Long-day Requirement for Floral Induction in <italic>Lythrum salicaria</italic> ‘Dwarf Pink’

Background and objective: Lythrum salicaria 'Dwarf Pink' is a dwarf-type cultivar of purple loosestrife with a potential ornamental value as a garden plant. This study was conducted to determine a photoperiodic requirement for floral induction and the effects of plant size on flowering characteristics in L. salicaria 'Dwarf Pink'.Methods: In experiment 1, seedlings with an average of 17.7 leaves were grown under five different photoperiods: 9/15, 12/12, 14/10, 16/8, and 24/0 h (day/night hours) for 12 weeks. In experiment 2, the same daylengths were delivered to two groups with different leaf counts of 8.5 (stage 1) and 21.1 (stage 2).Results: In experiment 1, after four weeks of photoperiod treatments, floral buds started to develop under 16/8 and 24/0 h of long-day conditions, and these plants showed 100% flowering. There were no significant differences in the days to visible bud between the plants under 16/8 and 24/0 h. Plants under 9/15 and 12/12 h did not flower. Although floral induction was observed under 14/10 h, the flowering percentage was 17% at the end of the experiment. In experiment 2, plant size did not induce significant differences in the days to flowering and the number of nodes. Under the same long-day condition, when the plant size was large, the number of inflorescences and branches was less than the small plants. However, in both experiments, plant height and the number of branches significantly increased with increasing photoperiod.Conclusion: These results indicated that L. salicaria 'Dwarf Pink' could be categorized as an obligate long-day plant, and under the long-day conditions, the plant size did not affect the days to flowering. However, when the plant size was significantly large under long-day conditions, the number of inflorescences and the number of branches decreased, affecting the quality of flowering.