Winter is, broadly speaking, the ideal time to prune most trees and shrubs.
Deeply dormant, these woody plants experience less stress when pruned in the winter months, and with cool temperatures, fewer pathogens are active and able to make their way into pruning cuts.
A few woody plants, however, are best left through winter and pruned in another season. When you are performing your annual structural pruning, pass by these woodies for their own good.
One of relatively few groups of regionally relevant landscape trees that winter-prune poorly, maples of all kinds push sap vigorously during the winter and early spring months. This is with good reason: they are among the first trees to flower in spring and provide excellent early-season forage for insects. If pruned in winter, they will “bleed” sap profusely, leading to unsightly bark streaking. This flow of sugar-rich sap can also encourage the growth of undesirable organisms on the tree. This sap, of course, is why maples are tapped during the late winter, since it is the raw material for making maple syrup. Instead of winter pruning these trees, postpone structural work until the leaves are fully expanded. Trees will require deep, occasional watering after a summer pruning.
Just like maples, birch of many kinds push sap from root to shoot during the winter and spring seasons to fuel their early spring bloom. I learned this lesson the hard way after thinking I was doing the right thing by winter pruning a large river birch (Betula occidentalis) in one of my gardens, only to find that it oozed sap for over a month; unsightly mold growth followed on the trunk where the sap flowed over the tree’s bark. I now summer prune my birch and have had good results, but I always waterRead more on finegardening.com