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Arboriculture & Urban Forestry Online
Volume 32, Issue 1 — January 2006


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.

Keywords: Gravel; root depth; root diameter; urban design; vertical root barriers

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