Preventing Grade Change Damage to 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. Grade changes - both raising and lowering the soil grade - in the root zone of a tree can be fatal.

Roots are Important

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.

tree with grade changeEffects of Grade Changes on Trees

Of all the soil disturbances previously mentioned, grade changes and their impact on tree roots can be some of the most problematic. 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.

Lowering the Grade

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 outside of the tree's root zone. If soil removal becomes absolutely necessary, grade changes should be limited to areas outside the branch spread of trees.

Raising the Grade

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 around any tree, 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 expensive and not always successful. 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 sprucecatalpasilver maple
green ashEastern cottonwoodswamp white oak
river birchred mapleblack willow
Table 2. Fill-Intolerant Trees
white firironwoodred oak
white pinelindenwhite oak
scotch pinesugar mapleserviceberry

Symptoms of Grade Changes on Trees

Trees that have experienced a change in soil grade will see symptoms develop over a period of months to many years.  Initial symptoms include reduced growth, yellowing of leaves, crown thinning, delayed bud break in spring, and early fall color and leaf drop in the fall.  Over time twig dieback often occurs with epicormic shoots developing on the trunk and major limbs.  Severe damage includes dieback of large branches and entire sections of the crown eventually leading to tree death.  The severity and progression of the symptoms depends on the amount of grade change, species of tree, tree age, and overall health of the tree before the grade change.

This article adapated from an article that originally appeared in the July 14, 1995 issue of Horticulture and Home Pest News, pp. 106-107.

Last reviewed:
May 2022