Phenological growth stages of ‘Barcelona’ hazelnut (Corylus avellana L.) described using an extended BBCH scale

A wide range of hazelnut (Corylus avellana) cultivars are produced around the world. Cultivars are often listed by the time of bloom, from early to mid and late season. To date, phenological stages have been recorded using alphanumerical codes, although data analysis is difficult with such codes. This study converted the alphanumerical codes to fully numerical codes using the Biologische Bundesanstalt Bundessortenamt und Chemische Industrie (BBCH) scale. A hazelnut BBCH scale was created for the cultivar 'Barcelona', which is considered a reference for phenological stages. The scale was constructed with at least two digits (0 to 9) for the main growth stages and three digits for female flowering and fruit development. The first number indicates the general growth stage, the second number indicates the growth stage at a specific time, and the third digit, when present, indicates the proportion at that stage on the tree. The hazelnut BBCH scale describes in 38 stages within 7 of the 10 available stages. The BBCH stages used are 5 (inflorescence emergence), 6 (flowering), 0 (bud development), 1 (leaf development), 7 (fruit development), 8 (fruit ripening), and 9 (senescence). The hazelnut is unusual in that it flowers in winter. Thus, the scale was extended from flower initiation (spring to summer of the year before harvest), male and female flowering (winter before harvest) and bud and leaf development (spring before harvest). During the spring before harvest, and summer before harvest, fruit developmental stages and fruit ripening are monitored and finally, senescence finalises the hazelnut life cycle in autumn after the harvest. Thus, the BBCH hazelnut cycle has a duration of at least 18 months and gives a good idea of hazelnut development. Finally, this new scale will add new information on the kernel development stages which were not evaluated in the former phenological scale. It will help producers to have a better idea of the fruit ripening and potential harvest date.