Arboriculture & Urban Forestry 34(2): March 2008 85 Figure 1. A linear root barrier. HISTORY OF ROOT BARRIERS Roots cause both direct and indirect damage to infrastructure. Direct damage is related to the forces exerted by expanding roots and typically affects only light infrastructure such as roads, curbs, and sidewalks. Indirect damage is associated with soil movement resulting from changes in moisture content precipi- tated by roots. In cases of indirect damage, concrete cutoff walls prevent root growth in undesired areas. A typical application of cutoff walls is to exclude roots from soils near the foundations of buildings. This is necessary because root moisture absorption causes soil shrinking and differential settling, which causes sig- nificant structural damage to foundations and walls. Cutoff walls are especially prevalent in areas characterized by plastic clays such as London, U.K. They are installed by digging deep trenches between trees and the desired protection area. These trenches, often many meters deep, are filled with concrete slurry, thus creating a subterranean wall. In extreme cases, cutoff walls are secured to the bedrock. In cases of direct damage, tree roots affect roads, curbs, and sidewalks. To prevent root growth near these elements of infra- structure on a city scale, concrete cutoff walls are impractical and so other means are necessary. To address this large-scale need for root management, commercial root barriers were de- veloped during the 1970s (Randrup et al. 2001). Since then, their use has risen substantially. A survey of 137 cities in California has shown that confidence in root barriers rose markedly during the 1990s; the percentage of respondents who believed linear and circular barriers were effective rose to 52% and 40%, respec- tively. By contrast, the belief that inhibitor-style barriers are effective has declined to 33% (Thompson 2006). Formal root barrier studies have been conducted for approxi- mately 40 years, dating back to Wilson (1967) who studied root behavior after contact with simulated barriers. He noted that there appeared to be little tendency for roots to become trained to the direction of the barrier. This was the first evidence of the difficulties inherent in attempting to control root growth. ROOT BARRIER DESIGN Root barrier designs have changed over time and although dif- ferences in design are often subtle, they can be the difference between an effective and ineffective barrier. Many barriers cause roots to circle, thus precipitating the introduction of vertical ribs to deflect root growth downward. Whole barriers were heaved Figure 2. A circular root barrier. upward by soil movement over time, so new designs have in- corporated wings or tabs meant to lock the barriers in place. The connectors used to join segments of root barriers are a challeng- ing design consideration; if they fail, roots will penetrate the gap in the barrier. Some designs incorporate a separate connector, some use a chemical bonding agent, and others still have inte- grated couplings. The top edge of barriers, which is aboveground level, has been observed to deteriorate rapidly as a result of pedestrian and vehicular damage. Some newer barriers have in- corporated a wide, rugged upper edge to combat this issue. These and other practical differences were reviewed and contrasted by Barker and Peper (1995) who identified vertical ribs, durability, ease of installation, connector type, and material type as crucial characteristics of effective barrier design. In addition to the commercial options, numerous “homemade” versions have been tested. These include polyethylene tubing, Figure 3. A three-dimensional root barrier. ©2008 International Society of Arboriculture
March 2008
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