Part 2: Mechanical and Weather Damage
By Mary Free, Certified Extension Master Gardener
Mechanical damage is the most preventable of bark injuries because it is often due to impatience, inattention, or ignorance. While pruning is essential to maintain a robust tree or shrub, improper pruning can be detrimental to a plant’s health. While Americans devote considerable time cultivating their lawns, careless maintenance practices can damage trees that take decades, not weeks, to grow. While we cannot control the weather, which can cause irreparable bark injury, we can control what plants we purchase and where we choose to locate them—the right plant in the right place increases its chances of surviving weather extremes.
Pruning can resolve problems with branches that are dead, deceased, weak, broken, co-dominant, narrow-angled, rubbing against each other, or sprouting from the trunk or its base. Selective pruning can rejuvenate a shrub. It can increase light under a tree canopy as well as flowering and fruit production. Trees and shrubs should be pruned at the proper times of the year, using proper tools and methods. Complex pruning on a large tree may require the services of a certified arborist.
Poor pruning techniques can strip or damage the bark, stunt growth (especially that of very young trees), and leave trees vulnerable to infection or infestation. Failure to follow the three-step pruning process (as described in “Winter Is the Time for Pruning”) is a common way to injure trees. Neglecting to make an undercut first usually results in a heavy limb stripping bark from the branch collar or trunk as it falls away. If the collar is wounded, then the branch defenses that stop the spread of decay and seal the wound will not activate.
Lawn mowers, tractors, string trimmers, and weed wackers also cause mechanical damage to trees and shrubs. Repeated rubbing or gouging of the trunks or stems by this equipment can damage bark. For example, when lawn mowers injure the trunk of Cornus florida (flowering dogwood), the tree becomes particularly vulnerable to attack by borers. This can be prevented by removing turf, and laying mulch about 2–3 inches deep, starting from at least 6–12 inches away from the trunk of a lawn tree out over the root zone, to 6–12 inches beyond the drip line. Mulch helps to retain moisture, and prevents soil compaction from riding mowers and foot traffic. Ground covers, which can grow in partial to full shade, can be planted under a tree to provide a protective circle, and is more attractive than turf, which performs best in full sun. If roots protrude into the lawn, then trim the nearby grass by hand or set the mower blades to the highest setting. Mulch beds and borders, or edge with hardscape for protection.
If bark has peeled (when it is not a normal characteristic, like that of birch trees), split, or cracked, then it may be a sign of cold injury. For trees, such injury is referred to as frost cracking; for shrubs, it is referred to as bark split. This is likely to occur when temperatures are much lower than normal in early fall, winter, or late spring. Sudden drops to below freezing temperatures before plants have time to harden in fall, extremes between day and nighttime temperatures in winter, and rapid thawing in early spring can damage plant tissues. If water within a tree branch or trunk freezes, just like a water pipe, it can split or burst. Some cracks may close as the weather warms, even as damaged wood fibers remain broken. As a result, some injured branches or plants may die in early spring; others may take years to die.
Sunscald or southwest injury occurs in winter with extreme changes in temperature. During the day, the sun warms the south or southwest sides of trunk tissues so that they become less cold-hardy. When the sun sets and temperatures suddenly drop, the bark tissues cannot reacclimate in time to resist freezing. Deciduous trees are the most vulnerable, especially if they are young, newly planted, or thin-barked. Among the most vulnerable species are maples and fruit trees. Birches, which also have thin barks, use the compound betulin, which gives the tree its white color, to reflect sun rays and reduce winter sunscald.
To prevent permanent damage to wounded plants, gently clear dead or loose bark from the injury, and prune out split or cracked areas as far as the unaffected growth, once the threat of late spring freeze passes. To prevent further stress, water injured plants during dry spells in the summer. The best solution is to grow reliably hardy plants, preferably natives, that have a better chance of withstanding low or rapidly changing winter temperatures. Do not fertilize in late summer (new growth may fail to harden before temperatures fall). Be sure to mulch. Mulch controls moisture and soil temperature. Water before temperatures freeze, to keep the soil warmer and to maintain plant hydration. Plant vulnerable trees where they will be shaded by structures or evergreens, and not on the south or west side of a building or near reflective surfaces.
Tree trunks and branches exposed to high temperatures and solar radiation, especially on the south and southwest sides, can suffer injury similar to sunscald. Symptoms of sunburn, which may take months to manifest, include discolored and then dried, cracked, and peeling bark. The most susceptible trees are thin-barked, like birches, young fruit trees, and maples. Specimens previously shaded that are suddenly exposed to direct sunlight, or those grown on tree farms or nurseries in less intense light are particularly vulnerable to sunburn. To decrease the chance or degree of sunburn and to prevent further stress on injured trees, water them during summer dry spells. Also, refrain from planting trees in the summer.
