Chill Unit and Growing Degree Hour Requirements for Vegetative Bud Break in Six Apple Rootstocks

Estimates were determined for chill unit (CU) and growing degree hour (GDH) requirements for vegetative bud break in 6 apple (Malus × domestica Brokh.) rootstocks: Antonovka 313, MM 111, MM 106, M.7a, M.26, and M.9. Rooted layers were lifted in the fall, potted, and kept in a cold room at 4°C for various lengths of time. plants then were moved to a greenhouse, and the percentage of bud break was determined for various GDH intervals. Prediction equations were determined for the percentage of bud break vs. chill unit accumulation and growing degree hour accumulation. M7a had the lowest chill unit and growing degree hour requirements for 50% bud break (590 CU and 4278 GDH). MM 106 required the most chilling (1220 CU), and M.26 the highest number of growing degree hours (6138 GDH) for 50% bud break.