Landscape Crossroads  

This mailer has been provided as an avenue to disperse information pertinent to public agencies and the landscape architecture profession, in hopes of fostering greater understanding and collaboration. Topics address issues that affect the built environment within which we live.



"The creation of a thousand forests is in one acorn." - Ralph Waldo Emerson"

Do root barriers really deflect roots and prevent damage to adjacent infrastructure? I hear this question quite frequently from private clients and from public entities. The question arises because of the exceedingly high costs due to maintaining and repairing damaged infrastructure caused by tree roots. According to McPherson (2002) damage of this nature exceeds $135 million dollars per year. Though this short summary of my findings will not provide any definitive answers it may arm you with information to make your own decision as to the effectiveness of root barrier products.

Many studies have been performed to determine root barrier effectiveness. Few of them have actually focused on the direct correlation between root barriers and subsequent concrete damage. Despite much speculation on actual causes of concrete cracking and lifting, available reports show positive results that root barriers aid in root diversion and potential prevention of concrete cracking and lifting. The following bullet points highlight findings and suggested practices related to root barriers. This information is derived from studies referenced at the end of this article:

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  1. 50% of 200 California forestry programs using root barriers report them to be effective or partially effective in preventing sidewalk damage (Gilman, 1996).

  2. Most of the same forestry programs report that Species selection is 90% effective (Gilman, 1996).

  3. Roots are directed under vertical plastic root barriers quite effectively but may resurface within 3-4 feet of the barrier depending on soil conditions and plant genetics (Costello, Elmore, Steinmaus, 1997).

  4. In well drained soils, 12" deep plastic barriers may not reduce root quantities within the top 4" of soil on the far side of concrete walks. Compacted or poorly drained soils show some reduction in roots within this zone. Deeper barriers should have a more profound effect (Gilman, 2006).

  5. Gravel sub-base treatments were shown to reduce root quantities within the top 4" of soil on the far side of concrete walks in both well drained and poorly drained soils. This is the only treatment in either soil type that resulted in the absence of roots within the top 4" of soil (Gilman, 2006).

  6. Plastic barriers tend to be effective at mechanically redirecting roots, while chemically impregnated barriers tend to suppress root growth all together (Smiley, 2005).

The key to remember is that there are still many studies being performed to assist us in our decisions effecting the urban forest. We can only make decisions based on the most current information available.

Literature Cited:

Smiley, 2005 -Root Growth Near Vertical Root Barriers.
Gilman, 1996 - Root Barriers Affect Root Distribution.
Gilman, 2006 - Deflecting Roots near Sidewalks.
Costello, Clyde, Steinmaus, 1997 - Tree Root Response to Circling Root Barriers.

Author: Chad Kennedy, Landscape Architect

This informational article provided by O'Dell Engineering - 1165 Scenic Drive, Suite A, Modesto CA 95350