Bark can be stripped by lightning strikes. Likely casualties are the tallest tree or the lone tree in an area, those on the top of hills, close to water, and near transformers, electrical wiring, or plumbing. Oaks are among the most vulnerable; birch among the least. The effects of lightning strikes on trees vary, depending on the intensity and location of the strike and the water content and species of tree. Sometimes a strike occurs without leaving any physical evidence and yet the tree has been so traumatized internally or below ground, that it will die soon thereafter; other times the tree will violently explode apart or burn. It is not uncommon to see a trunk wound where the bark has been stripped from the upper canopy right to the base. In these cases, where just one side of the tree has been hit, the chances of a tree surviving are good.
Unless there are obvious signs that the tree is dead or dying and poses a hazard, it is recommended that you wait a period of time—about 2–6 months (Clatterbuck) to 1 year (DeRosa, 1983)—to observe the tree’s reaction to the strike before employing costly measures. The most important immediate responses are to gently cut away loose bark to where it is firmly attached to the branch or trunk, prune any broken branches, and make sure that the injured tree is sufficiently watered so that it can begin the healing process. Large trees may take decades to heal, so monitoring for insect damage and disease will also be required.
This 95-foot oak was struck by lightning twice. The first time was over a decade ago; the second within the last couple of years, from which some charring is still visible. It was not until a year or so after the second strike, that an arborist was consulted. He pruned some of the branches in the upper canopy but recommended no further action, since the tree appeared otherwise healthy and seemed to be healing itself. © 2019 Mary Free
No matter how much care you take in planting and maintaining the vigor of your trees and shrubs, you can do only so much to prepare for intense thunder and snow storms, hurricanes, tornadoes, and other violent weather events. Quick action after these occurrences can prevent further damage. For example, after a snow storm you can use a broom to sweep snow upward and off of branches to prevent breakage. Cracked or broken branches should be pruned soon thereafter. As mentioned in Part 1, a tree is likely to survive bark damage that is less than 25 percent of the way around the tree trunk. But the more bark stripped around the tree, the more it is at risk of death. Also at high risk are trees with severe damage to more than 30 percent of the main branches. (Bradley, 2018) Trees that present a threat to life or property should be attended to as soon as possible and if they are large, they should be dealt with by an arborist.
When deciding whether or not to remove a damaged tree, consider the severity of the damage, its health (before the event), age, functionality, desirability, and its potential service as a snag to benefit wildlife. Of course, if it presents a hazard, it should be removed as soon as possible. When trees or shrubs are removed, consider replacing them with natives, which are more acclimated to local environments.
If you have concerns about a tree, the online resources listed below may be helpful. You can also contact your local extension office. In Arlington and Alexandria, Virginia, you can contact the Master Gardener Help Desk at (703) 228-6414 or firstname.lastname@example.org.
- Bradley, Lucy and Barbara Fair. 2018. “North Carolina Extension Gardener Handbook, Chapter 11: Woody Ornamentals.” North Carolina State Extension. http://content.ces.ncsu.edu/11-woody-ornamentals
- Clatterbuck, Wayne K. et al. “Understanding Lightning & Associated Tree Damage, Tree Care Kit.” Texas A&M AgriLife Extension. https://agrilife.org/treecarekit/after-the-storm/understanding-lightning-associated-tree-damage/
- DeRosa, Ernest W. February 1983. “Journal of Arborculture 9(2), Lightning and Trees.” p. 52. http://joa.isa-arbor.com/request.asp?JournalID=1&ArticleID=1862&Type=2
- “Flowering Dogwood.” University of Kentucky Cooperative Extension Service. Department of Horticulture. http://www.uky.edu/hort/Flowering-Dogwood
- French, Susan C. and Bonnie Lee Appleton. 2009. “A Guide to Successful Pruning, Deciduous Tree Pruning Calendar.” Virginia Cooperative Extension Publication 430-460. https://pubs.ext.vt.edu/content/dam/pubs_ext_vt_edu/430/430-460/430-460_pdf.pdf
- French, Susan C. and Bonnie Lee Appleton. 2009. “A Guide to Successful Pruning, Shrub Pruning Calendar.” Virginia Cooperative Extension Publication 430-462. http://pubs.ext.vt.edu/content/dam/pubs_ext_vt_edu/430/430-462/430-462_pdf.pdf
- Koci, Joel and P. Eric Wiseman. “Hiring an Arborist to Care for Your Landscape Trees.” Virginia Cooperative Extension Publication ANR-131NP
- Ophardt, Marianne C. and Rita L. Hummel. 2016. “Environmental Injury: Sunscald and Sunburn on Trees.” Washington State University Extension. http://cru.cahe.wsu.edu/CEPublications/FS197E/FS197E.pdf
- Purcell, Lindsey. July 2015. “Tree Pruning Essentials.” Purdue Extension Publication FNR-506-W. https://extension.purdue.edu/extmedia/FNR/FNR-506-W.pdf
- Relf, Diane and Bonnie Appleton. 2015. “Managing Winter Injury to Trees and Shrubs.” Virginia Cooperative Extension Publication 426-500. p. 4. https://pubs.ext.vt.edu/content/dam/pubs_ext_vt_edu/426/426-500/426-500_pdf.pdf