Prediction of flowering rose shoot development based on air temperature and thermal units

Abstract

Developmental events of rose shoots were observed on Rosa hybrida ‘Cara Mia’ plants under a wide range of air temperature and light conditions. The dates of occurrence of ‘bud break’, the unfolding of each leaf, the occurrence of ‘visible flower bud’, and suitability for harvest were observed for 152 shoots while hourly averages of air temperature and photosynthetic photon flux density (PPFD) were recorded continuously. Temperature affected the rate of rose shoot development while mean daily integrated PPFD levels did not. Thermal units (heat units) were computed and found to be suitable for tracking most phases of rose crop development. The base temperature, needed for computing thermal units, was found to be 5.2°C. While the duration of the phase from cutting to bud break could not be predicted reliably using thermal units, the duration from bud break to either visible bud or harvest was predicted reasonably well. The thermal units were 260 ± 30 and 545 ± 45°C day, respectively. The occurrence of unfolding of each leaf was also found to occur at specific thermal unit thresholds.