Growing-degree-days (GDDs) in a uniform phenology scale (BBCH) for two walnut ( Juglans regia L.) cultivars

Descriptors are widely used for monitoring the phenology and pomology of plants. Among them, the BBCH (Biologische Bundesanstalt, Bundessortenamt und Chemische Industrie) scale is the most important. We evaluated the growth of 'Chandler' and 'Persia' walnut cultivars to create a BBCH descriptor for the crop, and estimate the growing-degree days (GDDs) of different growth stages. The GDDs (°C-days) were measured using 0°C as a base temperature (Tb) for male flowering and 5.5°C for other phenological stages in both cultivars. Growth stages were divided into eight principal stages, each with multiple substages. Stage 0 includes the bud sprouting, while stage 9 occurs when the leaves develop and the tree enters the dormancy. Both cultivars had a similar heat unit requirement at the bud quiescent stage (84 and 71°C-days for 'Chandler' and 'Persia', respectively); however, 'Chandler' needs higher GDDs accumulation to reach the harvest (3876°C-days compared to 3539°C-days for 'Persia'). The results suggest that 'Persia' can be cultivated in colder areas at higher altitudes or latitudes compared with 'Chandler'.