Though trees contribute much to the landscape aesthetically, environmentally and even economically, roots growing into waste-collection pipes cause more sewer damage than almost any other issue, according to the University of Kentucky. Some evergreen trees, or trees that keep their foliage year-round, cause more sewer damage than others. When choosing trees to plant near a sewer system, avoid tall, fast-growing species with deep, vertical taproots, as these tend to cause the most problems. Instead, select small, slow-growing trees with a more horizontal root system.
Evergreen tree roots cause damage when they grow into sewer lines, causing both leaks and blockages. Roots invade sewer pipes because they contain oxygen, moisture and nutrients, three things that trees need to thrive, according to the University of Tennessee. Unfortunately, when sewer lines are blocked or compromised, they tend to leak toxic waste and hazardous gases, causing a potential public health issues. Along with health and environmental costs, damage to sewers by evergreen-tree roots also presents a steep economic cost to both municipalities and homeowners. According to the Michigan State University Extension, a survey of 15 cities indicated that each tree planted by the city causes an average of £1.0 per year in sewer damage.
Damage from evergreen-tree roots can be prevented by creating an unfavourable planting environment around sewer pipes. Root barriers, usually thick sheets of plastic, metal, wood -- or landscape fabric treated with a root-stopping herbicide -- direct root growth away from pipes. These barriers are difficult to install, especially once a tree is established. A more effective and time-consuming method is to plant small, slow-growing evergreens with shallow root systems. If you wish to plant a faster-growing tree, expect to replace it every eight to 10 years.
Certain evergreens are especially prone to causing sewer damage, whether through their deep taproots or fast growth rate. Trees to avoid include evergreen ash (Fraxinus uhdei) and evergreen oaks (Quercus spp.) -- such as the coast live oak (Q. agrifolia), cork (Q. suber), emory (Q. emoryi), holly (Q. ilex), Japanese evergreen (Q. acuta), leather (Q. durata) and live oak (Q. virginiana). Other fast-growing evergreens to avoid include gums (Ecualyptus spp.), loblolly pine (Pinus taeda), northern bayberry (Myrica pensylvanica), tree bamboo (Bambusa vulgaris vittata), slah pine (Pinus elliottii), southern red cedar (Juniperus silicicola), southern waxmyrtle (Myrica cerifera) and strangler fig (Ficus area).
Though no evergreen tree is completely "sewer-safe," a few species tend to cause less damage. These include Carolina cherry laurel (Prunus caroliniana), Croonenburg American holly (Ilex opaca "Croonenburg"), cypress (Cupressus spp.), foster holly (Ilex x attenuata) and Little Gem magnolias (Magnolia grandiflora "Little Gem"). Other slow-growing evergreens include Canadian hemlock (Tsuga canadensis), Carolina hemlock (T. caroliniana), compact Korean pine (Pinus koraiensis "Compact Glauca"), mugo pine (Pinus mugo) and southern Japanese hemlock (T. sieboldii).
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- Michigan State University; Reducing Damage Caused by Tree Roots; Bert Cregg; March 2011
- University of Tennesee Extension; Choosing "Sewer Safer" Trees; Brett Ward, et al.
- University of Kentucky; Sewer Root Control; Blake Newton, et al.
- Virginia Cooperative Extension; Planting on Your Septic Drain Field; Susan Day, et al.
- Urban Forest Ecosystems Institute: All Trees List
- North Carolina State University; Trees; Erv Evans