Control of Floral Initiation in Florists’ Hydrangea

Abstract

Abstract Plants of Hydrangea macrophylla ‘Rose Supreme’, ‘Merritt's Supreme’, and ‘Sister Therese’ produced inflorescence primordia more effectively under 8-hr photoperiods than under continuous photoperiod (CP) at 24°C. Inflorescence primordia were present on all plants under 8-hr photoperiods after 16 weeks, whereas plants under CP remained vegetative. Plants under CP sustained internode elongation throughout the experiment, whereas plants under 8-hr photoperiods remained short with little increase in number of expanded leaf pairs. Continuous photoperiod inhibited floral initiation of ‘Rose Supreme’ and ‘Merritt's Supreme’ plants at 24°C, yet had little effect at 18°C. ‘Sister Therese’ plants bloomed freely, regardless of photoperiod. Daminozide (3 biweekly foliar sprays of 5000 mg/liter) inhibited floral initiation of ‘Rose Supreme’ and ‘Sister Therese’ plants. Plants of all 3 cultivars flowered more rapidly at 24° than at 18°, whereas photoperiod had no effect on flowering date. Inflorescences were larger and plants were taller at 24°C than at 18°C. Continuous photoperiod increased inflorescence diameter and plant height at 24°C but had little effect at 18°C. Plants treated with daminozide were shorter than untreated plants, regardless of temperature or photoperiod.