Root intrusion can pose a serious threat to the long-term viability of a new septic system. Assess the design plan and property to determine if tree removal is necessary.


It seems every year we receive questions about what to do about trees growing on and around septic system components, most often the drainfield area. One question was whether trees survive if they are in contact with fill used to complete mound construction.

The direct answer is no, they will not survive if fill is brought up around the tree and the bark is in direct contact with soil. It may take a year or two but a fungus will develop in the bark and ultimately the tree will die. In other landscaping situations, trees can be protected from this fate by creating an open area around the base of the tree out of rocks or interlocking landscape blocks. If the bark is open to the air the tree should be able to survive if it or its root system has not been damaged during the installation process.

The larger question is whether the trees should be left in the mound or the drainfield for belowground systems. There are a couple of issues of concern about leaving trees in the area. First, root intrusion may threaten any of the system components. Second, when the tree or trees shade the area, it may be difficult to establish and maintain any other type of vegetation, which leaves the site subject to erosion, particularly for above-ground at-grades and mounds.

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TREE SPECIES PLAYS A ROLE

For mounds and at-grades, our recommendation has always been that the trees should be removed from the area under the footprint of the mound. A potential exception to this would be when the fill extends beyond the typical footprint to cosmetically blend the mound into the landscape.

Type of tree and the site characteristics will help direct the decision whether to keep the tree or not. Trees that are water-seeking/loving and will send roots out in search of water should be removed due to the root intrusion concerns. Where we live, this includes species like white pine and sugar maple. These trees should also be 20 feet or more away from any sewage tanks or other system components to reduce the potential for root intrusion.

We should mention that these are the very kinds of trees that homeowners do not want to lose out of their yards! Before work starts, it is important to explain with them why the trees need to be removed. We have seen more than one irate homeowner rip into an installer because they did not realize their trees would be removed during installation.

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For mounds and at-grade systems, tree removal should consist of cutting them off as close to the ground as possible and leaving the remaining stumps in place, with the soil around them scarified and sand fill placed over the top. This is to protect the infiltrative surface of the soil from smearing or compaction reducing the infiltrative capacity of the soil.

Assess trees in the vicinity of belowground drainfields to determine if they will limit your ability to excavate or place trenches in the right location and if they pose the potential to interfere with piping or parts of the system. In gravity-fed trench systems — where we rely on development of the biomat for distribution of effluent — trees can be closer to the trench. Roots will generally not enter the trench when it is dry since there is no water or nutrients, and they won’t enter when the trench contains sewage because it lacks oxygen.

The decision about keeping trees next to gravity trenches comes down to whether the tree is in the way of installation or so close that the root system will be irreparably damaged during excavation and will ultimately die due to the disturbance.

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LIMIT COMPACTION

Stump removal depends on the location of the system and site characteristics. Three methods to remove stumps are stump grinding, hand digging or using a backhoe. Stump grinding will remove the upper portion of the stump but may not remove the root parts deep enough to excavate. Hand digging is very hard and difficult work. And no, a half a stick of dynamite is not a solution! Using the backhoe is what we have seen done most often.

With any of these methods, the key is to limit potential compaction that will affect the infiltration area of the system. If it cannot be done without damaging the drainfield area, stumps should be left. Also note that the objective is not to remove all tree roots. Depending on the tree size, root systems can be extensive. The stump should be removed to a depth of about 12 inches and the rest of the roots left to decay naturally over time.

Another side note is that in some regions numerous large rocks will be found, such as northern parts of the country that were glaciated. Decisions need to be made on removal to facilitate installation. The same precautions of not smearing and compacting the treatment area apply. Where possible they should be left in place and worked around. If the area is over-excavated due to rock removal, additional fill will be needed. Typically, clean sand is used as the replacement fill, but, as usual, know your local regulations and requirements.

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Jim Anderson, Ph.D., and David Gustafson, P.E., are connected with the University of Minnesota onsite wastewater treatment education program. David is extension onsite sewage treatment educator. Jim is former director of the university's Water Resources Center and is now an emeritus professor. Readers are welcome to submit questions or article suggestions to Jim and David. Write to ander945@umn.edu.


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