316 around clay or concrete than around polyvinyl chlo- ride (PVC) pipes (McPherson and Peper 1994). Most roots enter clay and concrete pipes through breaks, loose joints, or failed rubber gaskets, and smaller pipes (diameter 22.5 to 40 cm) are most often pene- trated, especially in sandy soils (Watson et al. 2014). Water mains have sufficient water pressure to prevent root growth, but this is not the case for sewers and drains that rely on gravity to move their contents (Roberts et al. 2006). Roots will grow along moisture gradients created by leaky pipes or by condensation around the pipes (Coder 1998; Randrup et al. 2001; Roberts et al. 2006). Once they have penetrated pipes, roots grow rapidly, causing blockages, but over time the block- age may cause anaerobic conditions, and the roots die back (Ridgers et al. 2006). Damage may occur on shrinkable clays, when roots grow under pipes and increase in diameter, if pipes are encircled by roots, or if strong winds cause sufficient root movement to break pipes (Watson et al. 2014). When pipes are warmer than the surrounding soil, fine root activity is greater (Kurpaska et al. 2005; Kawasaki et al. 2014). Older sewers were made of vitrified clay, brick, and concrete, while more modern systems are made of plastic, ductile iron, steel, and reinforced concrete, which have adequate compressive strength and suffi- cient tensile strength to reduce cracking and damage. Between the world wars in England, clay packed joints and cement seals dried over time and cracked (Rolf et al. 1995; Randrup et al. 2001). In the 1930s, joints of drainage pipes contained copper wire to pre- vent root growth, but these eventually failed (Roberts et al. 2006). Clay and concrete pipes without rubber joint gaskets are likely to have faulty joints (McPher- son and Peper 1994; Rolf et al. 1995). In Sweden, most intrusions occurred in pipes of 225 to 400 mm diameter with yarn and cement sealed joints laid before 1950. The rubber seals of the 1960s were more effective, and the roots growing in pipes from that era are probably due to shoddy work. In Melbourne, the most common occurrences of roots in pipes were in thirty- to sixty-year-old clay pipes joined with rubber rings located at a depth of less than two meters (Pohls 2001). These seals expand and break, and joints were the most common sites of root intrusion (Coder 1998; Stal and Rolf 1998). PVC plastic and fiberglass pipes that are laid correctly with welded joints are rarely invaded by roots if they are ©2019 International Society of Arboriculture Moore et al.: Root penetration of PVC pipes installed according to correct standards on a com- pacted bed of gravel (McPherson and Peper 1994; Coder 1998; Randrup et al. 2001). While roots rarely break pipes, prevailing winds affecting the root plate may place stress on pipes on the windward side of the tree. Roots growing on the windward side of the tree are under tension and refer this tension to pipes lying over them, which may crack on their upper surface, especially if the roots are of a large diameter. Pipes directly under the trunk and pipes on the leeward side of the tree are not at risk of damage (Roberts et al. 2006). Different species of trees are implicated in pipe damage in different parts of the world. In Melbourne, the roots of commonly planted eucalypts and Mela- leuca species were most frequently found in pipes (Pohls 2001), while in Sweden, willow (Salix) and poplar (Populus) were the main cause of damage (Rolf et al. 1995; Stal and Rolf 1998). Pipes contain- ing stagnant water are not conducive to root growth due to a lack of oxygen, but willow and swamp cypress (Taxodium) can grow in these conditions, and pipes that have water flow twenty-four hours a day, are more likely to have root growth than drains that carry water occasionally (Roberts et al. 2006). Reduced root growth can be achieved by annual cutting, flooding pipes with boiling water after cut- ting roots, or using copper sulphate, which is only taken up a short distance into the root before the root dies, preventing further absorption without killing the tree (Rolf et al. 1995; Randrup et al. 2001). Exposure to copper sulphate requires days to weeks to kill roots, which is often impractical. Chemicals such as dichlo- benil, oryzalin, and trifluralin control root growth, but affect water quality (McPherson and Peper 1994; Pohls 2001). Metham (methylcarbamadithioic acid) has been used for control but is toxic to sewage treat- ment nitrifiers and was replaced by less toxic but more costly glufosinate. Herbicides have been used to control root growth for three to five years, but many have faced environmental bans in Europe (Roberts et al. 2006). Two experiments investigated the penetration of PVC pipes by the roots of different tree species. The first experiment investigated the penetration of PVC pipes by roots of six commonly planted street trees. The pipes had cracks of different widths cut into them and contained potable water, stormwater, sandy soil, and soil plus stormwater. The research aimed to
November 2019
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