Root Barrier and Extension Casing Effects on Chinese Hackberry

"In California, repairing sidewalk damage associated with tree roots exceeds $62 million annually. Efforts to reduce sidewalk damage have provided street tree managers with a variety of root barrier products. In this study, 3 types of root barriers were installed and evaluated to determine whether 1) internal vertical ribs prevented circling roots and 2) root development would be significantly reduced in the top 30 cm (12 in.) of soil. The 3 barriers tested included 1) a modified production container, partially left in place when planted (extension casing), 2) a commercial product with vertical ribs spaced 15 cm (6 in.) apart, intended to prevent circling roots, and 3) a commercial product with vertical ribs spaced 12.5 cm (5 in.) apart. Root diameter, depth, and dry weight were measured for Chinese hackberry (Celtis sinensis) planted and grown for 3 years with and without root barriers. The extension casing reduced root biomass in the top 33 cm (13 in.) of soil by 50% compared to trees without barriers (controls). Mean root diameter of the 6 largest roots outside of the casing was also significantly smaller. Mean root diameters and biomass of controls were similar to those for the 2 commercial barriers. Roots on the trees with root barriers escaped beyond the barrier walls then grew upwards to depths similar to the controls (37 cm [14.5 in.]). Although casings reduced total root biomass and diameter, they appeared to encourage circling root growth. Circling was diminished when commercial barriers with internal vertical ribs were used. Mean top growth was similar among treatments."