Japanese maples (Acer palmatum) are valued for their compact size, ferny leaves and brilliant autumn foliage. The mature trees may reach a height of up to 40 feet, depending on variety. Summer foliage may be green, golden or deep red. Japanese maples are relatively pest and disease resistant, and grow well in U.S. Department of Agriculture plant hardiness zones 4 through 8. Trimming a Japanese maple will enhance its form and beauty.
When to Prune
Start trimming Japanese maples two or three years after planting. Dead wood can be cut off anytime. You can recognize dead wood by its dark or gray coloration and brittleness.
The ideal times to prune Japanese maples are early to mid winter, when the tree is dormant, and mid to late summer. Late winter and early summer are the worst times to prune, because pruning cuts at these times will cause sap to leak out. The sap may attract pests and harm the tree. Pruning cuts in winter may help you correct and improve branch patterns, because the absence of leaves allows for better visibility. Prune in summer to shape the form of the foliage.
Use only sharp and clean pruning tools to trim live branches on your Japanese maple. Minimize how much you cut off each time, because each cut is a wound and may weaken the tree. Prune gradually; trim only a portion of the live branches slated for removal. It may take several years to achieve the desired form. Do not paint or seal pruning cuts; studies show better healing of unsealed cuts.
"Limbing up" is a pruning method where you remove the lowest branches and expose more of the lower trunk. This pruning method is suited for taller upright Japanese maples. Don't stress the tree by removing all the branches in one year; spread pruning out over two or three years. Cut branches up to the branch collar, and don't leave an unsightly stub. Branch collars are the swellings where a branch meets the tree trunk.
Layer pruning is a method that creates non-overlapping branch layers from bottom to the top of the tree. Use this method to prune single-stemmed Japanese maples. Before starting to layer prune, remove dead and broken branches. Remove all branches that cross from one horizontal layer to another. To achieve layering, you may need to make a cut, called the "selective heading cut." This cut removes the main branch growing between two side branches or buds. Cut as close to the side branches or buds as you can. Throughout the pruning process, take a few steps back periodically to look at the branch pattern before continuing to prune. Layer pruning is best done during the dormant period when the branch pattern is clearly visible.
Smaller or weeping Japanese maples, such as the laceleaf varieties, are best pruned by the "shell" method. To shell prune, trim the Japanese maple such that the top layer of foliage covers the whole tree, creating a protective "shell." The small Japanese laceleaf maples have fragile branches; use care not to produce gaping holes in the canopy by accidentally breaking twigs. As with other types of pruning methods, remove dead and broken branches and frequently move away to observe the work in progress. Don't prune out more than one-fifth of the tree's canopy each year. The best time to shell prune is summer.
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