As others have noted, winter is the best time. Actually, late winter-early
spring works best for me in central NJ (just a little north of you).

Magnolia grandiflora, the evergreen magnolia, takes well to any sort of
pruning. You can cut branches back to bare stubs and new growth will develop.
If you just want to make the tree more dense, you can wait until buds begin to
develop in the spring and remove the terminal bud of each branch. This will
induce 2-5 dormant buds to develop at the end of the branch. You do not need to
prune flower buds. Branch tips that produce a flower bud will naturally produce
multiple new vegitative growths.

--beeky

Jim Hamer wrote:

Can anyone tell me the best time to prune my five year old Magnolia? Can I
prune now? Should I wait until winter? We are warm enough so that it is an
evergreen (doesn't lose leaves in the fall). I live outside of Philadelphia
Pa.

Thanks for any info you can give me.