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The truth about adding soil over the roots of existing landscape trees.
By Rebecca Koetter and Gary R. Johnson
No, the title "Will Fill Kill?" has nothing to do with people's propensity to stuff their digestive systems during Thanksgiving! Instead, it directs attention to the three most common questions from homeowners about relandscaping and construction activities around their trees:
• What is fill soil?
• What are the potentially harmful effects on tree roots and tree health? And,
• Are there options available to avoid harming trees?
Fill is a term commonly used by building contractors, landscape architects and designers that refers to the addition of soil or other materials (e.g., sand, gravel, debris) to raise the level of a landscape. Fill is strictly a physical change and is not normally a method to improve soil nutrition, aeration or moisture movement. Fill is used to level irregular landscapes. It's used to fill up larger depressions that may be the result of settling soils or previous excavations. Or, fill may be used to create a foundation for sidewalks, patios or driveways.
Fill (sometimes referred to as misplaced soil) can be any sort or mixture of mineral materials (from large rocks to sand), soil (from clay to silt) or debris (bricks, concrete, or other artifacts). Often, it's a random concoction of these elements: mixes of soils, clay and gravel, chunks of concrete mixed with clay and sand. Organic matter that does add nutrition to inert soils is generally not part of fill soils. The point is, fill soil is an unreliable part of the landscape and in areas where it's part of new construction it quite often alters water movement and contributes to unacceptable compaction levels (Day, 1999).
When a significant amount of fill is added to a landscape, soil conditions will change as will the root growth potential of existing trees. It has been estimated that 80-90% of all tree problems are related to soil and its effects on root growth potential and health of the trees' entire root systems (Smiley et al., 1998).
A primer on tree roots.
Overall, tree root systems are often misunderstood. Many believe that tree roots extend tens or scores of feet deep into the ground, and those trees have one moisture-seeking tap root that reaches the water table. However, most trees have a short tap root stage of life and most roots are within the top three feet of the soil. In addition, fine roots (those that absorb most of the water and nutrients from the soil) are found within the top 12 inches of the soil (VanDerZanden et al., 2001). Research has revealed that as little as four to six inches of fill placed over the roots of some tree species has caused serious life-damaging conditions (University of Rhode Island, 2005), yet seven FEET of fill placed over the roots of one tree caused no apparent damage (Costello et al., 2004). How can this be?
To begin with, fill alters air, nutrient and moisture situations. However, research has shown that these situations may or may not be solely responsible for tree damage. Table 1 shows the variability of effects on tree root health based on fill texture and depth. Other factors that are at least as important as the quality and depth of fill include the tolerance of the existing trees: tree species, tree age and overall tree health.
In general, these factors moderate or aggravate the effects of fill:
• Species with a vigorous growth habit (e.g., red maple, green ash) and those that survive in wetter environments may grow rapidly enough to recover from the fill event (Table 2).
• Younger trees recover from damage quicker than older trees of the same species. And,
• Trees in good health respond better to injury than trees in poor health of the same species (Costello et al., 2004)
Table 1: Soil Fill by Texture Class than can be added with varying degrees of effects on root health.
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Initiation of root damage by soil fill (inches)
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Massive root damage by soil fill (inches)
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Coder, Kim D.1996. Construction Damage Assessments: Trees and Sites. University of Georgia.
FOR96-39. |
Table 2: A Partial Listing of Trees that are more likely to survive the addition of fill over their root systems.
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Northern White Cedar (Thuja occidentalis )
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Green Ash (Fraxinus pennsylvanicum )
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River Birch (Betula nigra)
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Eastern Cottonwood (Populus deltoides )
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Red and Silver Maple, Boxelder (Acer spp.)
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Bicolor Oak (Quercus macrocarpa )
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Black and White Willow (Salix spp.) |
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More than the Fill, more than the Trees.
In addition to the obvious (namely, the fill and the trees) there are still more factors to consider. The soil composition (i.e., the texture as well as the structure) of the original landscape (also called the base soil) is very influential on a tree's tolerance. Another important factor is the volume of the tree's root area that will be directly impacted by the fill (Tusler et al., 1998). Base soils that are more prone to compaction can emphasize the detrimental effects of fill because the existing tree roots are growing in the base soil. Finer textured base soils such as silts and clay soil are more vulnerable to compaction by the equipment that brought in and spread the fill soil (e.g., trucks and tractors). The mere weight of six inches of fill is often enough to significantly compact a vulnerable base soil.
The texture and structure of the fill soil compared to the texture and structure of the base soil can determine the amount of water that will reach existing tree roots. A coarse textured fill over a compacted, fine textured base soil (or vice-versa) can result in a situation that makes it very difficult for both oxygen and moisture to uniformly penetrate the soils where the tree roots currently exist and where they need to grow.
Is it a tree worth saving, a battle worth fighting?
With this information on the effects of varying depths and textures of fill soil, varying textures and structures of base soils, and a tree's relative tolerance to changes, the next step should be a determination of whether the tree or trees in question are worthy of preservation and protection tactics. Diseased or dying trees should be removed simply because the chances of a tree in that condition surviving are slight. Removing those trees would open up space for new trees after the filling operation is over. However, if the tree is relatively healthy and is a key element in the appearance and function of the landscape, and if money is not an issue, then all necessary steps to save the tree may be taken.
