Arboriculture & Urban Forestry 45(6): November 2019 Arboriculture & Urban Forestry 2019. 45(6):315–327 URBAN FORESTRY ARBORICULTURE Scientific Journal of the International Society of Arboriculture & Root Penetration of Polyvinyl Chloride (PVC) Stormwater and Sewer Pipes By Gregory M. Moore, Susan Bendel, and Peter B. May Abstract. Two experiments investigated factors influencing root penetration of polyvinyl chloride (PVC) pipes. Eucalyptus leucoxylon, Allo- casuarina littoralis, Lophostemon confertus, Callistemon salignus, Acer palmatum, and Pyrus calleryana seedlings were grown in containers containing 150-mm lengths of sealed 75-mm PVC stormwater pipe with cracks 0.04 mm, 0.66 mm, or 1.48 mm wide on their upper surface. The buried pipes contained water, water and stormwater, soil, or soil and stormwater. There were six replicates and 432 plants. There was no significant difference in the mass of roots entering the pipes for the two larger crack widths with 70% of pipes penetrated and strong growth inside the pipes. While the roots of all species penetrated cracks greater than 0.66 mm, only roots of C. salignus, E. leucoxylon, and L. confer- tus penetrated 0.04 mm cracks. Roots penetrated 50 to 60% of pipes containing soil, or soil and stormwater, and 40% of pipes containing water, or water and stormwater were penetrated. The plants with roots penetrating pipes containing water and stormwater grew tallest. No roots pen- etrated the welded caps of the stormwater pipes. A second experiment using E. leucoxylon, Melaleuca ericifolia, Ficus macrophylla, A. littoralis, and Salix fragilis investigated root penetration of different sized holes in polycarbonate plates. The plates, installed in containers with growing medium above and below, had either 2 × 4 mm holes, 8 × 2 mm holes, 127 × 0.5 mm holes, or a mixture of holes (1 × 4 mm, 2 × 2 mm and 32 × 0.5 mm holes), total pore area in all being 25.14 mm2 . Below the plates, the growing medium was capillary irrigated with stormwater or water. All species grew through 0.5-mm holes and had strong root growth below the plates. When irrigated with stormwater, all species were taller and had greater biomass, and most species had a greater root mass below the plates. In general and regardless of hole size, the more holes in the plates, the more roots penetrated them. Properly installed PVC pipes are impenetrable, but the width and number of openings in a pipe influ- ence the capacity for penetration and subsequent root growth so protocols minimizing damage to pipes should be enforced. Since species have different capacities for penetrating stormwater pipes, appropriate species selection for urban environments where damaged pipes may occur could reduce incidences of pipe damage. Keywords. Root Penetration; PVC Pipes; Tree Roots and Pipes; Street Trees and Pipes. 315 INTRODUCTION Trees are important parts of urban infrastructure, pro- viding environmental, human health, and social ben- efits (Lohr et al. 2004). Trees evolved in forests and woodlands, not in urban soils in narrow verges between foot paths and roads where they share root- ing space with sewerage and stormwater pipes and other utilities. These unnatural rooting spaces lead to compromises in tree health and conflicts with infrastructure. Disturbed and often compacted soils have reduced oxygen diffusion and drainage, limiting tree root growth (Kozlowski 1992; Craul 1994; Lichter and Lindsey 1994; Bradshaw et al. 1995). Backfill around pipes is often less compacted than the surrounding soil, mak- ing it suitable for roots, which can then penetrate pipes if there are cracks larger than the root tips (Rolf et al. 1995; Stal and Rolf 1998; Pohls 2001). Roots require pore spaces larger than the growing tip for elongation, although they can push through pores, enlarging them in friable, uncompacted soil (Roberts et al. 2006; Urban 2008). Tree roots can grow into cracked pipes, creating costly problems for local gov- ernments (Lesser 2001; Dodge and Geiger 2003; Lohr et al. 2004). Trees are often removed if their roots cause even minor infrastructure damage (Han- nah and Yau 1990), which usually involves the removal of a valuable, mature tree that is then replaced with a younger specimen. Moisture condenses on the outside of clay pipes (Roberts et al. 2006), and opportunistic roots attracted to the moist and fertile environment which exists at leaks can enter the pipes at cracks rather than causing cracks (Ridgers et al. 2006). There is greater condensation ©2019 International Society of Arboriculture
November 2019
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