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The time of rest completion of 'Apache', 'Arapaho', 'Chickasaw', 'Darrow', 'Kiowa', 'Navaho', and 'Shawnee' blackberry (Rubus subgenus Rubus Watson) buds was compared and various models for estimating chilling were evaluated. 'Kiowa' and 'Arapaho' buds had the shortest rest periods, while those for 'Shawnee', 'Navaho', and 'Chickasaw' buds were intermediate. 'Apache' and 'Darrow' buds had the longest rest periods. The model that ac- counted for the variation in percent budbreak among cultivars and temperatures during two dormant periods had the following two components: 1) a chilling inception temperature of -2.2 °C and 2) weighted chilling hours that accumulated after the chilling inception tempera- ture. The chilling hours in this model were weighted as follows: 0 to 9.1 °C = 1; 9.2 to12.4 °C = 0.5; 12.5 to 15.9 °C = 0; 16 to 18 °C = -0.5; >18 °C = -1. This study also elucidated that a blackberry model with a chilling inception temperature of -2.2 °C estimated chilling more accurately than one with chilling inception just after the maximum negative accumulation of chill units as used in the Utah chilling model. Also, temperatures between 0 and 2.4 °C must be weighted more heavily in a blackberry model than in the Utah peach model to ac- curately estimate chilling and rest completion. Many temperate zone fruit plants require a period of low temperatures during dormancy to grow vegetatively and produce fl owers in the spring. The number of hours necessary to fulfi ll the chilling requirement varies among fruit crops (Westwood, 1993) and cultivars (Dale et al., 2003). Several models have been used to estimate the chilling requirements and to predict the time of rest completion. In 1932, Hutchins proposed a model for peach that ac- cumulated chilling hours at temperatures ≤7 °C (Weinberger, 1950). Chandler et al. (1937) studied chilling of deciduous fruit trees, small fruit crops, and ornamentals, and Lamb (1948) examined effective chilling temperatures in 'Latham' raspberry. Both studies indicated that temperatures from 0 to 7 °C were most effective in satisfying the cold requirement. For peach, temperatures as low as 1.5 °C have been used to accumulate units in the Utah chilling model (Richardson et al., 1974), and temperatures as warm as 13 °C have been used to accumulate units in chilling models in Israel (Erez et al., 1990). As temperatures increase to 10 °C, about twice as many hours are required for rest completion of peach than at 6 o

A Chilling Model to Estimate Rest Completion of Erect Blackberries