The primary objective of pruning is to produce and maintain healthy trees. With that said, pruning is only effective when properly applied. The following examines the pruning techniques that are most beneficial to trees, and what they entail.
General cleaning is one of the most routine pruning practices. It involves the removal of any dead, dying, diseased, or weakly attached branches present in a tree. This reduces the number of safety hazards present in the tree, and promotes the development of healthy branches.
Crown thinning is the selective removal of branches from the crown of a tree. The intention of crown thinning is to maintain or develop a tree’s natural structure and form. Proper crown thinning helps trees avoid stress by increasing light penetration and air circulation throughout the crown. It also prevents the development of weak limbs, codominant stems, and epicormic sprouts. This reduces the potential for branch failure, and helps retain the tree’s natural shape.
When thinning a tree’s crown, no more than a quarter of the crown’s living growth should be removed at any one time. Branches with u-shaped angles should be retained. They help establish strong branch structure, and produce healthy growth. Branches with narrow, v-shaped angles of attachment are more precarious, often forming included bark. Included bark is a structural defect that prevents the development of strong branch unions, increasing the potential for cracking and tearing at the stem.
Codominant stems are branches that grow closely together, typically emerging from the same position. They are another frequent source of included bark. Removing some of the lateral branches from a codominant stem can reduce its growth enough to allow the other stem to assume the terminal role. As a general rule, lateral branches should measure no more than one half to three quarters of the diameter of the stem they are attached to. They should also be distributed evenly, especially on younger trees.
Avoid excessive pruning of the crown. It can destroy the tree’s structure, and produce lion’s tails: small tufts of foliage that are caused by the removal of the tree’s inner lateral branches. If branch growth is overly vigorous, and additional pruning is required, it should be performed gradually, over a number of years.
Crown raising is the practice of removing the lower branches from a tree. This type of pruning is generally applied to create clearance for pedestrians, vehicles, and buildings. It also ensures the removal of hazardous branches and epicormic sprouts from the lower section of the tree.
When raising a tree’s crown, always maintain at least two thirds of its growth in order to avoid creating structural weaknesses. Removing too many lower branches can deplete the tree’s resources, hindering its development.
Crown reduction, also called drop-crotch pruning, is a type of pruning method often applied to reduce the size of a tree. It is commonly used on trees that have grown too large for their permitted space. Crown reduction is accomplished by pruning the leaders and branch terminals back to the stem, and replacing them with secondary branches. The secondary branches must be large and strong enough to assume the terminal roles. Those selected for this purpose should be at least one third the diameter of the cut stem. Crown reduction pruning should not be administered on any trees with pyramidal growth forms, as it can destroy their form and branch structure.
Crown reduction is not a desirable pruning practice. It produces large pruning wounds in the stem that do not mend quickly, and are prone to decay. However, when necessary, it is a preferred alternative to pruning practices such as topping or lion’s tailing.
Tips for Pruning Live Branches:
When determining the proper place to cut a branch, begin by searching for the branch collar and the branch bark ridge. The branch collar resembles a large bulge, and can be found growing from the stem tissue at the underside of the branch. The branch bark ridge is located on the upper surface, running parallel to the branch attachment, along the stem of the tree.
Once these have been discovered, make the pruning cut just outside the branch bark ridge, and angle it downwards, away from the stem of the tree and the branch collar. Cut as close as possible to the stem, but avoid slicing into the branch bark ridge. Pruning cuts should always avoid damaging stem tissue, and removing the branch collar or branch bark ridge. If only branch tissue is cut when pruning, it decreases the risk of infection, and allows the wound to seal more effectively.
Use the three cut method for removing larger branches. This approach is crucial to preventing ripping or tearing of the bark as the branch is separated from the tree. To prevent the growth of epicormic sprouts, or dieback of the stem, be sure to make the pruning cut at a lateral branch that is at least one third of the diameter of the stem. The first cut should be made on the underside of the stem, well above the branch crotch. Begin the second cut inside the branch crotch, staying well above the branch bark ridge, and cut through the stem just above the notch. Finally, cut the remaining stub just inside the branch bark ridge.
Tips for Pruning Dead Branches:
Dead branches should be pruned in a similar fashion to live branches. The branch collar and branch bark ridge are easier to distinguish on dead branches since they continue to grow. Perform the pruning cut just outside the ring of callus that has formed, letting the branch tumble safely to the ground. Larger branches should be removed using the three cut method.