When to Prune
It really depends in part on why you are pruning. Removal of deadwood and or light pruning can be done at anytime. Here are some guidelines to follow but keep in mind different species may have special requirements
Pruning while trees are dormant is the most common time to prune. This practice results in a burst of new growth in the spring at leaf emergent. It is also best to wait until the coldest part of winter has passed to prune. Some species of trees such as Walnut and Birch trees may leak sap from the pruning wounds once sap begins to flow. This is not harmful to the tree and will end when the tree leafs out
To slow the growth of select branches or redirect a branch you should prune soon after the seasonal growth. The slowing is caused by reducing the amount of leaves therefore reducing the amount of food made and sent to the roots. Summer pruning can also be done for corrective purposes. Defective limbs and limbs which are to heavy from the weight of leaves can be easily spotted.
Pruning flowering trees to enhance flowering
If the tree blooms in spring prune when their flowers fall off. Trees that flower in mid to late summer should be pruned in winter or early spring.
When NOT to prune
Fall- Because the healing of wounds is slower on fall cuts and decay fungi spores spread in the fall making the tree at a higher risk of being infected. In addition to spreading fungi, trees are exerting energy to prepare for winter. You do not want the tree to redirect resources into healing during this time.