Pruning Japanese Maple Tree
My tree is about 5' tall and 7' wide; I have no idea how to prune it.
Washington County Oregon
The best way to prune Japanese maples (Acer palmatum and Acer japonica) is to prune as little as possible.
There are several reasons why pruning Japanese maples should be minimal.
First, at their best, these trees naturally develop an individual, sweeping but irregular "line" that moves gracefully from the ground to the trunk to the main branches to the secondary branches, right out to the smallest twigs. ("Line" is a term dancers use to describe the body in motion; unlike most trees, the "line" of a good Japanese maple can often be described with a single, smooth, continuous sweep of the hand.) This sounds pretty esoteric, but the point is that a bad pruning cut can severely damage or destroy the smooth sweep of the tree.
Second, every species of tree has a "pruning budget"--an amount of foliage that can safely be removed before the plant responds by sending out scores of long, whip-like water sprouts. For Japanese maples, this "budget" is very small. Removing branches that constitute more than about 5% of the total foliage of the plant in any one season is very likely to result in an instant profusion of unsightly whips. And if cutting these off exceeds that same "budget", the result is even more unsightly, leggy growth. You can see how this can easily get completely out of hand.
The worst kind of cut is to cut off part of the length of any branch, as the tree responds by immediately generating three to six scattershot replacement branches right from where the cut was made. By the same token, Japanese maples should never be sheared, as this will immediately turn them from images of grace to dense, graceless shrubs, with branches going in every direction.
When a cut MUST be made, to remove a damaged or dead branch, to open a tree so it's structure can be more clearly seen, or to create a "window" in the tree through which a special view can be seen from a specific location, then that cut should remove a branch where it springs from a larger branch or trunk.
It is never appropriate to use pruning to control the height or width of a Japanese Maple. It is impossible to do so without completely destroying their graceful shape. (There are over 500 named cultivars, each with a fairly predictable height and spread. Pick one that is size-appropriate by nature.) It IS appropriate to use pruning to strengthen the natural line of the tree by removing extraneous or confusing branches--always removing them by cutting them off where they join a larger branch or the main trunk.
The most useful pruning you can do, especially with the weeping forms of these trees, is to wait until the tree is completely dormant (no leaves) and then gently comb the small branches with your bare hands, which will effectively break off all the tiny dead branches that tend to collect over time, thus opening up the view of the tree's main branch structure, and restoring the tree's fresh appearance.
Since you live close, you might be interested in taking a one-day class in pruning Japanese maples taught by the gardeners at the Japanese Garden in Portland. The instructors are world-class, and the classes always include actual pruning work (on potted trees provided by local nurseries, not on the permanent specimens from the Garden!) Unfortunately, the Garden is temporarily closed for construction until March of 2016. Keep an eye on their website; classes are usually offered several times each year. http://japanesegarden.com/