Answer: Keeping Japanese maples small isn't too difficult, but it does take regular pruning. Early spring, just as the buds begin to swell on the branches is the best time to prune. This way, new growth will cover the pruning cuts and there won't be the risk of sunscald.
Begin by removing any missplaced branches and by thinning out the stems growing on the branches you want to keep. Throughout the spring, as your maples continue to send out new growth, either rub out the buds or prune off the stems that are making the canopy too high. Usually by mid-summer new growth will stop and the tree will redirect its energy into manufacturing carbohydrates to feed the roots so there won't be much pruning required.
If the trees are fairly overgrown, plan to pruning them back over 2-3 years. The goal is to keep them under control, but not to remove more than about one third of the live material in any one year. Cutting back more severely can stress the trees and make them more vulnerable to insect and disease problems.
Best wishes with your maples.
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