Extension Communications
3614 Administrative Services Building
Ames, Iowa 50011-3614
(515) 294-9915
8/12/02
Contacts:
Chris Feeley, Forestry, (515) 294-6739, cfeeley@iastate.edu
Jean McGuire, Continuing Education and Communication Services, (515) 294-7033,
jmcguire@iastate.edu
Yard and Garden Column for the Week Beginning August 16, 2002
Keeping the Bark on Your Trees
Chris Feeley
Extension Forester
Iowa State University
Trunk injuries, cracks and cankers can seriously compromise the health and survivability of trees. In the past year, I have seen many instances of trunk injuries in shade and ornamental trees. In a few cases, large numbers of trees were affected and eventually wilted and died. In this article I will review some of the causes and treatments of trunk injury.
So what happens to the tree when the outer bark is damaged? To answer this question we need to look at the physiology of the tree. The outer bark protects all inner tissues from injury and infection. The vascular cambium lies just inside the bark, and consists of actively dividing cells that produce new xylem (wood) to the inside and new phloem to the outside. The xylem and phloem cells are the lifelines of the tree. Xylem cells transport water and nutrients absorbed by the roots throughout the tree. Phloem cells transport the "food" for the tree, the sugars, amino acids, vitamins, hormones and stored food. Injuring the trunk of the tree disrupts the vascular system weakening or killing the tree.
Cankers caused by pathogens. Cankers are caused by the death of trunk tissue. Pathogenic organisms can cause many types of cankers in trees. Some kill bark; while a few can kill the wood. Many cankers appear as sunken areas or lesions in the bark. Other types appear as round, elliptical or long, narrow 'target' cankers with surrounding ridges of callus tissue.
Most disease organisms that form annual and perennial cankers are relatively weak. Trees have the ability to wall off annual cankers in one year's time. The host and disease reach a standoff in the perennial cankers. Each year the tree walls off the invader during the growing season, but during the dormant season the pathogen invades new tissue. In contrast, some types of canker-causing organisms are very strong pathogens that infect and girdle the trunk killing the cells that form wood and the cells that form bark. Aggressive cankers of this type are rare in Iowa.
Frost cracks and sun scald. Frost cracks appear as deep vertical cracks in the bark and wood extending several feet up the tree and several inches deep into the tree. The colder the temperature the wider the cracks may open. However, sudden drops in temperature do not appear to increase the appearance of frost cracks when compared with slow temperature changes. The current view is that frost alone does not cause the deep cracks. Many scientists believe that frost cracks are caused by a combination of factors including injury (bark wounds or improper pruning), cold temperature, planting depth and drought. Proper planting, pruning, watering and wrapping of newly planted trees will help to eliminate frost cracks.
Sunscald develops on the south or southwest side of trees. In the winter, the temperature of the sun-warmed side of the trunk can exceed the air temperature by 20 F, causing the bark tissue to heat and freeze. The heating and freezing cycle kills the bark causing a localized wound and eventually the bark cracks and falls away. Both sun scald and frost cracks cause a vertical wound, but sunscald damage does not extend into the wood. Thin-barked trees are most often affected and light colored tree wrap is recommended for newly planted trees.
Insect causes of bark injury. Injury from some insect pests may mimic wounds caused by canker diseases and sunscald. Wood-boring insects that feed on the inner tissue cause a sunken lesion on the bark. In most cases, wood-boring insects are present because the tree is in poor health or stressed. Matching the tree to the planting site, proper planting and proper care will greatly reduce the chances of having a wood-boring insect problem.
Cicadas are another insect that causes injury to the bark by making slits to insert their eggs directly into the sapwood of twigs and branches. The slits look very similar to a small canker. Cicada damage rarely harms the tree. In most cases, just a few small twigs die.
Lawnmowers and weed trimmer damage. Injury from lawnmowers and weed trimmers is by far the most common bark damage in Iowa. Fortunately, it is very easy to control. Just keep the lawnmowers and trimmers away from the tree trunk. The best method to protect the tree is to use organic mulch. Mulch to a depth of two to four inches deep, but do not pile the mulch up against the tree. Instead, leave a space of a few inches between the mulch and the trunk. Mulching will keep the competing grasses under control, conserve soil moisture, keep the lawn mowers and weed trimmers away, and add some organic matter back to the soil. For those who do not like the look of organic mulches, consider spraying a ring around the tree with Roundup to kill the grass next to the trunk of the tree.
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ml: isugarden
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