Chilling requirement for breaking dormancy and flowering in Paeonia lactiflora 'Taebaek' and 'Mulsurae'

Dormant rootstocks of Paeonia lactiflora 'Taebaek' and 'Mulsurae' (native Korean cultivars) were stored at three chilling temperatures (constant 0, 5, or 10A degrees C) for different durations (0, 3, 6, 9, or 12 weeks) to determine the chilling requirements for dormancy breaking and flowering. For both cultivars, percent sprouting, numbers of shoots and flowers, and height during flowering increased as the plants were exposed to lower chilling temperatures or longer chilling durations, whereas the time required for sprouting gradually decreased as the duration of the chilling treatment increased. However, there were no significant differences in these parameters between the two cultivars when they were chilled for 9 weeks or more. A chilling treatment at 0A degrees C for 6 weeks [1,008 hours chill unit (CU)] produced more number of shoots and flowers and required fewer days to achieve sprouting and flowering than at 5A degrees C for 6 weeks (706 hours CU). Temperatures of 5A degrees C and 10A degrees C afforded only 70% and 40%, respectively, when compared to a cumulative chill unit at 0A degrees C. 'Taebaek' required more chilling to sprout consistently and achieve its potential shoot numbers than 'Mulsurae'. Chilling for 6 weeks at 0 and 9 weeks at 5A degrees C (1,058 hours CU) were necessary to break dormancy and to induce flowering in P. lactiflora 'Taebaek' and 'Mulsurae', respectively.