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The chilling requirements of the University of Arkansas blackberry cultivars Apache, Ouachita, and Prime-Jim*, and the primocane-fruiting selections APF-25, APF-27, APF-40, APF-42, APF-44, APF-46, APF-52, and APF-53 were investigated using stem cuttings from field-grown plants. A biophenometer was used to measure chilling (hours below 7 °C) in the field and 12-node cuttings of lateral shoots were taken from the cultivars every 100 hours up to 1000 hours below 7 °C. However, only 500 chilling hours had occurred at the time of this writing, and the response of budbreak to higher chilling levels could not be reported. The cuttings were placed in a mistchamber in the greenhouse with a daylength of 16 hours and air temperature of 26–29 °C. Percent budbreak was measured weekly. The cultivar × chilling interaction was significant ( P  = 0.05). `Apache' and `Ouachita' showed little or no budbreak up to 500 h, indicating a higher chilling requirement. The chilling requirement of Prime-Jim was determined to be between 300 h and 400 h, and that of the APF selections appeared to be between 300 h and 500 h. The chilling requirement of APF-53 could not be determined since budbreak was consistent at all levels of chilling up to 500 h. In general, the primocane-fruiting genotypes appeared to require less chilling than floricane-fruiting `Apache' and `Ouachita', and they would therefore be more suitable for low-chill locations.

CHILLING REQUIREMENTS OF SELECTED FLORICANE AND PRIMOCANE-FRUITING BLACKBERRIES