University of Maryland Extension

Grade Changes

grade changes

Adding or removing soil over tree roots can seriously damage them and jeopardize the health of the tree. Increasing the soil depth over tree roots changes the oxygen exchange between the roots and the soil surface, causing dieback of tree branches. Additional soil may also affect the permeability of water to the root system. Clay soils cause the most injury because the fine texture blocks the movement of air and water. Decreased oxygen levels are a common problem with clay soil additions of more than an inch. The deeper the fill the more marked is the damage. Up to several inches of gravelly or sandy soils may cause fewer problems since water and oxygen permeates them more readily. However, the addition of fill over existing soil will lead to a problem called lithologic discontinuity, layers of soil of differing textures. This condition will cause drainage, temperature and air exchange differences that may cause root problems in the existing soils. Slow warming of soil the next spring along with low oxygen levels may lead drought damage by mid summer. Diseases such as crown rot may develop if the trunks of certain tree species remain moist due to increased soil moisture retention.

too much soil around trunk
Backfill around tree trunk at construction site.

Lowering the grade may be as disastrous as raising the grade around trees. Most tree roots occur within the top three feet of the surface and most of the feeder roots are within the top six inches. Lowering the grade around trees will lead to root loss due to removal of the small feeder roots that are present in the upper layers of soil. Soil removal will also expose the remaining feeder roots to higher or lower temperatures that may lead to root death. Severed and weakened roots will not be able to perform functions such as anchorage, water transport or nutrient absorption.

Trees very sensitive
to grade changes
Trees less sensitive to grade changesTrees least sensitive
to grade changes
sugar maple, beech, dogwood, oak, tulip tree, pines, sprucebirch, hickory, hemlockelm, poplar, willow, planetree, pin oak, locust
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