Native Plants

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Tuesday - September 04, 2012

From: Austin, TX
Region: Southwest
Topic: Problem Plants, Erosion Control
Title: Exposed Tree Roots in Austin
Answered by: Brigid & Larry Larson

QUESTION:

I have a large ash tree with a lot of mud at the top of a sloping yard. I want to build a small retaining wall with the ground leveled above. This would entail covering exposed tree roots with 4-18 inches of dirt. Will this hurt the tree?

ANSWER:

Mr Smarty Plants is a bit mixed on your question. Common knowledge has it that it is harmful to a tree to cover its roots, yet if the roots are exposed anyway it is tempting to cover them a bit.

This article from the Colorado Extension describes the root mass, both that it extends beyond the tree crown and that the majority of the action is within 6 to 24” of the surface.

The issue with covering the roots involves compaction of the soil. The article above states ”Changes in soil depth around trees can also cause injury to root systems. The addition of only 4 to 6 inches of soil over a root zone drastically reduces the amount of oxygen and water available to the roots”

A great solution to your desire is to build a “Tree Well”, where the roots are covered with very coarse rock or mulch to preserve their air and water access. This publication from the West Virginia Extension states 18” as the depth where plain dirt fill would be too deep. It gives excellent recommendations on building a tree well.

From the Image Gallery

Green ash
Fraxinus pennsylvanica

Texas ash
Fraxinus albicans

Arizona ash
Fraxinus velutina