Pruning deciduous ornamentals is done to generally improve tree structure and performance. It may involve training trees to certain shapes, and (or) removal of potentially weak branches. Narrow angled branches with included bark are generally weak and should be removed as early as possible. In effect, bark forms deep in the crotch and prevents the formation of strong wood structure between branches. As the branches get larger and heavier, the chance of failure increases.
The most common pruning techniques for deciduous ornamental trees and shrubs are crown cleaning, crown thinning, crown reduction and crown raising. The crown is comprised of the branches and leaves. Crown cleaning involves the removal of weak, damaged or diseased branches. For many woody plants, this may be the only type of pruning necessary. Crown thinning involves the removal of branches for the purposes of increasing light penetration and air circulation within the canopy as well as reducing the weight of selected limbs. Over-thinning should be avoided as it can result in sunscald, and watersprout production.
Crown reduction reduces the height or length of branches by cutting them back to lower laterals or to locations between branches. This process is known as heading back and is useful in stimulating shrubs to release lateral buds to become bushier. When reducing the size of a branch, prune to a lateral that has a diameter of no less than one third of the removed branch. In trees, this practice is generally used to control size and is sometimes known as topping; however, it is rarely an acceptable pruning technique as it may weaken the tree and promote poor structure. Typically the trees may grow vigorously in height to replace the lost material.
Crown raising involves the removal of lower branches that otherwise obstruct the passage of people, equipment etc.
When pruning deciduous ornamentals, timing can influence wound closure and recovery as well as the potential for pathogen infections and insect invasions. A general rule of thumb in Manitoba is that it is best to prune deciduous trees in late winter or early spring, while the plants are still dormant. However, pruning can be done at any time during the dormant season and in the summer as well. Trees such as birch and maple that exhibit a heavy flow of sap and tend to "bleed" if pruned in fall or before bud break. Although this bleeding has not proven to cause extensive harm to the tree, it may be more desirable to prune these trees in early summer after growth has stopped. In Manitoba, due to the devastating Dutch elm disease that threatens the native American elm, pruning these trees is prohibited between April 1 and July 31. Siberian elms must not be pruned between April 1 and June 30. These pruning restrictions coincide with the active period of the elm bark beetle, the carrier of the disease spores from tree to tree. Pruning during this active period attracts the beetles, thus facilitating the spread of the disease.
Timing can also affect flowering. Spring flowering trees and shrubs typically form their flower initials in late summer in buds that overwinter and hence they flower on the previous year's wood. If they are pruned in late summer, fall, late winter, or even early spring, the flower buds may be removed. A good time to prune spring flowering shrubs is soon after flowering, before they have had a chance to set the flower buds for the next year. Common examples of spring flowering ornamentals include lilac, forsythia and flowering plum. Summer flowering trees and shrubs typically bloom on the current year's growth. Thus, these are best pruned after flowering or in the dormant period but not after new growth has been initiated, as pruning will continuously remove newly formed flowers. Examples include alpine currant, dogwood, summer-flowering spiraeas and honeysuckle.
For removal of entire branches, pruning cuts should be made just outside the branch collar (swollen area at the underside of the branch-stem union) as opposed to the once recommended flush cut. Avoid leaving stubs as these will only die back, attracting insects and pathogens. Moroever, it is difficult, if not impossible, for the tree to 'heal over' such stubs. When removing large branches, a three-cut procedure should be used. The first cut should be made on the branch underside approximately 45 cm from the trunk or parent branch and should not penetrate more than half of the branch. The undercut is designed to minimize the breaking and tearing of the bark. The second cut is the removal cut. It should be initiated on the top of the branch approximately 2.5 cm beyond the undercut (away from the trunk). The branch will then break off at this point. The final cut removes the stub and should be made just outside the branch collar as describe above.
When performing any type of pruning activity, it is imperative that no more than one-quarter of the foliage be removed in a single growing season. Pruning tools should be disinfected between trees (between each cut if working with diseased material) in order to prevent the spread of disease. Wound dressings, though often recommended, have not been proven to hasten healing or decrease decay. If a wound dressing is desired for aesthetic reasons, ensure that it is registered for use on trees.
Note that there are many good books and web sites that described pruning in detail.
Relevant web sites:
Joyce, D. 2001. Pruning and Training Plants: A Complete Guide. Firefly Books
Lilly, S.J. 2001. Arborists' Certification Study Guide. International Society of Arboriculture, U.S.A. 222 pages