Paclobutrazol foliar sprays to suppress growth on landscape shrubs

Abstract

This study was conducted to determine the effects of Paclobutrazol (PBZ, Cambistat) foliar sprays on the growth of four shrub species (Abelia × grandiflora 'Edward Goucher', Ligustrum japonicum 'Rotundifolium', Ligustrum sinense 'Variegatum', Loropetalum chinensis) in the southeastern United States. After the shrubs were established, they were sheared to a defined size and sprayed with 1000 ppm PBZ, 4000 ppm PBZ or left untreated. Paclobutrazol was effective at reducing growth on the four shrub species tested. Total biomass of new leaves and twigs collected 20 weeks after treatment was significantly less with PBZ treatment when compared to the nontreated control in all species except L. sinense. There were no differences in biomass that can be attributed to PBZ between the 1000 ppm and 4000 ppm treatments in any species tested. The speed of effectiveness and longevity of this treatment varied with species. In general, the faster growing the shrub species, the greater benefit and the faster the response from spray applications of PBZ. This growth reduction should provide a desirable appearance, less pruning and faster clean up when PBZ is applied to fast growing shrubs. © 2009 International Society of Arboriculture.