Flowering quality of azalea (Rhododendron simsii) following treatments with plant growth regulators

The ornamental value of florist azalea depends strongly on its synchronous flowering. This quality demand is however not always met. A clear cut direct cause is seldom found; interaction between several elements related to the culture conditions should be at the base. In this experiment the application of plant growth regulators (PGR) were tested on two cultivars, ‘H. Vogel’ (early flowering) and ‘Kint’ (late flowering). Six applications with 2.25 g/L chloormequat were combined with 4 applications with 0.012 g/L paclobutrazol. A control group was not treated with any PGR. Microscopic observations showed that flower initiation in PGR treated plants was enhanced by 12 days for ‘H. Vogel’, but was unchanged for ‘Kint’. Differentiation was only slightly enhanced (2-4 days) compared to the control plants. When the floral primordia were fully developed cold treatments (7°C for 2 to 8 weeks) to fulfill flower bud dormancy requirements were applied. Results show that the late flowering cultivar ‘Mw. Kint’ has a stronger dormancy than the early flowering cultivar ‘H. Vogel’. Subsequently a longer period at 7°C is necessary for ideal flowering. For both ‘Mw. Kint’ and ‘H. Vogel’, PGR treated plants have a stronger dormancy, resulting in delayed flowering when forcing the plants at 21°C.