18 Gilman: Deflecting Roots Near Sidewalks DEFLECTING ROOTS NEAR SIDEWALKS Edward F. Gilman Abstract. Concrete sidewalks 10 cm (4 in) thick measuring 1.2 m (4.5 ft) wide by 5 m (16.5 ft) long were installed in spring 1996 with and without barriers designed to deflect roots. Forty-eight Platanus occidentalis from #15 containers were planted 0.75 m (30 in) from sidewalks and irrigated regularly to encourage rapid growth. Identical studies were installed on one well-drained and one poorly drained site located about 18 km (11.2 miles) apart. Barriers included 30 cm (12 in) deep DeepRoot, Biobarrier®, polyethylene (6 mil), a clean gravel layer (15 cm [6 in] deep; 2 to 3 cm [0.8 to 1.2 in] diameter) under the walk, and a control without a barrier. Roots were excavated 8 years after planting. No roots grew in the gravel in the well-drained site, resulting in a significantly deeper root system (19 cm [7.6 in]) under the walks than all other treatments (11 cm [4.4 in]). Vertical root barriers did not increase root depth compared to the control on the well- drained soil. Gravel under the walk and Biobarrier were most effective on poorly drained soil. DeepRoot was the least effective vertical barrier on the poorly drained site; Biobarrier was the most effective. Treatments had no effect on diameter of roots growing under the sidewalks. Roots deflected by the vertical barriers were forced deeper into the soil, but many returned to the surface by the time they reached the opposite side of the walk. Gravel under the sidewalk appears to hold promise for reducing sidewalk damage, especially on well-drained sites. Key Words. Gravel; root depth; root diameter; urban design; vertical root barriers. Roots growing under concrete walks have been associated with extensive sidewalk damage (Rajani 2002), although one study casts doubt on the cause of sidewalk damage (Sydnor et al. 2000). Certain tree species and cultivars cause less sidewalk damage by developing a small trunk flare (Wagar and Barker 1983), maturing at a small size (Costello and Jones 2003), developing a relatively deep root system (Francis et al. 1996), and other factors (Randrup et al 2001). Nursery production method may also affect surface rooting (Marshall and Gilman 1997) and, therefore, damage potential. There are many observations of root damage to sidewalks (Costello and Jones 2003); however, there are only a few controlled studies of newly planted tree response to physical barriers designed to deflect roots (Wagar 1985; Barker 1995a, 1995b; Peper 1998). Physical and chemical vertical barriers deflect roots downward or laterally (Randrup et al 2001). Few roots grow through the commercial barriers (Kopinga 1994). Wagar (1985) showed that a plastic barrier deflected roots to deeper soil layers; however, after growing under the barrier, roots in a poorly drained clay soil quickly grew back to the soil surface. The barrier was most effective ©2006 International Society of Arboriculture in the parts of the field containing well-drained soil, where roots returned to the surface more gradually. Gilman (1996) and Costello et al. (1997) also found that roots often return to the soil surface after growing under vertical barriers. Barker (1995a, 1995b) showed that in an alluvial, well- drained, silty clay loam soil, roots deflected down by a polyethylene plastic sheet did not grow up toward the soil surface within 1 m (3.3 ft) of the barrier. Other reports indicate that barriers can deflect roots growing from pruned roots on established trees; the authors speculated that this could potentially reduce damage to sidewalks (Wagar and Barker 1993). Approximately 25% of the nearly 200 urban forestry programs using root barriers in California, U.S., report them to be effective in reducing sidewalk damage; another 25% report that they are partially effective (Bernhardt and Swiecki 1993). Therefore, about half those that tried barriers question their usefulness. Most responding to the survey said species selection was nearly 90% effective in reducing sidewalk damage, with small- statured trees less damaging than large-growing ones. Kopinga (1994) in the Netherlands showed that gravel under asphalt bicycle paths reduced rooting directly under the path and extended its life. Others suggest that a gravel sub-base can prevent roots from growing under walks by creating an air environment too dry for root growth (Coder 1998). The purpose of this study was to evaluate effective- ness of root barriers and gravel near and under sidewalks on well-drained soil and poorly drained soil. MATERIALS AND METHODS Sidewalk sections 10 cm (4 in) thick measuring 4.6 × 1.2 m (16.5 × 4.5 ft) were installed in USDA (1990) hardiness zone 8 in Gainesville, Florida, U.S., May 1996, on a well-drained (Millhopper fine sand ([loamy, siliceous, hyperthermic Grossarenic Paleudults]) site and a poorly drained site (Arrendondo fine sand). About 8 cm (3.2 in) of soil was removed from each 4.6 × 1.2 m (16.5 × 4.5 ft) area before pouring concrete sidewalks. Sidewalks were not reinforced. Sidewalk sections were placed 9.2 m (30.4 ft) apart in a row (Figure 1). One row of sidewalks was 1.5 m (5 ft) away from and parallel to a second row, with a tree planted between them. A third row was 1.5 m (5 ft) away from and parallel to a fourth row, with a tree planted between them. Each row of sidewalks acted as a replicate. Treatments, including no sidewalk control, were randomly arranged within a row. Experimental design was a randomized complete block design on 2 soil types × 6 treatments × 4 replicates = 48 trees.
January 2006
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