As a general rule, it is cheaper and more effective in the long run to prevent damage from initially taking place. Research has shown that little success can be expected by removing fill that has been present around the tree for more than two years (Cue et al., 2002). The initial costs may be high, but time, energy and money will be saved if proactive actions are taken.
Preventing damage from fill.
The decisions have been made: fill will be added, but fill must not kill! Nothing is absolute, but the following tactics have been used to successfully add fill. In addition, using all these tactics is most effective.
1) Minimize the amount of roots covered. Tree "wells" can very effectively protect existing tree roots from being smothered by fill...IF they are constructed in the right place. At a minimum, locate the wells as near as possible to the dripline (the edge of the branch spread) of the tree. As research on tree wells has shown, the larger the well, the better (Costello et al., 2004). A tree well constructed directly around a tree trunk or within a few feet isn't much more than a very attractive brick coffin. Often, tree wells will need drainage holes or pipes if the fill is very deep and/or if standing water problems are to be avoided.
2) Choose fill wisely. Tree health will be determined by the amount of fill used and the ability of the tree roots to grow into the newly added fill. Use the least amount of fill that is absolutely necessary. Choose fill that is less damaging to roots (Table 1). Select fill that has a similar texture to the base soil. Better conditions for tree health are achieved when only a small portion of the roots are affected and the fill soil is similar in texture to the base soil (Costello et al., 2004).
3) Take care of the base soil. Avoid driving heavy equipment over the base soil, especially during the times of the year when the base soil is thawed and moist. This is the period when soils are most vulnerable to compaction, and finer textured soils such as clays can be irreversibly compacted. If you must use heavy equipment, do the grading when the base soil is frozen or very dry. If you can't do the filling during those times of the year, apply the fill with the long arms of a backhoe and then grade it.
4) Aeration systems and layers of coarse gravel under the fill may not help at all. There is little research-based evidence that aeration systems under fill soils are consistently effective, and they are very expensive. Likewise, coarsely ground rock layers that separate base soils from fine textured fill soils may restrict water movement by creating a layered stratum that does not allow normal water percolation. The above tactics would be expensive with good intentions; however the benefits would be marginal and the chances that more damage could result are high.
In Summary
Many existing trees are unwittingly lost due to filling operations associated with new landscapes and construction activities. This doesn't always need to happen if a few simple steps are followed:
1. If the tree is priceless, irreplaceable...don't fill.
2. If the tree is worth saving and has a chance of making it (i.e., good genes, younger, healthier), then continue with the following steps. KEEP that tree healthy during and after the fill operation.
3. Choose a fill that will be less restrictive to water and oxygen from Table 1 (e.g., a sandy loam is better than clay).
4. Choose a fill that has a similar texture to the base soil.
5. Apply the least amount of fill necessary.
6. Affect the smallest volume of the root system as necessary. If possible, construct a well that will keep fill away from the roots of the tree.
7. Do not compact the base soil during the fill operation. Avoid heavy equipment use, especially during times of the year when the base soil is wet.
8. Finally, because so many considerations and assessments must be done in order to save and keep trees healthy, it may be worth your time and money to have a professional who is a specialist in tree preservation assess and monitor the entire fill project.
References Cited:
Coder, Kim D. 1996. Construction Damage Assessments: Trees and Sites. University of Georgia.
Costello, L.R. and Susan D. Day. 2004. A New Look at the Impact and Management of Fill Soil around Trees, pp. 25-29. Arborist News.
Cue, K.P. and S. Josiah. 2002. Landscaping around established trees. University of Nebraska-Lincoln Extension. G1452
Day, Susan. 1999. Growth and Physiology of Several Urban Tree Species in Soils Disturbed by Construction Fill or Compaction. Dissertation submitted to the Faculty of Virginia Polytechnic Institute and State University.
Johnson, Gary R. 1999. Protecting Trees from Construction Damage: A Homeowner's Guide. University of Minnesota Extension Service, FO-6135.
Smiley, E.T., T.R. Martin and Bruce R. Fraedrich. 1998. Tree root failures. Landscape
Below Ground II: Proceedings of an International Workshop on Tree Root Development in Urban Soils. D. Neely and G. Watson, Eds. International Society of Arboriculture, Champaign, IL.
Tusler, P.E., J.D. MacDonald and L.R. Costello. 1998. Fill soil effects on soil aeration. Landscape Below Ground II: Proceedings of an International Workshop on Tree Root Development in Urban Soils. D. Neely and G. Watson, Eds.
University of Rhode Island, (n.d.). Maple Tree Decline. Retrieved June 16, 2005 from Greenshare Factsheets.
VanDerZanden, A.M. and J. McNeilan. 2001. Conserving Water in the Garden: Landscape and Lawn Care. Orgeon State University Extension Service Website: http://eesc.orst.edu/agcomwebfile/EdMat/html/EC/EC1531/EC1531.html.
Rebecca Koetter is a Graduate Research Assistant in the Department of Forest Resources, and Gary R. Johnson is a Professor of Urban and Community Forestry in the Department of Forest Resources, University of Minnesota. Posted December 27th, 2005.
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