Q. What is the best time to prune cut-leaf Japanese maples? Should I prune out a lot of the thin branches, inside and outside the tree?
A. Cut-leaf Japanese maples (Acer palmatum var. dissectum) tend to be slow-growing plants that do not require much pruning. They should be carefully trained when they are young to establish an attractive growth habit and sturdy limb structure. In subsequent years, a bit of pruning to remove dead or damaged branches and to eliminate branches that cross each other should be all that is necessary.
A number of tree species, including maples, tend to "bleed" or lose a lot of sap if they are pruned in late winter or early spring, which is when deciduous trees are typically pruned. Although this does not really hurt the tree, it often alarms the person doing the pruning. If you have to do any major pruning and shaping, it is best to do it before the tree leafs out so that you have a clear view of its framework and what needs to be done. It will bleed, but that does not harm it.
Do not use any type of pruning paint or tar to try to stanch the sap flow. Research has shown that these products actually interfere with the tree's ability to heal the wound on its own. Once you have the tree in good shape, you can delay pruning until early to mid-June in future years when it is less likely to bleed.
Start pruning by removing any obviously dead or badly damaged branches. Then remove branches that cross, rub other branches and create wounds in the bark. There are many cultivars of cut-leaf Japanese maples, and they have distinct growth habits. However, they often have a graceful weeping shape that you should maintain as you prune.
Be sure to make thinning cuts, those that remove a small branch at its point of origin on a major branch, rather than heading cuts. Heading cuts are those that cut a small branch back to a pair of buds. Too many heading cuts will create a dense tangle of growth toward the ends of branches that will shade the interior of the tree and destroy its naturally graceful growth habit.