Flower development and effects of a cold treatment and a supplemental gibberellic acid application on flowering of Helleborus niger and Helleborus x ericsmithii

The increased horticultural interest in Helleborus requires the development of techniques for flowering control. Therefore, we have determined flower initiation and differentiation under natural conditions for Helleborus niger L. and the hybrid Helleborus x ericsmithii B. Mathew. Once flower primordia were present in the underground buds of the plants, the effect of a cold treatment (336, 504 or 672 chilling units (CU)) and a supplemental gibberellic acid (GA3) application (500 mg L−1) on pre-cooled plants was examined. The cold treatment resulted in a progressive decrease of the time to flower. Moreover, when pre-cooled plants received a supplemental GA3 application early flowering was further enhanced and these plants had a higher number of flowers with longer flower stems and larger flowers.

► Histologicial sections illustrate the shift from a vegetative to a generative development in Helleborus during late summer-early autumn.
► Cold treatment after floral differentiation promoted an earlier flowering in Helleborus niger and H. x ericsmithii.
► A supplemental treatment with gibberellic acid on pre-cooled plants further enhanced this earliness and increased flower number, flower diameter and flower stem length of Helleborus niger and H. x ericsmithii.