There is considerable debate on the pruning of a Japanese maple. Some people even suggest that if you have one of these beautiful and delicate trees you should leave it alone, never pruning it. This wisegeek author must confess that following this path for ten years has resulted in a very healthy and happy Japanese maple with no ill effects — which perhaps was a bit lucky. Others argue that pruning a Japanese maple should occur in late fall or early winter, after all the leaves have fallen and when the tree is in its dormant period. Some suggest light spring pruning, especially if you want to create the healthiest branches for your tree.
In most cases, light spring pruning involves cutting back a few of the new growth branches to the first two leaves on the branch. This can result in stronger and heartier branches and it can help avoid branches that will cross over each other and rub together. Any major branch removal should occur in the fall or winter, and there are several procedures to follow.
In the past, many experts who advocated major pruning strongly recommend using a sealant when branches from the tree were removed. Subsequent studies have shown, however, that wound dressing is not necessary if the pruning is done correctly. Experts recommend pruning cleanly back to the main trunk, a lateral branch, or a bud, and not leaving a stub, which heals more slowly.
Before beginning to prune your Japanese maple, take a good look at the structure of the branches. If you see any old or dead branches, or those that look sickly, these should come off. Also evaluate the tree for branches that cross each other, rub together, or any branches that just appear weak.
If you will need to take off quite a few branches, some master gardeners suggest you only prune some of them, and develop a two to three year plan to fully prune the tree. Take off about a third or a half of the problem branches, then wait until the next winter to remove the rest. Each cut to the tree can cause a bit of trauma for the tree, which may make the tree less hearty.
As you prune, you want to be certain you are making clean cuts of the branches, and never twisting or tearing branches off. A clean single cut heals more quickly and provides less shock to the Japanese maple. Choose sharp pruning sheers, which you may want to dip in a bleach solution first to avoid transferring any plant diseases to the tree.