Answer: Many gardeners believe you should prune whenever the pruning tools are sharp. Arborists, however, have scientific studies to rely upon when deciding when to prune. The timing varies with species and with the effect you want to achieve. In general, large cuts should be made in the late winter or early spring before bud break when hydrostatic pressure is greater than atmospheric pressure (i.e. when the sap pressure is positive). A wound made at this time will cause the tree to "bleed". Although some believe the bleeding to be unsightly, it has a positive benefit in reducing the probability of disease-causing organisms to enter the wounds. In the spring the rate of callus production is more rapid, further reducing the chance of invasion.
Other acceptable pruning times include winter and midsummer. Spring pruning after budbreak is undesirable because the bark is tender and damage to the bark is likely. In addition, the food reserves of the tree are being directed toward new growth, leaving less energy available for wound repair. In areas where oak wilt disease is present, it is especially important not to prune oaks between mid April and early July.
Pruning in the late summer is also undesirable because it interferes with food storage necessary for growth the following spring. Additionally, a number of important decay fungi have been observed to produce their largest numbers of spores during the fall. So, the long answer to your question is -- you can remove dead or diseased branches, or any that present a safety hazard at any time. Crown pruning can be done in late winter, early spring, or midsummer.
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