Tree Root Behavior and their Interactions with Individual Sewage Treatment System (ISTS)

To ensure a properly functioning soil based septic system whether an in-ground system or mound, a suitable vegetative covers must be established. The purpose of this vegetation ranges from removing moisture and nutrients from the soil, preventing soil erosion, to acting as an insulating layer.

The University of Minnesota's Onsite Sewage Treatment Program (OSTP) recommends herbaceous plants such as turf grasses, wildflowers, and native grasses as suitable cover for septic systems in Minnesota. Turf grasses have fibrous root systems that hold soil in place, require maintenance similar to a lawn, and are available in numerous varieties including shade-tolerant to suit site conditions. Wildflowers and native grasses are an attractive alternative to turf grass, while providing many of the same benefits including fibrous roots, low maintenance (once established), and tolerant of dry soil conditions.

While it is true that no dedicated scientific research has contributed to our understanding of rooting and septic systems in Minnesota and beyond, this should not dismiss existing tree rooting research findings, informal surveys and anecdotal evidence via field observations recorded while troubleshooting septic systems.

Just the Facts

The following are some of the facts regarding septic system functioning and tree rooting behaviors.

  • Septic systems have oxygen: The fact is that compliant septic systems in Minnesota effectively treat septic tank effluent by utilizing oxygen-loving soil bacteria among other beneficiary aspects of oxygenated soil.
  • Septic systems are shallow: Significant components of a septic system, distribution pipes and distribution media (rock, chambers, etc.), are installed within 6-12" from final grade.
  • Septic systems require erosion protection: Septic systems require adequate erosion protection to ensure that the 6-12" of soil cover remains. Newly planted trees and deciduous trees provide minimal protection from raindrop impact and surface runoff, especially during early spring and late fall. If trees are selected as the vegetative cover, mulch, cover crops, or other soil conservation strategies must be established and maintained during tree dormancy over the entire life of the septic system (20+ years).
  • Dense vegetative cover insulates: Vegetative cover is critical to insulating the system over the winter. Well-established vegetation helps hold snow close to the soil surface where it insulates the septic tank, piping, and soil treatment area. Snow helps keep the heat of the sewage and soil from escaping, keeping the frost depths shallow. In the absence of snow cover, a dense vegetative cover acts as an insulating layer helping prevent the septic system components from freezing.
  • Tree rooting depth varies: Depending on site specific conditions and tree species, tree rooting depth can vary from 12 inches to depths of 48 inches. (See additional discussion under Tree Rooting Behaviors.)

Additional Concerns

Additional justification exists as to why establishment of trees and woody plants over a septic system are a significant concern. Careful consideration and understanding of existing research has aided in this understanding.

  • Roots (i.e. structural roots) penetrate through the soil treatment area below the sewage effluent and create macro pores for effluent to travel without proper treatment.
  • Tree throw or the blow down of trees in a storm, uprooting the tree and disrupting the soil which is protecting the system. Structural damage to the septic system infrastructure may also result from uprooting. Creating a clearing for the septic system's soil treatment area in a forested area may increase wind velocities in this clearing and enhance the likelihood of tree throw. Property owners and septic system designers should carefully consider prevailing winds directions, preponderance of the prevailing winds, shelterbelt establishment, etc. when siting a septic system.
  • Disease or natural mortality of a tree that is planted on or around the septic system can cause erosion, structural disturbance to the ISTS, creation of macropores where partly treated septic tank effluent may travel to our water supplies, and may cause additional concerns.

Tree Rooting Behaviors

Although no direct research has been done looking at how tree and other plant roots affect septic systems, we can draw on scientific literature to tell us a bit about tree root growth behavior. Understanding these additional facts is critical when considering the types of vegetative cover established on and around a septic system.

Tree rooting characteristics: Tree rooting characteristics have been documented in several studies. Tree root growth is highly dependent upon the soil environment and species.

  • Tree Root Depth: The University of Florida Extension (Gilman, 2003) has found that the majority of fine roots exist within the top 12 inches of the soil. However, this was not found to be true when soil textures are considered. In Colorado (Sillick and Jacobi, 2006), greater than 90% of tree roots, by mass, were found within the top 12" and top 36" in clay soils and sandy soils, respectively. In a Utah study of aspen trees, their root mass was found within the upper 48" in a sandy loam soil (Gifford, 1966).
  • Lateral Root Spread: A University of Colorado Extension publication (Sillick and Jacobi, 2006) compared lateral root spreading in different textured soils and found that trees studied in sandy soils had lateral spread 2-3 times the drip line, while clay soils had lateral rooting up to five times the drip line. This is a more detailed analysis than a study in Florida that documents lateral rooting of trees beyond the drip line (Gilman, 2003).
  • Taproot: The same University of Florida (Gilman, 2003) study revealed that many times a taproot does not exist. Taproot development depends on the trees species, tree age, soil, and if the tree has grown naturally or been transplanted from a nursery. Nursery trees commonly have their taproot cut to initiate lateral rooting growth. Once planted, nursery trees can form many taproots or none. This depends on the soil texture and soil compaction level.
  • Sinker Root: Sinker root development is a rooting structure that develops from nursery trees. These roots have been documented to vertically penetrate into the soil until water is found, where a root mass will develop. Aspen trees in Utah were found to possess these sinker roots at depths exceeding 114" into the soil (Gifford, 1966).
  • Species: Yet another factor that influencing tree root growth is genera. A paper in the Journal of Arboriculture (2003) looked at 4 different types of trees and their root growth activity. It was found that lilac and pear tress had longer root growth then serviceberry and crab apples. There was a direct correlation between the tree diameter at breast height of the tree and the width and depth of the spread of its roots.

Research is needed to know exactly how tree roots function when planted or exist around septic systems. Until that time we can utilize the current knowledge about tree roots and make well-informed assumptions on how these roots may affect a septic system.

Recommendation

Shrubs and trees on and around septic systems in Minnesota present numerous limitations to the treatment and proper functioning of system components, including failing to provide adequate year-around erosion control, requiring additional homeowner maintenance (mulching, disease prevention, etc.), and interfering with septic system infrastructure with varying rooting depths and rooting structures that depend on site, soil, origin of the tree/shrub and tree/shrub species. The OSTP does not recommend planting trees or shrubs on or around septic systems due to these stated reasons. For additional recommendations and proper maintenance of septic systems, please refer to the Septic System Owner's Guide (Olson et al., 2006).

References Cited

Gerhold, H.D., and A.D. Johnson. 2003. Root dimensions of landscape tree cultivars. Journal of Arboriculture 29(6): 322-325.

Gifford, Gerald F. 1966. Aspen root studies on three sites in northern Utah. American Midland Naturalist 75(1): 132-141.

Gilman, Edward F. 2003. Where are tree roots? University of Florida. IFAS Extension. ENH137.

Olson, K., D. Gustafson, S. Christopherson, and B. Liukkonen. 2006. Septic system owner's guide. (verified 1/30/2007). University of Minnesota Extension Publication PC-06583-GO.

Silllick, J.M., and W.R. Jacobi. 2006. Healthy roots and healthy trees. Colorado State University Cooperative Extension. No. 2.926.