2019 ASHS Annual Conference

Paclobutrazol is a tree growth regulator that is widely used by arborists, particularly to control height growth of trees near utilities. Paclobutrazol is also increasingly marketed as stress mitigation treatment in urban trees due its fungistatic properties effects and effects on plant drought tolerance. In this study we applied paclobutrazol (tradename Cambistat) as a soil drench to Callery pear (Pyrus calleryana) trees planted as street trees at two sites in downtown Lansing, MI. We assessed shoot and leaf growth and physiological responses for two years following application. Paclobutrazol reduced leaf size and increased SPAD chlorophyll content in both years. Net photosynthetic rate increased in the first year following application but was unaffected two years after application. The results suggest that paclobutrazol application my help to offset urban stresses by reducing leaf size resulting in increased nutrient content (as indicated by SPAD index) and increased photosynthetic efficiency.