Wednesday, July 16, 2008

Landscape - Tree Wounds

The following is a good article on dealing with wounds in trees from Kentucky.

Windstorms, heavy thunderstorms, snow loads, and layers of ice are occasional features of Delaware weather that can result in many broken tree limbs and downed trees in the landscape. Much of the fallen wood comes down because the interior of the branch or tree was decayed, but branches with no decay also break and fall. Wood decay in trees almost always begins with an injury to the tree.

Wounds of many types can occur on landscape trees. Weather-related broken branches are significant, but bark injuries, pruning stubs, "too flush" pruning cuts, and cut or damaged roots are also associated with decay problems. One of the most frequent causes of damage to trees in the landscape comes from lawn equipment. Mowers and string trimmers can damage the bark, and if continued, will result in visible wounds at the base of the trunk. Besides restricting the movement of water and nutrients, these wounds become points of entry for insects and wood decay microorganisms.

When an injury or break in the bark exposes the underlying wood, bacteria and fungi in the air, in nearby soil, and on the bark contaminate the wound surface. At the same time, the tree responds to the wound by producing chemical and physical barriers in an attempt to block the invasion of microorganisms and to seal off the damaged area. Organisms which are able to overcome these protective barriers can then colonize and invade the wounded tissues. Among these organisms are the wood decay fungi.

Not all wounds result in extensive decay since trees are frequently able to successfully "compartmentalize" or "wall-off" the decayed area. In many cases, the formation of internal barriers to fungal movement and infection can prevent the decay fungi from spreading. The ability of a tree to internally compartmentalize decay differs from one individual tree to another, although it is also influenced to some extent by tree vigor. Wound-wood provides an external barrier to decay once the wound has completely closed over. The formation of wound-wood may be an indicator of relative tree vigor but it is not necessarily indicative of the tree's resistance to the internal spread of decay. Extensive internal decay may exist behind a well-sealed wound.

The severity of the wound, the tree's vigor and the tree's inherent ability to compartmentalize are important factors in determining the rate the tree is able to seal off the wounded area. Other factors such as time of the year, type of organisms present, and position of the wound also play a role. A healthy tree will normally respond more quickly than one that is stressed. Small wounds may take a growing season to close, while larger wounds may require several growing seasons to close.

The presence of mushrooms at the base of the tree, or conks (bracket or shelf-like fungal structures) on the trunk or branches are the most certain indicators of decay. The absence of these obvious fungal structures (also referred to as "fruiting bodies"), however, does not mean the tree is free of decay; fruiting bodies of some decay organisms do not appear until decay is well advanced while others may go unnoticed because they are small, short-lived, hidden or produced infrequently. Other indicators of decay include old wounds, hollowed out areas, and abnormal swellings or bulges. Decayed wood is usually soft, white, spongy, stringy, and friable; or brown and brittle. Since decay structurally weakens the wood, affected trees become susceptible to wind or other storm damage.

Control. There are no controls or cures once wood decay has begun. Decaying trees should be removed when they become potentially hazardous.

Preventive Measures.

Protect trees and shrubs from injuries due to human activities: Choose a planting site that is away from potential causes of wounds (i.e., away from walkways, driveways, roads). Give the tree plenty of space for growth to maturity. Protect the tree from lawn equipment by controlling the grass and weed growth at the base of the tree. Hand weeding is good, but labor intensive; applying a layer of mulch around, but not against the trunk is most helpful. A plastic tree guard will also protect the trunk, but it should be removed when the trunk diameter approaches that of the tree guard.

Use proper pruning techniques: Prune out injured and diseased branches as soon as they are found. Prune as close as possible to the connecting branch or trunk without cutting into the branch collar. Never leave pruning stubs because these will seldom close over. Do not top trees (refer to the UK publication, ID-55, "WARNING: Topping is Hazardous to Your Tree's Health!").

Practice sanitation: Remove prunings from the tree and do not leave dead wood nearby.

Treat wounds properly and immediately.

Treating recent incidental wounds:

If immediately after the wounding event, the bark and cambium are still moist, carefully press the bark back onto the trunk, making sure the pieces are fitted into their original positions on the tree. If possible, cover the wound with plastic and shade it from the sun to keep it from drying. Secure the bark piece(s) in place using soft cloth strips tied around the tree.

Carefully break away any dry, loose, injured bark. Using a sharp knife, cut back to healthy bark. Make a clean edge between the vigorous bark and exposed wood; even if the wound shape is irregular, avoid cutting into healthy bark.

Treating pruning wounds:

Wound dressings are primarily cosmetic and do not stop decay. A product called Lac Balsam is used by some arborists and may stimulate callus formation. Dressings are needed where spread of oak wilt disease is probable. Otherwise, painting over wounds is generally not recommended.

Treating old wounds:

If callus (wound-wood) has begun to form, carefully remove the old bark until the wound-wood zone is found. Do not cut into the fresh growth or shape the wound.

If wound-wood is absent, treat the wound as if it were a recent injury.

Reprinted from "WOUNDS AND WOOD DECAY OF TREES" By John Hartman in the July 30, 2007 edition of the Kentucky Pest News from the University of Kentucky, College of Agriculture.

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