Magnolia x loebneri 'Leonard Messel' was one of my favorites while studying up at NYBG in 2005.
How would I prune a magnolia in our small backyard? It's a multi- stemmed type with white flowers. It really wants to be large, but the space just won't permit it. I'm considering pollarding it, as some of our neighbors have. When is the best time for pruning? Before flowering or after?
To be perfectly honest with you, I wouldn't prune a magnolia if I could help it. Magnolias are not the fastest growers nor are they considered fast or expedient when it comes to callusing fresh cuts. I can easily picture the type of magnolia you have in your backyard. That growth habit is simply how the magnolia wants to grow and if you try and make a number of cuts to provide it with a different shape I'm just too fearful that you are going to be opening too many wounds where pests and diseases will be able to attack. Now, if your magnolia has deadwood, crossed branches that seem to be harming the bark, or any diseased wood, then that wood you do want to prune out of the tree. You want to remove any wood that may be depleting the energy reserves of the tree, but remember that you only ever want to prune what you have to and no more. Especially as a tree matures, they should be pruned less and less because it takes more energy and time to callus those wounds. In that respect trees are like humans. A 5-year old that clocks himself on the edge of a table will be back playing in no time; clock an adult over the head and we have to sit down, take some advil, and are still feeling it hours later. In many ways the same holds true for trees and their ability to compartmentalize decay and redirect their energy to other stems and branches.
If you find that you do have to do some pruning of your magnolia, wait until late spring or summer, definitely after the flower. If you prune now you will sacrifice some of the flower buds that have already formed, and I'm sure you want to enjoy as much bloom as you can. Also, if you make cuts that the tree has to callus, then it has to pull from a finite amount of stored energy that the tree has already allocated to spring flowering. You never want to prune a tree when it is actively leafing out because its energy levels are most depleted at that time and you don't want to stress the tree out any more. After the tree has leafed out then it is rebuilding those energy reserves and has a better chance of callusing cuts quickly and efficiently. In the case of your magnolia, it will then also have ample time to set the flower buds for the following spring.
Lastly, pollarding is a labor intensive commitment and even though the outcome can be fabulous looking it is not suitable for every kind of tree. Usually you want to pick a fast growing tree that is pretty tough to begin with. I have seen and/or I have pollarded planetrees, certain maples, Paulownia, Cotinus, and a number of others that presently escape me. To establish a good pollard you have to repeatedly remove new shoots of growth every winter during dormancy. Already I've mentioned that magnolias are not the fastest at callusing cuts and compartmentalizing decay. Not to mention that cutting off the previous-years shoots in winter means that you would never see those beautiful white flowers again. So, sorry to burst your bubble, but pollarding a magnolia? Maybe not.