Total Root and Shoot Biomass Inhibited by Paclobutrazol Application on Common Landscape Trees

Abstract Background Paclobutrazol (PBZ) is used in the arboriculture industry to reduce the growth of trees. It works by inhibiting gibberellin biosynthesis, a group of phytohormones associated with cell elongation. A substantial amount of variation exists within the literature as to the impact of PBZ on woody plant root systems. The purpose of this study was to assess the impact of PBZ on belowground growth and biomass allocation among plant species with varying levels of PBZ sensitivity in a controlled setting. Methods We treated containerized silver maple, white oak, pecan, laurel oak, and stone pine trees with Cambistat ® at the full label rate, one category lower, two categories lower, or water-only controls. After a 14-month incubation period, leaf, stem, and root tissue dry mass were quantified, root:shoot ratios were calculated, the length of the longest root quantified, and total root length of a subset of replicates was estimated. Species were statistically analyzed separately and collectively to assess trends. Results Paclobutrazol application resulted in significantly lower root dry mass and total root length for all species analyzed, and significantly reduced longest root length of all species except for silver maple. Across species and dosage combinations, we saw few dose effects on any response variable and no major trends in root:shoot ratios. Conclusion The impact of PBZ on trees in the landscape appears to be influenced by a number of factors, but we observed relatively consistent results on belowground biomass when growing conditions were uniform in our controlled experiment.