Submitted to: HortScience
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/19/2008
Publication Date: 11/20/2008
Citation: Black, B., Frisby, J., Lewers, K.S., Takeda, F., Finn, C.E. 2008. Heat unit model for predicting bloom dates in Rubus. HortScience. 43(7):2000-2004.
Interpretive Summary: One way to reduce crop loss due to spring frost is to plant blackberry varieties that bloom after the latest expected frost date for the farmer’s location. No information is available to help farmers make blackberry cultivar choices based on their flowering dates. We recorded flowering and fruiting dates of 125 blackberry varieties and breeding selections in the field for three years in addition to spring air temperatures. We also recorded flowering and fruiting dates of two blackberry varieties grown in controlled environments. We tested a new mathematical model that includes accumulated hours between critical air temperatures that predicted the flowering time for different blackberry varieties. Researchers, state extension agents, and nursery owners can now input local spring air temperatures into this model to determine bloom dates for blackberry varieties for the farmer’s location. This information is useful for selecting varieties that will not flower until after the threat of frost is low.
Technical Abstract: Plants of Navaho and Apache blackberry cultivars were maintained at 10, 15, 20, 25, 30 or 35 degrees C in growth chambers to determine optimum temperature for anthesis (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 of 25.6 degrees C for cv. Apache and 29.2 degrees C for cv. 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 curvilinear growing degree hour (GDH) model, heat units to bloom varied among cultivars in the collection from 10,600 GDH for cv. Chickasaw’ to 22,300 GDH for cv. Merton Thornless.