University of Maryland Extension

Grade Changes

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

Key Points

  • Grade changes are often necessary during new building construction.
  • Any addition or subtraction from the natural soil grade will adversely affect trees and shrubs. 
  • Sometimes very young plants adjust to minor soil grade changes. However, older, well-established trees and shrubs are very sensitive to grade changes around their root systems.
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

Problems Caused by Grade Changes

  • Soil placed over roots hampers the exchange of oxygen and carbon dioxide between roots and the soil surface causing dieback and decline or even death of trees and shrubs. 

  • Excess soil placed against the trunk keeps the area moist increasing decay of the lower trunk bark.

  • Additional soil may also negatively affect the ability of water to reach the root system.

  • Clayey soils cause the most injury because clay particles are small and closer together slowing down the movement of air and water. Decreased oxygen levels are a common problem with clayey soil additions of more than an inch. The deeper the fill the more of a chance of damage. 

  • Up to several inches of gravelly or sandy soils may cause fewer problems since water and oxygen filters through them more readily.

  • The addition of fill over existing soil will lead to a problem of layers of soil of differing textures (lithologic discontinuity). This condition will cause drainage, temperature, and air exchange differences that may cause root problems in the existing soils. 
  • Lowering the grade may be as serious as raising the grade around trees as this leads to root loss due to the removal of the small feeder roots that are present in the upper layers of soil. Most tree roots occur within the top three feet of the surface and most of the feeder roots are within the top six inches. 

  • 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.

  • When distress symptoms become severe, it is usually too late for any recovery, even after the excess soil has been removed. If the distress symptoms are not too severe, recovery may occur if the excess soil over the root system is carefully removed.

  • Arborists can perform a procedure called 'root collar excavation' using an air spade to remove excess soil piled on roots and against tree trunks. 

Prevention

  • Plan to construct tree wells and install perforated pipe systems to provide the roots with needed oxygen if a soil grade must be changed. A consultation with an arborist about protecting specimen trees before construction can be very helpful.

  • Three inches of soil (less if the soil is high in clay) is the maximum depth that can safely be added without making special provisions.

Rev. 2019

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