by Simmy Parker
Working around existing trees or under their canopies can be harmful to the trees. If you are planning to do work under the tree canopy or dripline of the tree it will be beneficial to read this article first and decide if the tree you will be working under is worth saving and how to protect it from stress.
Trees have an extensive root system that is required to maintain the health of the tree. The majority of roots are located within the top 12 inches of soil and extend past the dripline of the tree. However, the main area of concern is within the dripline of the tree's canopy. The dripline can be determined by dropping an imaginary line from the outermost leaves to the ground, as seen in figure 1. This area makes a circle around the tree and is considered the protected root zone. Any work in this area should be done with extreme care.
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The easiest way to avoid damage is to reconsider the work to be done. Remember there is usually more than one way to do a job. If you plan to build a patio with pavers that will require cutting or lowering the grade underneath a tree, consider instead building a wood deck. Wood decks are built above the soil line and the only disturbance to the root system will be digging the foundation post holes. This option decreases root disturbance and soil compaction and will more than likely keep the tree healthy and alive.
Remember to contact your utility companies to find out where it is safe to dig before digging.
Age of Tree. Younger trees will survive changes better than older, more mature trees. If the tree is overly mature and in poor condition, it might be best to remove it and replace it with a new tree.
Sensitivity to Root Disturbance. Trees vary in sensitivity when their roots are cut or the soil around their roots disturbed.
Sensitivity to Soil Compaction. Trees also vary in sensitivity to the soil around their roots being compacted. For example, using heavy equipment or raising the soil grade often causes soil compaction. Basically, soil compaction decreases the amount of oxygen in the soil that trees need to survive.
Health of Tree. If trees are already stressed and unhealthy in appearance the likelihood of surviving a major change in their current environment (loss of roots or grade change) can be minimal. On the other hand, a healthy, vigorous tree should live even after losing half of its root system. However, it is often difficult for a person to determine whether a tree is healthy. Following are some guidelines to help you decide:
Young trees are generally healthier than older trees.
Diseased trees increase the chance of loss. A good way to determine if a tree is diseased is to check whether:the tree has excessive dieback of the canopy or branches, the tree turns to fall colors early, the tree loses leaves early in the season.
Ratio of canopy (leafed out part of tree, deciduous trees only) vs. trunk. A healthy tree has 60% canopy to 40% trunk (see figure 2a). An unhealthy tree has 40% or less canopy to 60% or more trunk (see figure 2b).Severity of Change Necessary. If the amount of grade change or root loss is too severe, remove the tree and replace it after changes have been made.Some trees are more sensitive to root disturbance than others. Try to avoid disturbing more roots than necessary no matter how tolerant the tree is of root disturbance. The less the soil is disturbed around and underneath a tree, the healthier and more vigorous the tree will be. See the table in the publication Protecting Trees from Construction Damage: A Homeowner's Guide, published by the University of Minnesota Extension. Check under root disturbance for your specific type of tree. Keep in mind that if your tree is tolerant of root disturbance, but is unhealthy, it might be best to consider the tree as being intermediate or sensitive before cutting away roots. On the other hand, if your tree is young and healthy yet considered sensitive, it might be possible to make the maximum cut.
These usually involve removing the entire root system, 12 inches or deeper.
Trenches should only be dug on one side of the tree. To find the minimum distance from the tree trunk that the trench can be dug, determine diameter of tree at breast height (dbh) by measuring the diameter or width of the tree at 4.5 feet off the ground. Multiply the dbh by 0.5 feet to get minimum distance from tree to start trenching.
dbh x 0.5 feet = minimum distance from tree to start trenching (in feet)Keep in mind this is the minimum distance, however, it is always best for the health of the tree to maximize this distance. So if possible go a few feet beyond this amount or, preferably, go outside the dripline.
Augering When augering for fence posts, deck footings, etc., multiply dbh (see above) by 1 foot
dbh x 1 foot = minimum distance from tree to auger (in feet).Again this is a minimum distance. If possible, extend this outward a few feet or outside the dripline.
If it is necessary to fill on only one side of the tree, the best method is to use the equation for Trenching mentioned above. However, if the situation requires filling closer to the tree than recommended, do not fill closer than 3 feet from the trunk of the tree.
Aerate within the protected root zone before and after grade changes. Starting 3 feet from the base of the tree, drill 2-inch diameter holes into the soil about 1 to 1-1/2 feet deep. Drill the holes in a circle 3 feet away from the trunk, spacing them two feet apart, then go out 5 feet and drill again in a circle. Continue every 2 feet within the protected root zone (see figure 5). Use the porous soil referred to previously to fill the holes.
References:
Miller, N. L., D. M. Rathke, and G. R. Johnson, Protecting Trees from Construction Damage: A Homeowner's Guide, (FO-6135-GO) University of Minnesota Extension Service, St. Paul, MN. FO-7848-C, 1998.
Lichter, J. M. and P. A. Lindsey, "Soil Compaction and Site Construction: Assessment and Case Studies," The Landscape Below Ground: Proceedings of an International Workshop on Tree Root Development in Urban Soils, pp. 126-129, 1993.
Miller, F. D., Jr., "The Effect of Trenching on Growth and Overall Plant Health of Selected Species of Shade Trees," The Landscape Below Ground: Proceedings of an International Workshop on Tree Root Development in Urban Soils, pp. 157-164, 1993.
The Morton Arboretum, Lisle, Illinois, Tree Care Handbook, Nature Study Publishers, Rochester, NY 14616, 1994.
Harris, R. W., J. R. Clark and N. P. Matheny, Arboriculture: Integrated Management of Landscape Trees, Shrubs and Vines, Prentice-Hall, Inc., Upper Saddle River, NJ 07458, 1999.
Prosser, T., "Construction Damage to Trees,"Minnesota Horticulturist, Vol. 118, No. 5: pages 12-13, May 1990.
Personal Interview with Gary Johnson, Extension Educator and Associate Professor Urban and Community Forestry. Urban Forestry Building. May 18, 1999.
About Simmy Parker
Simmy is an outdoor expert who loves to spend time in the wilderness. She received a BS degree in Civil Engineering at Sacramento State University, and has put her skills to use by helping design and build some of the most impressive structures in the world. However, Simmy's true passion lies in sharing her love of nature with others, and she spends much of her free time leading hikes and teaching people about the flora and fauna that can be found all around them.
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