Root barriers have been around for many years. In theory a properly placed root barrier will hold back or redirect root growth. Often root barriers are used to keep roots from growing under sidewalks or other infrastructure.
There are three basic types of root barriers:1
1. root traps or screens — these consist of screens, welded fiber sheets and woven fabrics . The holes should be large enough to allow the root tip to grow through but strong enough to strangle and girdle the root as it grows and tries to enlarge. Hole size is general around 1/16 to 1/26 of an inch square with the smaller openings tending to be better.
2. root deflectors — these are barriers made of solid plastic or metal. Barrier thicknesses have been suggested to be more effective if they are at least 0.15 mm (15 mils) thick. Thicker barriers are easier to install without damage and can handle more intensive site impacts over time.
3. chemical inhibitors — this type of barrier is a textile that is impregnated or covered with a chemical root growth inhibitor. One commonly used chemical is cupric carbonate (CuCO3). Another widely used chemical root inhibitor is trifluralin, an herbicide that inhibits root tip expansion. Both are effective but gradually loose their concentration and effectiveness over time. The speed of degradation is dependent upon soil temperature and soil moisture conditions. In Florida these chemicals will break down more quickly than in other parts of the country.
In the last 15 years a number of research projects have tested the efficacy of root barriers. And many of these research projects studied the effects of root barriers in preventing tree root damage to sidewalks. A Dutch study of 16 different root barrier treatments found that roots escaped the barriers in every case.2 In all cases the roots eventually grew back to the soil surface. The reason root growth is near the soil surface is roots require oxygen and the oxygen level near the soil surface is higher. The researcher concluded that in order to be effective, the root barrier should both extend below the likely rooting depth and protrude above the ground level and be UV resistant.
Dr. Tom Smiley has done a recent study of root barriers. His article in the publication Tree Care Industry in May, 2008 said that roots grew under all the the root barriers tested but he noted that the roots that came back to the surface on the other side of the barrier roots tended to be “smaller in diameter at least near the barrier.”3
Dr. Ed Gilman has studied root barriers in the late 1990ʼs and has continued to review the research literature. He concludes, “Experiences and research cast doubt on the usefulness of root barriers.”4 He also observed that digging the trench for the installation of the root barrier often makes the soil more amenable to root growth making it easier not more difficult for roots to grow downward and under the root barrier. Photos of his research are in Figures 1 and 2.
Even though roots grow under and sometimes over root barriers, often the slight diversion of the roots beneath a relatively narrow sidewalk is enough to allow protection to the sidewalk from root damage. In the case of a larger surface such as a driveway, temporary downward diversion of roots would not necessarily solve the potential for root damage to the driveway surface.
Conclusion and Recommendations
Root barriers tend to reduce the number of offending roots. And the deeper the root barrier, the greater the root control. They are more likely to work best for sidewalks rather than driveways. But much depends on the installation
Although root barriers of all kinds tend to fail sooner or later and are dependent upon good installation, there are some positive findings in the research that may help prevent or slow down root damage to infrastructure. Based upon the research results, I can make some recommendations:
A Deeper Root Barrier Is More Effective — A deeper (longer) root barrier will provide more protection for longer period of time. I recommend you select a root barrier that is at least 30 inches deep. A 36-inch or 48-inch deep root barrier would be even better. I also prefer the stiffer root barriers that can be snapped or fastened tightly together. One such root barrier is DeepRoot (http://www.deeproot.com/products/root-barrier/sizes-types) And other root barrier suppliers can be found on Google or Bing.
The Root Barrier Should Protrude Above Grade — Be sure the root barrier protrudes upward at least two inches above grade so roots will not grow over the top of the root barrier. Care must be taken during installation or during landscape maintenance that the top edge of the barrier is not broken back to the soil level allowing roots to grow over the top. Excessive organic mulch over the root barrier may also allow for roots to grow over the top of the root barrier.
Compact Backfill — Once the trench has been dug and the root barrier has been installed, consider compacting the soil that is used to backfill the trench. Also consider placing a small footer at the base of the root barrier that will protrude back toward the tree.
Use Clean Gravel To Separate the Roots From The Root Barricade — Research in reducing root damage to sidewalks has found that placing a layer of clean, washed gravel beneath a sidewalk will prevent roots from growing too close to the bottom of the sidewalk. The stones will also spread as roots thicken allowing for more root expansion without damage. Using this same concept, I suggest using clean washed gravel to backfill the vertical trench after installing the root barricade. Because of the air spaces in the clean gravel, roots cannot grow into the gravel that is placed vertically in front of the root barrier.
Consider Root Pruning — First of all root barriers are more appropriate for new tree plantings where an existing root structure will not have to be cut. But to install a root barrier on existing trees, some roots will have to be pruned if a root barrier is to be installed. Cutting roots too close to the tree trunk can make the tree unstable creating much graver problems such as tree failure than infrastructure problems that you are trying to solve. If the tree is too close to the sidewalk or driveway, then it may be better to remove the tree and plant another in a better location. If the tree is far enough from the infrastructure so the pruning of roots will not make the tree unstable, then root pruning may be an option if not too many roots are being cut and if roots are only cut on one side of the tree. Even if tree stability is not compromised by root pruning, cutting the roots may still damage tree health causing sparse foliage, upper branch dieback and eventual decline. The distance that is considered too close to cut roots is generally a distance from the tree trunk equal to three times the diameter of the tree. For example a 30 inch diameter tree should not have its root pruned closer to the trunk than 90 inches (3 x’s 30=90). A competent ISA Certified Arborist should always be part of the decision-making process when considering root pruning.
Mulch — The area beneath the dripline of each tree should be mulched with organic mulch (wood chips) to enhance the root environment. The mulch can be placed directly over the grass and natural soil. Do NOT use a weed mat. Slowly this mulch decays and becomes part of the soil profile much like the soil on the forest floor. This enhanced soil will promote new root growth.
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1. Dr. Kim Coder, Tree Root Growth Control Series: Root Control Barriers, University of Georgia, March, 1998, 5 pages.
2. John Roberts, Nick Jackson and Mark Smith, Tree Roots in the Built Environment, Centre for Ecology & Hydrology, Natural Environment Research Council, London, 2006, p. 386.
3. Dr. Tom Smiley, Growing Trees Near Sidewalks, Tree Care Industry, May, 2008, pp. 8 – 14.
4. Dr. Ed Gilman, Trees for Urban and Suburban Landscape, Delmar Publisher, New York,
1996, p. 32