Predicting Flowering of Apricot Cultivars Using Growing Degree Days

Heat requirement for flowering in apricot cultivars was estimated from field data using Growing Degree Days method (GDD). Ten years of phenological and temperature data for 42 apricot cultivars of diverse geographical origin and different flowering time, grown in Belgrade region, were used to conduct the study. Base temperatures for different cultivars were obtained by the RMSE (the smallest root mean square error between the observed and predicted number of days) method. The base temperatures calculated by this method differed among cultivars from -2.8 to 0.2 degrees C. GDD was calculated from 1 January to the beginning of flowering using base temperatures obtained by RMSE method, the Null method (base temperature set to 0 degrees C for all cultivars) and Fixed Value method (base temperature set to -1 degrees C for all cultivars). The best agreement with observations in terms of mean absolute error (MAE) was attained by RMSE method (MAE=2.7 days). However, the Null method (MAE=3.2 days) and Fixed Value method (MAE=3.0 days) gave also very good results. Taking -1 degrees C for the base temperature (the mean value of all cultivars studied), the accumulated GDD for flowering varied from 332 (in 'Tyrinthos') to 425 (in 'Stella').