84 Arboriculture & Urban Forestry 2008. 34(2):84–88. Morgenroth: Root Barrier Research A Review of Root Barrier Research Justin Morgenroth Abstract. A review of root barrier research from the past 40 years is presented. Research has resulted from the need to minimize conflicts between the expanding roots of trees and urban infrastructure such as roads, curbs, sidewalks, foundations, and under- ground utilities. The history of root barriers, naming conventions, and different classes are described. The results of experiments and surveys are examined, the successes and failures of different barriers are noted, and directions for future research are suggested. Key Words. Curbs; infrastructure conflicts; roads; root barriers; roots; sidewalks; street trees; urban trees. THE NEED FOR ROOT BARRIERS It is widely acknowledged that the roots of street trees in urban areas conflict with hardscape such as roads, sidewalks, and curbs. Damage to hardscape is internationally pervasive, affect- ing regions as diverse as North America (Wagar and Barker 1983; D’Amato et al. 2002a; Rajani 2002), Central America (Benavides Meza 1992; Francis et al. 1996), South America (Achinelli et al. 1997), and Europe (Wong et al. 1988; Kopinga 1994; Reichwein 2002). The mechanisms that result in conflict remain unclear. Some researchers argue that sidewalks induce root growth (D’Amato et al. 2002b). This is plausible given that soils directly beneath sidewalks possess better moisture and tem- perature regimes than surrounding soils (Craul 1992; Wagar and Franklin 1994). In contrast, most studies suggest that trees are the root of the problem. Perhaps this stems from the common observation that cracked or heaved sidewalks are often adjacent to trees and that roots are often located directly beneath cracks (Nicoll and Armstrong 1998). Sydnor et al. (2000) dispute this, reporting the incidence of cracks in sidewalks is not related to the presence of adjacent trees. Irrespective of the cause of the conflict, the resulting cracking and heaving of sidewalks and roads is highly undesirable. In addition to being aesthetically displeasing, heaved sidewalks are a safety liability and impose accessibility restrictions. The con- flicts are so pervasive that millions of dollars from urban forestry budgets are spent on mitigation and remediation annually (McPherson and Peper 1995). A tree-friendly approach to mini- mizing the inherent conflicts between street tree roots and adja- cent hardscape is desperately needed because current solutions too often result in damage to, or even removal of, street trees and thus degradation of the urban forest. One potential solution is the root barrier. This review provides the reader with an understand- ing of root barrier-related research conducted to date. ROOT BARRIER CLASSES Numerous researchers have suggested that root barriers are a potential solution to the conflicts between green and gray infra- structure (e.g., Hamilton 1984; Coder 1998; Randrup et al. 2001), although this is not universally accepted (e.g., Mead 1994). Root barriers are a physical or chemical impediment in- tended to limit root growth to designated areas away from in- frastructure. There are three main classes of root barrier: traps, inhibitors, and deflectors (Coder 1998). ©2008 International Society of Arboriculture Traps do not entirely inhibit root growth. Instead, they allow root tips to penetrate small holes, but subsequently preclude radial growth by girdling. Examples are woven nylon or copper screen, which are permeable and, as such, allow for lateral water movement and gas exchange (Coder 1998). A potential disad- vantage to this type of barrier is that by severely restricting large root development in one or more directions, traps may predis- pose trees to instability. Inhibitors are used to control root growth by means of chemi- cal intervention. Many of these barriers consist of fabric impreg- nated with a slowly released herbicide. Commonly, the herbicide used is trifluralin, which is considered to have no detrimental environmental impact beyond the root control area (Van Voris et al. 1988). As root tips approach the barrier, cell division is im- peded and thus root growth is controlled. Deflectors, often constructed from plastics, are physical im- pediments to root growth. They function by redirecting root growth away from infrastructure. Initial lateral root growth is deflected downward when roots come into contact with the bar- rier. In theory, by forcing roots to grow at depth, the forces that they exert will dissipate through a larger volume of soil before reaching the sidewalks above. This class of barrier is ideally suited for use adjacent to roads or sidewalks. Given their imper- meability, these barriers may impact air and water movement throughout the soil profile. Each of these three barrier types can be configured linearly or circularly. Linear barriers are installed in narrow trenches along the edge of a desired protection zone such as a sidewalk or curb (Figure 1). Alternatively, circular barriers are installed around the rootball of a newly planted tree (Figure 2). This circular configuration is often used in street tree pits or other restrictive spaces (Randrup et al. 2001). Linear configurations are more flexible because they can be used near new or existing trees. Conversely, circular configurations are used only for new plantings. One further distinction this author suggests is between two- dimensional barriers and three-dimensional barriers. The former is a vertically placed barrier extending downward, perpendicular to the soil surface. The latter will fill a three-dimensional volume underneath the surface it is intended to protect (Figure 3). An example of three-dimensional barriers is the coarse gravel sub- base tested by Gilman (2006). Three-dimensional barriers have also been referred to as air gaps (Coder 1998).
March 2008
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