Six-year evaluation of circular root barriers on two tree species

Abstract

The influence of four circular root barriers on surface root development and tree growth was evaluated on Liquidambar styraciflua(American sweetgum) and Ficus microcarpa (Indian laurel fig) in southern California, U.S. Six years after installation, root barriers had reduced the total number of roots growing in the surface 15 cm (6 in) of soil and nearly eliminated large (diameter 5 cm. [2 in] or greater) surface tree roots within a 120 cm (48 in) radius of the trunk. Various sizes and types of low-cost noncommercial barriers, including a container-grown tree's nursery container with the bottom removed, were equally effective in reducing the number of large-diameter surface roots. However, roots grew below barriers and returned to the surface soil when soil texture, bulk density, and water content were near optimum for root growth at the bottom of the barrier. Many small roots 1.25 cm (0.5 in) ≤ diameter less than 2.5 cm (0.5 in ≤ diameter to less than 1 in) were found growing in the upper 15 cm (6 in) of soil just beyond barriers within 18 to 60 cm (7 to 24 in) of the trunk. No barrier treatment reduced the number of small roots of either species beyond 60 cm (24 in) radius from the trunk. Keeping pavement at least 120 cm (48 in) away from trees would be as effective as a root barrier in reducing the possibility of damage from large surface roots. A # 15 nursery container serving as a root barrier reduced caliper increase of both species, whereas this treatment and the DeepRoot® barrier treatment reduced height increase of Liquidambar. © 2009 International Society of Arboriculture.