If your Japanese Maple requires major, structural pruning, the best time to do it is shortly before the plant "wakes up" in late winter, usually early to mid March in the SF Bay area. About a week or so before the tree's new leaves unfold is when the maple has the most energy to start growing protective callus tissue around any pruning wounds. However, pruning should never affect more than 25-30% of the maple. More radical pruning, especially of older trees, may seriously sap their strength. Also, if at all possible, it's best to avoid any pruning of the larger, lower limbs of established maples—wounds in these areas may never completely callus over, and dieback or rot could result.

– Margaret