Protecting Roots of Mature Trees

Trees are important fixtures in the urban and rural landscape. We value above-ground parts of trees for their spring flowers, cooling shade in summer, and vibrant leaf colors. But healthy root systems below ground are vital for tree vigor and longevity. Roots are responsible for water and mineral nutrient uptake, energy storage, and anchorage. If for any reason tree roots are damaged, tree health will be jeopardized.

Because roots work quietly out of sight underground, most people have a poor understanding of this important subterranean network. In general, roots grow where the resources of life (water, oxygen, and mineral nutrients) are available. They usually will not grow where there is no oxygen or where the soil is compacted and hard to penetrate. This need for oxygen explains why a majority of tree roots are located in the top 12 to 18 inches of soil. Root systems are also extensive. They often extend outward from the tree trunk to occupy an irregularly shaped area four to seven times larger than the crown (branch) spread. It is easy to see why any type of soil disturbance near trees can, and usually does cause damage. As trees mature in the landscape they attain a rather delicate balance with their surrounding environment. In fact, trees grow best in an environment of minimal change. Unfortunately, our urban, suburban, and even rural landscapes are places where drastic changes like driveway and sidewalk installation, grade changes, road widening, and utility trenching occur frequently. Such construction activities near trees can cause substantial root injury which may be fatal to established trees.

Of all the soil disturbances previously mentioned, grade changes and their impact on tree roots may be the least understood. Since roots are near the surface and depend on oxygen from the atmosphere, raising or lowering the soil level around an established tree can have serious impact. Scraping the soil away from a tree removes or injures important absorbing and transport roots, eliminates nutrient-rich topsoil, and exposes other roots to desiccating (drying) conditions. And if heavy equipment is used during the grading process, additional tree injury occurs because of soil compaction. Instead of lowering the grade, valuable trees might be protected by raising the grade elsewhere. If soil removal becomes absolutely necessary, grade changes should be limited to areas outside the branch spread of trees.

Soil fills which raise the grade around trees are equally harmful. Soil additions reduce the oxygen supply to roots, compact the soil, and often raise the water table. Soil additions six inches or less will probably not harm "fill-tolerant" trees (Table 1) especially if the fill material is good topsoil, high in organic matter and loamy in texture. But, irreparable damage will result if as little as two inches of clay soils are used as fill, particularly around "fill-intolerant" trees (Table 2). If fills deeper than six inches will occur, it is still best to limit those grade changes to areas outside the branch spread of the tree. In cases where significant soil additions will occur close to the trunk, elaborate aeration systems can be used to protect trees, however, these methods are largely unproven. Many times the most practical solution is to remove trees that will experience significant root injury from grade changes.

Table 1. Fill-tolerant trees

Colorado spruce catalpa silver maple
green ash Eastern cottonwood swamp white oak
river birch red maple black willow

Table 2. Fill-intolerant trees

white fir ironwood red oak
white pine linden white oak
scotch pine sugar maple serviceberry

This article originally appeared in the July 14, 1995 issue, pp. 1995 issue, pp. 106-107.


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