Within efficient production systems, ornamental crops need to be uniform in shape and size (Meijón et al., 2009). By regulating plant size and creating a uniform crop, growers can save space during production and transportation and reduce breakage and loss (Hayashi et al., 2001). Controlling plant architecture can also meet the demands of retailers and consumers for more compact, dense (Cameron et al., 2006; Lütken et al., 2012), and symmetrical plants (Glasgow et al., 1998). In addition, growers can manipulate flowering dates to extend shipping periods (Hayashi et al., 2001).
To achieve target uniformity, nursery producers manually prune and, less commonly, use chemical PGRs to enhance plant growth and plant architecture. Manual pruning can be labor intensive (Banko and Stefani, 1996) and expensive (Holland et al., 2007), yet, may be more environmentally sustainable than chemical control. Although PGRs are generally less labor intensive to apply than manual pruning, they can cause phytotoxicity (Meijón et al., 2009) and be perceived as digressing from sustainable production (Lütken et al., 2012).
Plant growth regulators have several modes of action, including branch inducing, chemical pinching (chemicals that suppress apical dominance), and ethylene generation. Plant responses to PGRs can vary across cultural and environmental conditions. In addition, the rates, timing, and number of applications required to elicit the desired response can vary among plant genera and species (Bruner et al., 2002; Kessler and Keever, 2008). For example, benzyladenine at 300 ppm increased BN in ‘Purple Haze’ anise hyssop (Agastache), but not in ‘Provence’ lavender (Lavandula ×intermedia) or ‘May Night’ sage (Salvia ×sylvestris) (Grossman et al., 2012), and 300 ppm benzyladenine and 1000 ppm ethephon increased BN of Marnier’s kalanchoe (Kalanchoe marnieriana), but not flower dust plant (K. pumila) (Currey and Erwin, 2012). These differing plant responses validate the importance of optimizing PGR chemistry, as well as rate and application timing based on the target crop, conditions, and desired effect on plant form.
Little Lime™ hydrangea is a new hydrangea cultivar that is increasingly popular, but is not considered attractive in its juvenile plant form because of sparse foliage and limited branching. Since hydrangea species account for an estimated $73 million in U.S. sales annually [U.S. Department of Agriculture (USDA), 2009], the use of PGRs to control growth could greatly benefit nurseries producing Little Lime™. Recently, dikegulac sodium, benzyladenine, and ethephon, all with different modes of action (DNA synthesis inhibitor, branch promoter, and ethylene generator, respectively), have shown success as PGRs on other hydrangea species (Hester et al., 2013), but efficacy on Little Lime™ is not known. With the variation in plant response, it is recommended to test PGRs on each plant cultivar individually (Currey and Erwin, 2012; Hilgers et al., 2005; Starman et al., 2004). Therefore, the objective of these experiments was to evaluate vegetative and floral development of Little Lime™ following application of dikegulac sodium, benzyladenine, or ethephon.
Bruner, L.L., Keever, G.J., Kessler, J.R. & Gilliam, C.H. 2002 Atrimmec suppresses shoot length and promotes branching of Lonicera ×heckrottii ‘Goldflame’ (Goldflame honeysuckle) J. Environ. Hort. 20 73 76
Cameron, R.W.F., Harrison-Murray, R.S., Atkinson, C.J. & Judd, H.L. 2006 Regulated deficit irrigation—a means to control growth in woody ornamentals J. Hort. Sci. Biotechnol. 81 435 443
Currey, C.J. & Erwin, J.E. 2012 Foliar applications of plant growth regulators affect stem elongation and branching of 11 kalanchoe species HortTechnology 22 338 344
Farris, M.E., Keever, G.J., Kessler, J.R. & Olive, J.W. 2009 Benzyladenine and cyclanilide promote shoot development and flowering of ‘Moonbeam’ coreopsis J. Environ. Hort. 27 176 182
Glasgow, T.E., Bilderback, T.E., Johnson, T., Perry, K.B. & Safley, C.D. 1998 Evaluating consumer perceptions of plant quality Proc. Southern Nursery Assn. Res. Conf. 43 497 500
Grossman, M., Freeborn, J., Scoggins, H. & Latimer, J. 2012 Benzyladenine increases branching but reduces root growth of herbaceous perennial liners HortScience 47 1085 1090
Hammond, H.E., Schoellhorn, R.K., Wilson, S.B. & Norcini, J.G. 2007 Differing blanketflower cultivar and ecotype responses to plant growth regulators HortTechnology 17 552 556
Hayashi, T., Heins, R.D., Cameron, A.C. & Carlson, W.H. 2001 Ethephon influences flowering, height, and branching of several herbaceous perennials Scientia Hort. 91 305 323
Hester, K.A., Bi, G., Czarnota, M.A., Fulcher, A., Keever, G.J., Lieth, J.H., Orsi, J.D., Whipker, B.E., Sullivan, K. & Palmer, C.L. 2013 Impact of Augeo, Configure and Florel on hydrangea branching J. Environ. Hort. 31 27 29
Holland, A.S., Keever, G.J., Kessler, J.R. & Dane, F. 2007 Single cyclanilide applications promote branching of woody ornamentals J. Environ. Hort. 25 139 144
Jeffers, A.H., Palma, M.A., Klingeman, W.E., Hall, C.R., Buckley, D.S. & Kopsell, D.A. 2009 Assessments of bare-root liner quality and purchasing decisions made by green industry professionals HortScience 44 717 724
Kessler, J.R. & Keever, G.J. 2008 Plant growth retardants affect growth and flowering of Achillea × ‘Coronation Gold’ J. Environ. Hort. 26 24 28
Lütken, H., Clarke, J.L. & Müller, R. 2012 Genetic engineering and sustainable production of ornamentals: Current status and future directions Plant Cell Rpt. 31 1141 1157
Meijón, M., Rodríquez, R., Cañal, M. & Feito, I. 2009 Improvement of compactness and floral quality in azalea by means of application of plant growth regulators Scientia Hort. 119 169 175
Moffatt, B., Pethe, C. & Laloue, M. 1991 Metabolism of benzyladenine is impaired in a mutant of Arabidopsis thaliana lacking adenine phosphoribosyltransferase activity Plant Physiol. 95 900 908
Townsley-Brascamp, W. & Marr, N.E. 1995 Evaluation and analysis of consumer preferences for outdoor ornamental plants Acta Hort. 391 199 206
U.S. Department of Agriculture 2009 2009 Census of horticultural specialties. 5 Jan. 2013. <http://www.agcensus.usda.gov/Publications/2007/Online_Highlights/Census_of_Horticulture_Specialties/hortic_1_023_023.pdf>