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Index : Trees
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- 31. Protecting Trees From Construction Injury - Top
- Just because trees look sturdy and strong doesn't mean that they can tolerate a great deal of abuse. Mother Nature dishes out her share with damaging wind, scorching heat and withering drought. But by far, people do the most damage, sometime without even realizing it.
Construction injury is one of the most common ways trees are damaged. Because trees do not replace injured tissue (heal) like an animal does, a wound or injury to its trunk, branches, or roots weakens it. In a weakened state, trees have a harder time fending off insects, diseases, drought and other stresses.
Soil compaction from heavy construction equipment damages trees in unseen ways. The shear weight of heavy equipment running over the roots can injure them. A single pass from a bulldozer of dump truck can significantly damage roots. Equipment should not be permitted to operate within the dripline of those trees to be saved. The dripline is the outer edge of the branches.
Because 50% of a tree's root system is located between it's trunk and it's dripline, fences should be constructed at the dripline. They should be sturdy enough to keep out equipment and vehicles, and their determined operators.
Tree roots extend out much further than the dripline. However, it is usually not practical to protect the entire root area. Protecting the area within the dripline and good irrigation management are the most important measures in helping prevent construction related tree decline.
Cuts and fills can also damage tree roots during construction. Cutting or scraping off soil can occur during the grading process. Fill soil may also be added. Because most feeder roots are located close to the soil surface, changing the grade by as little as 6 inches can cause extensive root injury. Cutting, physically removes roots. Filling covers the roots with excess soil. This disrupts the flow of oxygen and water into the soil; both vital for root survival.
Often when soil is to be filled around existing trees a wall, called a tree well, is constructed a few feet out from the trunk. If more that 3 or 4 inches of soil is added to the surrounding area, the well is of no benefit. If a tree well is to be used, it should be constructed no closer that the dripline. Soil should be graded away from the tree well to prevent runoff water from entering the well. Success has been reported in several cases where gravel was spread over an existing grade Vertical vent pipes were installed every 10 feet through the gravel to help carry oxygen out from the tree well. This spoke-wheel arrangement is complicated and best left to a certified arborist.
Trenching around trees to lay utility lines may be unavoidable, but it will injure the tree. The extent of injury depends on the percentage of roots severed. Unless the tree is twisted, roots on one side of a tree generally supply the same side of the crown with water and nutrients. When roots on one side of the tree are cut of injured in other ways, branches on that side will usually die.
If a tree survives the first 2 or 3 years following construction, it may still die from construction related injuries. Trees may decline quickly or slowly after construction of a home. Often, branches begin dying within a year or two due to severe root damage. The tree may be dead within 3 or 4 years. However, it is not uncommon for trees to show a slow decline over a 5 to 10 year period.
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Written by John Begeman, College of Agriculture and Life Sciences, the University of Arizona 626-5161.
Material originally appeared in Arizona Daily Star gardening column, on July 28, 1996
- Updated: July 28, 1996
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