Growth habit impacts the aesthetic value of an ornamental and the economics of the crop. Ornamental plants that grow too rampantly in the nursery often need to be pruned back before sale or shipment, causing additional labor costs. An ornamental that has a weak and open growth habit is often damaged during shipping. Although long-term breeding goals for a species or hybrid may include shorter stature and more compact growth, nurseries need interim solutions.
Plant growth regulators are often used to control plant height and growth habit to produce a more marketable plant. PGRs may also be used to promote or retard the onset of flowering. Application of paclobutrazol and chlormequat to pyrethrum (Chrysanthemum cinerariaefolium Vis.) was found to reduce plant height, but also caused plants to produce fewer flowers (Haque et al., 2007). Treatment of azaleas (Rhododendron simsii Planch.) with both chlormequat and paclobutrazol induced earlier flower bud formation (Christiaens et al., 2012). Treatment of Dissotis plumosa (a synonym for Dissotis rotundifolia) with chlormequat, daminozide, and ancymidol reduced growth in both greenhouse and field-grown plants (Criley, 1976). Paclobutrazol applied as a spray delayed flowering time in mini carnation (Dianthus caryophyllus f. spray Hort.) from 15 to 23 d but increased the number of blooms per flowering shoot (Atanassova et al., 2004). In Hibiscus rosa-sinensis L., paclobutrazol delayed flowering by 22 d compared with the control treatment (Nazarudin, 2012).
Effects on plant growth and flowering may differ among cultivars. When paclobutrazol was applied to Lilium L.A. hybrids, flowering in ‘Royal Respect’ was reduced at the highest concentration applied while flowering in ‘Ercolano’ was unaffected at any dosage. Plant height was also reduced more in ‘Royal Respect’ than in ‘Ercolano’ at every concentration of paclobutrazol (Francescangeli et al., 2007).
Effects of PGRs are often strongly dependent on treatment level. Pyrethrum treated at lower doses of 2-chloroethylphosphonic acid decreased both plant height and flower yield. However, in the same study, application of 2-chloroethylphosphonic acid at higher levels also decreased plant height but increased flower yield (Haque et al., 2007). China aster (Callistephus chinensis L. Nees) was found to produce fewer flowers at higher dosages of paclobutrazol (Mishra et al., 2005). In Lupinus varius L., days to flowering and number of flowers were negatively impacted at higher dosages of paclobutrazol by drench while time to flowering was positively impacted at the highest dosage by spray (Karaguzel et al., 2004). High dosages of PGRs may cause plants to cease growing entirely (Johansen et al., 1999).
Dissotis and Tibouchina are two genera in the Melastomataceae family that contains species of ornamental value. However, the growth habit of certain species and hybrids within these genera is not ideal for shipment or for economic nursery production. D. rotundifolia, a species from Africa, is a trailing plant that flowers profusely. The species roots easily and is extremely drought tolerant. D. rotundifolia grows vigorously and requires pruning to maintain it in a saleable state. Severe pruning prevents the species from blooming for several weeks afterward. Tibouchina fothergillae ×pilosa, a hybrid between two South American species, has purple flowers and attractive fuzzy foliage. The branches of T. fothergillae ×pilosa have a very open growth habit, making it difficult to ship. Paclobutrazol is a PGR that acts by inhibiting the biosynthesis of gibberellin (Lever, 1986). The compound has low solubility and has been found to persist in soil as long as 12 months after application (Lever, 1986). Paclobutrazol was chosen to test for its potential to control growth in D. rotundifolia and in T. fothergillae ×pilosa, as it has been used to control growth in other Melastomataceae species (Abdullah et al., 1998; Johansen et al., 1999; Roberts and Eaton, 1988). The objective of the experiment was to determine the effectiveness of paclobutrazol applied as a drench or as a foliar spray on controlling the growth and flowering of D. rotundifolia and T. fothergillae ×pilosa.
Abdullah, T., Malek, A.A. & Ahmad, S.H. 1998 Chemical manipulation of growth and flowering in potted Melastoma decemfidum and Tibouchina semidecandra Acta Hort. 454 297 301
Atanassova, B., Filipova, I. & Alexieva, V. 2004 Effect of plant growth regulators Alar, MEIA and paclobutrazol on the phenophase of flowering and some ornamental parameters of mini-carnation (D. caryophyllus f. spray Hort.) Bulg. J. Agr. Sci. 10 305 309
Barnes, J., Whipker, B., Buhler, W. & McCall, I. 2009 Osteospermum growth control with paclobutrazol substrate drenches. Proc. 36th Annu. Mtg. of the Plant Growth Regulat. Soc. Amer., Asheville, NC, 2–6 Aug. 2009. Plant Growth Regulat. Soc. Amer., Research Triangle Park
Berova, M. & Zlatev, Z. 2000 Physiological response and yield of paclobutrazol treated tomato plants (Lycopersicon esculentum Mill.) Plant Growth Regulat. 30 117 123
Christiaens, A., Van Labeke, M.C., Pauwels, E., Gobin, B., De Keyser, E. & De Riek, J. 2012 Flowering quality of azalea (Rhododendron simsii.) following treatments with plant growth regulators Acta Hort. 1 219 226
Criley, R.A. 1976 Growth retardants for control of ground covers. Hawaii Agr. Expt. Sta Dept. Paper
Francescangeli, N., Marinangeli, P. & Curvetto, N. 2007 Paclobutrazol for height control of two Lilium L.A. hybrids grown in pots. Span. J. Agr. Res. 5(3):425–430
Haque, S., Farooqi, A.H.A., Gupta, M.M., Sangwan, R.S. & Khan, A. 2007 Effect of ethrel, chlormequat chloride and paclobutrazol on growth and pyrethrins accumulation in Chrysanthemum cinerariaefolium Vis Plant Growth Regulat. 51 263 269
Intrieri, C., Silvestroni, O. & Turri, S. 1987 Uptake and transport of 14C-paclobutrazol on vinegrape seedlings Adv. Hort. Sci 1 1 15 19
Johansen, I.E., Findsen, S. & Andersen, A.S. 1999 Propagation, growth, retardation, flower induction, and postproduction performance of Tibouchina urvilleana (DC.) Cogn. Gartenbauwissenschaft 64 200 205
Karaguzel, O., Baktir, I., Cakmakci, S. & Ortacesme, V. 2004 Growth and flowering responses of Lupinus varius L. to paclobutrazol HortScience 39 1659 1663
Mishra, D.K., Mishra, H.R. & Yadava, L.P. 2005 Influence of paclobutrazol on early flowering and aesthetic value of China aster (Callistephus chinensis L. Nees) J. Appl. Hort. (Lucknow) 7 34 37
Nazarudin, A. 2012 Plant growth retardants effect on growth and flowering of potted Hibiscus rosa-sinensis L J. Trop. Plant Physiol. 4 29–40
Pepin, R.E. & Cole, J.C. 2014 Paclobutrazol, uniconazole, or flurprimidol applied at various concentrations as a substrate drench or through subirrigation have little effect on bee balm growth HortTechnology 24 313 317
Ruter, J.M. 1992 Growth and flowering response of butterfly-bush to paclobutrazol formulation and rate of application HortScience 27 929