Source: Nathan Winter, University of Minnesota Extension
Pruning trees and shrubs can be extremely beneficial to their health and beauty. Pruning trees during winter dormancy results in vigorous new growth in the spring, and can be used to eliminate branching problems the tree may have.
Apple trees, including flowering crabapples, mountain ash, hawthorns and shrub cotoneasters should be pruned in late winter from February to early April. Spring or summer pruning increases chances for infection and spread of the bacterial disease fireblight. Autumn or early winter pruning is more likely to result in drying and die-back at pruning sites. Oak trees, especially red oaks, should also be pruned before April to reduce their chance of developing oak wilt.
Pruning approaches include crown thinning, crown raising, and crown reduction. Crown thinning is primarily used in hardwoods to increase the amount of room for light and air to penetrate the tree. You still will want to maintain the trees natural shape, and form. Another form of crown thinning is to make sure there is only one dominant leader instead of two or more co-dominate leaders on the tree. This is the easiest to do while the tree is still young.
Crown raising is cutting off some of the bottom branches to permit travel underneath the tree. This could be for lawn mowers, people, and vehicles. Be sure not to raise the crown of the tree too high to avoid an excessively high crown. At least 50% of the tree’s height should be living crown.
Crown reduction is another approach to pruning. This method should be used only in a last resort when the tree has outgrown its permitted space. This approach should not be used on trees with a pyramidal growth form.
Topping and tipping pruning practices do more harm to trees than they help. Topping is pruning large upright branches between the nodes and is sometimes done to reduce the height of the tree. Tipping is pruning lateral branches between nodes to reduce the crown width. These practices result in sprouts and dead branches that will reduce the life of the tree. Use the crown reduction method as a last resort and avoid topping and tipping.
What about treating the wounds? Most of the time the tree sap, gums, and resins naturally work to decrease pathogens invading the trees. Therefore, there are very few circumstances when wound dressings are needed for pruning cuts. Often, they create more problems than they avoid. One scenario in which treating wounds is recommended is for oaks that are wounded between April and October. Remember; do not prune oaks between these months. Fresh wounds during that time attract the beetles that spread oak wilt, and leaves the trees very susceptible.
The University of Minnesota Extension Website contains a vast amount of information and can be found at the following link: http://www.extension.umn.edu/. Following is a great link to learn more about pruning trees and shrubs: http://z.umn.edu/pruningtrees15.