Ask a Master Gardener: Timing of pruning can affect plant health

Q: Does it really matter when I trim my shrubs and trees?

A: The general rule is that late winter, before new growth starts, is the best time for pruning. However, many plants should be pruned at specific times, some for aesthetic reasons, and others to reduce the risk of serious problems. For example, spring flowering trees and shrubs like lilacs should be pruned right after flowering because they start to set next year’s flower buds right after the current year’s flush is over. Pruning apple trees now would increase the chance of their contracting fireblight, a serious bacterial disease. Oaks should never be pruned in April, May or June to decrease the risk of fatal oak wilt.

We don’t encourage pruning anything after mid-summer because that can promote new growth that is too tender to handle our winters, but dead or diseased wood can be removed at any time. Using tar or other sealers is not recommended because studies show that they harm trees. The exception is for oaks that have been pruned or damaged during the risky months.

Our Extension Service has a good pruning overview at: www.extension.umn.edu/distribution/horticulture/dg0628.html. Be careful when looking outside of our region for pruning advice because timing, diseases and insects vary across the country.

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