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Pruning Large Established Trees

When to Prune

The late dormant season is best for most pruning. Pruning in late winter, just before spring growth starts, leaves fresh wounds exposed for only a short length of time before new growth begins the wound sealing process. Another advantage of dormant pruning is that it’s easier to make pruning decisions without leaves obscuring plant branch structure.

Pruning at the proper time can avoid certain disease and physiological problems:

• To avoid oak wilt disease DO NOT prune oaks during April, May, or June. If oaks are wounded or must be pruned during these months, apply wound dressing to mask the odor of freshly cut wood so the beetles that spread oak wilt will not be attracted to the trees.

• To avoid likelihood of stem cankers, prune honey locusts when they are still dormant in late winter. If they must be pruned in summer, avoid rainy or humid weather conditions.

• Prune apple trees, including flowering crabapples, mountain ash, hawthorns and shrub cotoneasters in late winter (February-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.

• Some trees have free-flowing sap that “bleeds” after late winter or early spring pruning. Though this bleeding causes little harm, it may still be a source of concern. To prevent bleeding, you could prune the following trees after their leaves are fully expanded in late spring or early summer. Never remove more than 1/4 of the live foliage. Examples include all maples, including box elder, as well as butternut, walnut, birch and its relatives, ironwood, and blue beech.

Trees  that bloom early in the growing season on last year’s growth should be pruned immediately after they finish blooming. These include apricot, Juneberry, flowering plum, cherry, and magnolia.

How to Prune

Leave the pruning of large trees to qualified tree care professionals who have the proper equipment. Consider the natural form of large trees whenever possible. Most hardwood trees have rounded crowns that lack a strong leader, and such trees may have many lateral branches.

The three most common types of tree pruning are:

   1. Crown Thinning. This is selectively removing branches on young trees throughout the crown. This promotes better form and health by increasing light penetration and air movement. Strong emphasis is on removing weak branches. (Don’t overdo it on mature trees.)

   2. Crown Raising. This is removing lower branches on developing or mature trees to allow more clearance above lawns, sidewalks, streets, etc.

   3. Crown Reduction. This is removing larger branches at the top of the tree to reduce its height. When done properly, crown reduction pruning is different from topping because branches are removed immediately above lateral branches, leaving no stubs. However, crown reduction is the least desirable pruning practice. It should be done only when absolutely necessary.

Proper Branch Pruningprune a small branch

• To shorten a branch or twig, cut it back to a side branch or make the cut about 1/4 inch above the bud.

• Always prune above a bud facing the outside of a plant to force the new branch to grow in that direction.

Pruning Large Branches pruning a large branch

• To remove large branches, three or four cuts will be necessary to avoid tearing the bark. Make the first cut on the underside of the branch about 18 inches from the trunk. Undercut one-third to one-half way through the branch. Make the second cut an inch further out on the branch; cut until the branch breaks free.

• Before making the final cut severing a branch from the main stem, identify the branch collar. The branch collar grows from the stem tissue around the base of the branch. Make pruning cuts so that only branch tissue (wood on the branch side of the collar) is removed. Be careful to prune just beyond the branch collar, but DON’T leave a stub. If the branch collar is left intact after pruning, the wound will seal more effectively and stem tissue probably will not decay.

• The third cut may be made by cutting down through the branch, severing it. If, during removal, there is a possibility of tearing the bark on the branch underside, make an undercut first and then saw through the branch.

• Research has shown wound dressing is not normally needed on pruning cuts. However, if wounds need to be covered to prevent insect transmission of certain diseases such as oak wilt, use latex rather than oil-based paint.

Source: University of Minnesota Horticutural Extension
Mike Zins and Deborah Brown, Pruning Trees and Shrubs, Item # 00628, 1997, All rights reserved

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