When to Prune a Maple Tree
The species’ tendency to lose sap from wounds makes gardeners question the wisdom of pruning maple trees. However, maple tree pruning is an essential part of maple tree maintenance.

In late winter, when the days are warm and the nights are cold, root pressure causes sap to flow from any wound made in the bark of the tree. This makes it look as if the tree is suffering.

However, maple tree pruning in winter generally won’t hurt a mature tree. You would have to remove an entire limb for the loss of sap to negatively impact a fully grown tree. If the tree is just a sapling, however, loss of sap may cause problems. You can avoid this issue if you wait until summer to prune maples.

Maple trees can be safely pruned in early summer after the leaves have leafed out or turned dark green. Maple trees will bleed less in later summer when sap is no longer running and the tree has received the bulk of its nutrition. You can determine when you want to prune from your maple tree leaves’ color and if they have hardened up (fully leafed out).

Once the leaf buds open, the sap is no longer under pressure and won’t leak out from pruning wounds. For this reason, many gardeners say that the best time for pruning maples is in summer after the tree is fully in leaf.

How to Prune Maple Trees
Regular maple tree pruning helps keep a tree the desired size and stops a tree from encroaching on its neighbors.

Pruning also assists the tree develop of a sound branch structure. Carefully removing branches can reduce or eliminate structural issues in a tree. It can also open up the center of the tree to let sun and air move through the canopy. This prevents certain types of diseases.

When you are pruning maple trees, it is always a good idea to remove broken, diseased, or dead branches. Otherwise, decay-producing fungi, such as cankers of various sorts, or growths called galls or burls, which can infect healthy parts of the trees.

The following articles may also be a help:
https://www.canr.msu.edu/news/tips_on_tree_and_shrub_pruning
https://mapleleavesforever.ca/pruning-young-sugar-maple-trees/
https://blogs.cornell.edu/nysipm/2016/04/07/trees-and-threes-prune-now-to-keep-trees-healthy/
https://www.purdue.edu/hla/sites/yardandgarden/late-winter-recommended-for-pruning-chores/