begins!
With any pruning for space, or misguided aesthetic reason, only prune the thinnest shoots, and then back by just one third at most. If the cut is taken back to a strong lateral shoot, then there is less chance of the watershoots developing.
Young Magnolias straight from the garden centre or nursery, have rarely had any formative pruning carried out. The reason being that Magnolias are normally sold by size - or even by the number of flower buds present! Formative pruning either reduces the size and bulk of the investment and normally takes off the valuable flower buds! (To test this out, simply have a look at the Magnolias on sale from late winter, and you will see the added value that is placed on flowerbuds.
When first bought, if the Magnolia looks a good shape, then leave well alone. If it is a stellata type, then don't tamper full stop! It is meant to be a mass of twigs. But prune out any water shoots that may have been allowed to develop in the Nursery accidentally of course!
The white magnolia clearly shows the framework of flowering lateral branches developed over the years by minimal pruning.
Magnolia grandiflora is the main evergreen Magnolia which - if allowed - will grow into a very large, broad, but slightly conical shrub - or small tree.
This summer flowering Magnolia - with huge cream white goblets of flowers -It is often grown against a house wall. A spectacular covering if you have the space! If grown as a large stand-alone shrub, then prune as for any other evergreen shrub. If trained against a wall, then prune shoots directly after flowering - back to a spur of around 12 long - that is to say back to around the third bud from the main framework.