Two mechanisms of remontant flowering in Hydrangea macrophylla

T. Harraß, M. Gundermann, C. Tränkner
Hydrangea macrophylla is a perennial shrub, which is used as ornamental crop plant due to its large and impressively colourful inflorescences. Transition from vegetative to reproductive growth occurs under cool temperatures and short-day conditions in autumn. Flower primordia stay in dormant buds over winter and blooming occurs in spring of the next season on apical tips of two-year-old shoots. In contrast, remontant H. macrophylla cultivars initiate flowers also on newly emerged, one-year-old shoots resulting in a continuous flowering phenotype. In this study, we determined the floral initiation on two-year and one-year old shoots of the remontant cultivars ‘Diva fiore’ and ‘Mak20’ and the non-remontant cultivar ‘Libelle’. At first flowering time, we found apical floral initiation in all three cultivars. Additionally, we detected gradual lateral flower formation. The highest lateral floral initiation rates were found at the 1st and 2nd node of generative shoots of the remontant cultivars, whereas lower nodes of generative shoots and all nodes of shoots without apical flowers produced vegetative buds. These lateral flower buds will bloom after withering of apical flowers, which results in a remontant, free-blooming phenotype. Next, we studied de novo floral initiation on newly emerged, one-year old shoots as second mechanism of remontancy. We found that ‘Diva fiore’ and ‘Mak20’ initiated new apical flowers during the growing season after plants had been pruned down to the rootstock after first flowering, yielding in flowering rates of up to 72 and 57%, respectively. Based on these results we confirmed a de novo floral initiation in the remontant cultivars ‘Diva fiore’ and ‘Mak20’ resulting in a re-blooming phenotype.
Harraß, T., Gundermann, M. and Tränkner, C. (2023). Two mechanisms of remontant flowering in Hydrangea macrophylla. Acta Hortic. 1383, 235-242
DOI: 10.17660/ActaHortic.2023.1383.28
floral initiation, flowering, free-blooming, re-blooming

Acta Horticulturae