Heat Unit Model for Predicting Bloom Dates in Rubus

'Navaho' and 'Apache' blackberry plants were maintained at 10, 15, 20, 25, 30, or 35 C in growth chambers to determine optimum temperature for budbreak and flowering (fewest days to flowering). In a separate experiment, bloom dates were observed for a collection of 117 Rubus genotypes over four seasons. Using these phenological data, predictive linear and curvilinear models were tested using a range of cardinal temperatures. The growth chamber experiment indicated optimum temperatures for bloom were 25.6 degrees C for 'Apache' and 29.2 degrees C for 'Navaho'. For the field observations, time to bloom was best: defined by a linear model with base and optimum temperatures of 6 and 25 degrees C and a curvilinear model defined by base and optimum temperatures of 4 and 27 degrees C, respectively. Based on the linear growing degree hour (GDH) model, heat units to bloom varied among cultivars in the collection from 9,200 GDH for 'Chickasaw' to 18,900 GDH for 'Merton Thornless'.