Arboriculture & Urban Forestry 35(6): November 2009 same degree as other branches. In this study of foliar applied PBZ, there were few errant or escape branches observed with any plant species or treatment. The new growth on treated plants was as uni- form or more uniform than the growth of the untreated controls. Abelia × grandiflora showed the greatest growth suppression 6 weeks after treatment with PBZ. By 18 weeks, there were no differences between either PBZ treatment rate and the control. However, with visual inspection there was noticeably less growth with the 4000 ppm rate. With this species, retreatment should be considered after 12 weeks with either application rate, because the growth rate of the treated plants increased noticeably at that point. The 4000 ppm rate provided more growth suppression on this spe- cies. To the eyes of the study’s evaluators, this growth suppression was beyond the point of aesthetic desirability. PBZ residue from this high application rate could also present aesthetic problems. Ligustrum japonicum ‘Rotundifolium’ treated with PBZ displayed growth suppression 6 weeks after treatment with both application rates. However, at 18 weeks, the 1000 ppm PBZ rate was no longer different than the nontreated control. The suppression continued longer and was more pronounced with the 4000 ppm rate of PBZ. No reapplication would be required at the high rate for 18 weeks of growth suppres- sion. If the 1000 ppm rate was applied, reapplication should be considered between 6 and 12 weeks after the first applica- tion when the growth rate of the treated plants accelerated. Ligustrum sinense ‘Variegatum’ treated with PBZ had no significant height growth differences until the 12-week mea- surements. Differences continued in week 18. There were no differences in response between the application rates ex- cept for width growth at 6 weeks. Therefore this species ap- pears to respond more slowly to PBZ than the other spe- cies tested. Re-application would not be required until after 18 weeks. This species had the lowest overall growth rate. Loropetalum chinensis treated with PBZ displayed growth differences at 6 weeks and the differences persisted for the re- mainder of the trial. At 6 and 12 weeks there were no differences between the application rates, however at 18 weeks the higher rate provided more suppression in shrub height growth. Reap- plication should be considered at 12 weeks with the 1000 ppm application rate and after 18 weeks with the 4000 ppm rate. In summary, paclobutrazol was effective at reducing growth on the four shrub species tested. The speed of effectiveness and longevity of this treatment varied with species and application rate. In general, the faster growing the shrub species, the greater benefit and the faster the response from spray applications of PBZ applied immediately after spring pruning. This growth reduction should provide a desirable appearance and less pruning and clean up after pruning when PBZ is applied to fast growing shrubs. Depending on the length of the growing season and the growth characteristics of the species being treated, one to three spray ap- plications per year may be required to manage growth of shrubs. In areas with short growing seasons, one application may last the entire season, making this a very cost effective service. In areas with longer growing seasons where multiple applications would be required, a soil drench application may be a more appropriate means of application of a plant growth regulator. While the high rate of PBZ in this trial often provided signifi- cantly greater growth control, this does not mean that the high 303 rate is the preferred. The high application rate may, especially in young plants, provide too much reduction in growth which may lead to an unnatural appearance (Figure 3), there may also be aes- thetic problems with residue on the leaf surface (Figure 2), and the difference in growth rate between the high and low application rates may not justify the higher cost of the higher application rate. So when selecting application rates, species response; appear- ance, cost, and, your plant growth goals should all be considered. Additional research is required to determine the impact of foliar application on growth over several years of application and to compare the differences between soil and foliar applica- tion. Since there were large differences in species response to PBZ application, treating untested varieties should be avoided. LITERATURE CITED Aron, Y., S.P. Monselise, R. Goren, and J. Costo. 1985. Chemical con- trol of vegetative growth in citrus trees by paclobutrazol. HortScience 20:96–98. Bai, S., W. Chaney, and Y. Qi. 2004. Response of cambial and shoot growth in trees treated with paclobutrazol. Journal of Arboriculture 30:137–145. Burch, P.L., R.H. Wells, and W.N. Kline. 1996. Red Maple and silver maple growth evaluated 10 years after application of paclobutrazol tree growth regulator. Journal of Arboriculture 22:61–66. Cid, M.C., and M. Caballero. 1993. Control of Canary Islands native shrub species for potted plant use. Acta Horticulturae 337:99–104. Davis, T.D. 1991. Regulation of tree growth and development with triaz- ole compounds. Journal of Arboriculture 17:167–170. Edgerton, L.J. 1986. Some effects of Paclobutrazol on growth and fruit- ing of apple, peach and cherry. Acta Horticulturae 179:467–472. Joustra, M.K. 1989. Application of growth regulators to ornamen- tal shrubs for use as interior decoration. Acta Horticulturae 251: 359–370. Looney, N.E., and J.E. McKellar. 1987. Effect of foliar—and soil sur- face—applied paclobutrazol on vegetative growth and fruit quality of sweet cherries. Journal of the American Society for Horticultural Science 112:71–76. Martin, C.A., W.P. Sharp, J.M. Ruter, and R.L. Garcia. 1994. Alterations in Leaf Morphology of Two Landscape Shrubs in Response to Dispa- rate Climate and Paclobutrazol. HortScience 29:1321–1325. Ruter, J.M. 1992. Growth and flowering response of butterfly-bush to pa- clobutrazol formulation and rate of application. HortScience 27:929. Welker, E. 1986. Bonzi [paclobutrazol] a new growth regulator for azalea Zierpflanzenbau 16:230–231. Acknowledgments. We would like to thank: Robert Bartlett Jr. and the F.A. Bartlett Tree Expert Co. for their support of this research. Shawn Bernick and Jim Zwack at Rainbow Treecare Scientific Advancement for the donation of the Cambistat used in this trial and for the guidance they provided. Dr. Glynn Percival of the Bartlett Tree Research Laboratory UK for assistance with the manuscript. ©2009 International Society of Arboriculture
November 2009
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