effects of chill hour accumulation on hydrogen cyanamide efficacy in rabbiteye and southern highbush blueberry cultivars

A controlled environment study was conducted to evaluate the effects of chill hour accumulation on the time of application and the resulting efficacy of the plant growth regulator, hydrogen cyanamide (H2CN2) in both rabbiteye (Vaccinium asheii Reade) and southern highbush (V. corymbosum) blueberry cultivars. Application of H2CN2 at the interval in which accruement of 75% of the individual chill-hour requirements of 'Bladen', 'Jubilee', 'Premier', and Tifblue' blueberry cultivars resulted in greater vegetative bud break than the 50% chill-hour application timing, or than their untreated checks. The 75% timing also resulted in a significant increase in the terminal growth of stems in 'Bladen' and 'Premier'. Consideration of a blueberry cultivar's exposure to chill-hours in application timing decisions should provide a greater degree of precision in optimizing vegetative bud break with H2CN2.