Photoperiodic Responses of Hydrangea

Following the termination of rest, 16-, 20-, or 24-hr photoperiods hastened flower development of 7 cultivars of hydrangeas [ Hydrangea macrophylla (Thunb.) Ser.] compared to those at 8- or 12-hr photoperiods. The interruption of a 16-hr dark period with 4- or 8-hr incandescent illumination was as effective as continuous light. Stem elongation and cyme expansion increased with length of photoperiod, while the interruption of the dark period had an intermediate effect upon these parameters. Plants undergoing continuous development without defoliation or a rest treatment responded similarly to rested and defoliated plants in rate and extent of growth and initiated flowers more uniformly at an 8-hr photoperiod than at longer photoperiods.