Temperature and photoperiod control of morphology and flowering time in two greenhouse grown Hydrangea macrophylla cultivars

Knowledge of the factors involved, and tools to control morphology and flowering are important in intensive and cost-efficient greenhouse production. Hydrangea macrophylla is an important flowering pot plant in Norway and is produced year-around in greenhouses. Due to problems in scheduling, a study was conducted to compare floral transition and morphology of two commercially important cultivars of Hydrangea (‘Early Blue’ and ‘Schneeball’) under different flower initiating treatments in growth chambers. Plants were grown with high pressure sodium lamps (HPS) at moderate temperature (17 °C) (MT) and high (24 °C) temperature. At high temperature, the effect of (1) irradiance under long day conditions (16 h lighting with 70 or 200 μmol m−2 s−1), and (2) short day (8 h lighting) was investigated. The short day treatment had similar light integral as the low irradiance long day treatment (SD: 8 h × 140 μmol m−2 s−1 and LD: 16 h × 70 μmol m−2 s−1 = 4.0 mol m−2 d−1). The intention was to test the effect of irradiance and SD on flower transition and morphology under high temperatures. The results clearly showed that MT is the strongest signal for floral transition. MT resulted in a rapid floral transition of the terminal buds and lateral flower buds. A short forcing period was required and the plants became short and compact without any use of chemical growth retardants. At high temperatures only SD had a promotive effect on flower transition and the response was found to be stronger in ‘Schneeball’ than ‘Early Blue’. In general, all the treatments under high temperatures required a long forcing time and the plants tended to be very tall with a low number of lateral flower buds.

Research highlights▶ Most of the ornamental pot plants production in Norway occurs in greenhouses. ▶ During the last few years energy prices have increased a lot and made the growers more concerned about energy consumption. ▶ It has become more and more important in intensive and cost-efficient greenhouse production of ornamentals to be able to accurately control flowering time. ▶ Thus, basic knowledge on the factors involved in the flowering process and better tools to control the process is necessary to be able to enhance the competitiveness of the Norwegian pot plant production. ▶ In this paper we show that moderate temperature is the fastest and safest way to control flowering and shoot elongation in Hydrangea macrophylla. ▶ The light climate is of minor importance.