Relation between floral initiations on apical buds of basal shoots after the autumn and cultivar differences in unseasonal flowering of hydrangea

Typically, hydrangea (Hydrangea spp.) plants produce flowers from early to mid-summer. However, their basal shoots often continue to produce flowers from late summer to autumn, and we call this unseasonal flowering. The flowering frequencies and flowering period durations of basal shoots were studied using 23 hydrangea cultivars and lines in 2017, 2018, and 2019. The flowering frequencies of basal shoots were relatively high in ‘Christmas’, ‘Endless Summer’, ‘Ezo’, ‘Rosea’, and ‘Sumida-no-hanabi’ in each year of the study. The flowering period durations of basal shoots ranged from one to six months among the cultivars and lines studied. Basal shoots of ‘Endless Summer’ and ‘Rosea’ continuously flowered during each year of the study period. The basal shoots of ‘Hatsushimo’, ‘Jyogasaki’, ‘Maihime’, ‘Masja’, ‘Ms. Hepburn’, ‘Uzu’, and No. 5 never flowered after August in any of the study years. Using ‘Masja’ and ‘Rosea’, axillary buds expected to develop into basal shoots were studied for flower bud initiation in November 2018. Flower bud initiation was observed in 8.9% and 77.4% of axillary buds of ‘Masja’ and ‘Rosea’, respectively. The number of nodes produced before flower bud initiation in the buds ranged from 10 to 12 and 8 to 13 in ‘Masja’ and ‘Rosea’, respectively. The numbers of nodes corresponded to those of the basal shoots that flowered up to August 2019 in both cultivars. The number of nodes produced by basal shoots of ‘Rosea’ that flowered after August 2019 ranged from 14 to 19, which was higher than those observed for the axillary buds expected to develop into basal shoots in November 2018. In conclusion, the flowering period duration of hydrangea basal shoots differs among cultivars. The floral initiations on the apical buds of the basal shoots after the previous autumn largely contribute to unseasonal flowering occurrences after August in hydrangea.