Soil-applied Paclobutrazol Controls Shoot Growth of Field and Container-grown Conifers

Field-grown and container-grown conifers represent a major portion of the landscape tree market in the Midwestern United States. Growers commonly prune or shear conifers as part of their standard nursery culture. In this project we examined the utility of soil-applied paclobutrazol to control height growth of conifers in a field trial and in a container study.  In the field trial, the effectiveness of soil-applied paclobutrazol on height growth varied with species and tree size.  Paclobutrazol reduced height growth of field-grown Norway spruce (Picea abies) trees by 50% compared with untreated control trees.  Paclobutrazol reduced height growth of small (< 1.3 m tall at treatment) blue spruce (Picea pungens) trees by 52% compared to control trees but did not affect growth of trees in two larger size classes (1.3-1.8 m and >1.8 m).  In the container study, soil drench with paclobutrazol did not affect shoot growth of Norway spruce, Serbian spruce (Picea omorika) or white pine (Pinus strobus) in the growing season following treatments but reduced growth the following year. For all species, there was no difference in the height growth between high and low rate of paclobutrazol.  This indicates that the low rate was sufficient to achieve height control and suggests that additional research to develop dose-response curves is needed.  In addition to growth control, treatment with paclobutrazol resulted in visibly greener plants.  Analysis of needle chlorophyll concentration demonstrated that application of paclobutrazol increased needle chlorophyll concentration of Norway spruce and white pine but not Serbian spruce.