Answer: Generally it's safe to prune during the inactive or dormant period, ideally late winter to early spring, as long as the trees are not frozen. It's not a good idea to prune in late summer or early fall because pruning stimulates growth and the new tender shoots may be damaged in the upcoming colder months.
However for every general rule there are exceptions, and maple is one of them. While it is not harmful to the tree to prune in late winter, maples are "bleeders" (the sap flows freely). All cuts during this time will "bleed." This is not harmful to the tree, but could create an untidy situation. Under such circumstances maples and other bleeders can be pruned in midsummer when the sap will not run.
If you need to remove an entire limb, cut it cleanly back to the trunk of the tree. Otherwise, remove small side branches by cutting them close to a main branch. Pruning in this way will help your tree retain its natural shape.
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