Experts and BHG readers answer.
Japanese maple makeover
Because Japanese maple (Acer palmatum) is grown for its attractive foliage rather than its flowers, you can prune it anytime from late winter to late summer. Most people like to prune these trees in early spring, just before they break dormancy. Not only can you see the outline of the tree better, but the plant gets a full growing season to adjust to the pruning. The downside to pruning at this time of year is that maples are heavy sap producers, so you might see "bleeding." Although this looks unsightly, it doesn't harm the tree. To avoid it, prune later in spring, after the leaves have appeared. Use sharp pruning tools to make clean cuts just outside the collar (the raised area of bark where a branch joins the main trunk). Avoid pruning off more than one-third of the branches in any one year. There is no need to apply tree paint to the cuts.