300 Smiley et al.: Paclobutrazol Foliar Sprays to Suppress Growth on Landscape Shrubs Arboriculture & Urban Forestry 2009. 35(6): 300–304 Paclobutrazol Foliar Sprays to Suppress Growth on Landscape Shrubs E. Thomas Smiley, Liza Holmes, and Bruce R. Fraedrich 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 treat- ment 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 applica- tions of PBZ. This growth reduction should provide a desirable appearance, less pruning and faster clean up when PBZ is applied to fast growing shrubs. Key Words. Abelia × grandiflora ‘Edward Goucher’; Bonzi; Cambistat; Ligustrum japonicum ‘Rotundifolium’; Ligustrum sinense ‘Variegatum’; Loropetalum chinensis; PBZ; Shrub Pruning. Shrub species are planted in the urban landscape to provide un- derstory growth, to act as screens, or for their flower produc- tion. While shrubs provide these and other landscape benefits, they often need to be pruned multiple times per year to maintain the size and shape appropriate for the landscape. This requires considerable resources from landscape managers. Therefore, with many fast growing shrub species, it would be desirable if new growth could be suppressed to reduce the number of times the shrubs need to be pruned and to reduce the volume of biomass that needs to be removed in the clean-up process. Paclobutrazol (PBZ) has been used successfully on trees to sup- press growth, especially after utility pruning (Davis 1991; Burch et al. 1996; Bai et al. 2004). One of the major benefits of PBZ application is the reduction in the time required to prune trees fol- lowing treatment. Time required for utility tree pruning was great- ly reduced and highly correlated with the reduction in biomass associated with growth regulator treatment (Burch et al. 1996). PBZ has been tested and used successfully on turf and fruit crops (Aron et al. 1985; Edgerton 1986; Looney and McK- ellar 1987). Shrub treatments have been tried, but were lim- ited mainly to soil drench applications on containerized plants (Welker 1986; Joustra 1989; Ruter 1992; Cid and Cabal- lero 1993). Foliar spray treatments of PBZ have been found to change the shape and thickness of some shrub leaves (Martin et al. 1994). Since there may be a substantial delay between the time of soil application of PBZ and growth reduction ef- fects (Bati et al. 2004), spray applications may be more desir- able, especially during the first year of a treatment program. This study was conducted to determine the ef- fects of PBZ foliar sprays on the growth of several land- scape shrub species in the southeastern United States. ©2009 International Society of Arboriculture MATERIALS AND METHODS Abelia × grandiflora ‘Edward Goucher’, Ligustrum japonicum ‘Rotundifolium’, Ligustrum sinense ‘Variegatum’, and Loro- petalum chinensis, four fast-growing shrub species commonly planted in the southeastern United States, were selected for test- ing. Twenty-one plants of each species were planted from 11.3 L (3 gal) containers into rows spaced 2.4 m (8 feet) apart with 4.6 m (15 ft) between rows, in July 2007 at the Bartlett Tree Re- search Laboratory in Charlotte, NC, U.S. (USDA Climate Zone 7). Species were planted within a single row. Fresh wood chip mulch was applied after planting and was maintained to a depth of 5–10 cm (2–4 in) during the trial. Water was provided as nec- essary using a soaker hose drip irrigation system. Weeds were suppressed with applications of glyphosate, as needed. All plants became established and were growing well by the fall of 2007. On May 20, 2008, between 25% and 33% of the foliage was pruned from all plants using a Husqvarna HE3 hedge trimmer (Jönköping, Sweden), to provide a baseline size for each plant. This was done by constructing a cubical structure from PVC pipe to define the final shape of the plant and to ensure that growth measurements and re-shearing would be to the same defined area (Figure 1). The structure was removed after shear- ing. L. chinensis was pruned to a height of 64 cm (25.25 in) and a width of 60 cm (23.5 in). All other species were pruned to a height of 33 cm (13 in) and a width of 47 cm (18.5 in). Each species was divided into seven blocks of three plants each. One plant within the block was assigned one of three treat- ments. Immediately after shearing, a foliar spray treatment was applied uniformly to the point of run-off using a backpack spray- er. Applications included a nonionic spreader sticker (Nufilm® IR, 96% Poly-1-p-menthene, a terpenic polymer, Miller Chemi-
November 2009
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