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Home HortScience Relationship Clarified between Peach Chilling Exposure, Budbreak
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BYRON, GA—To accommodate unpredictable winter conditions, peach growers in the southeastern United States need to choose varieties that correspond with the chilling regime for their specific location. This practice helps to ensure that peach tress get adequate chilling for normal development but bloom late enough to avoid damaging frosts. For years, scientists have sought to establish a better understanding of the effects of temperature on dormant bud development. Although many horticultural "chilling models" have been developed that advance the understanding of the process, a model that can predict time of bloom across a wide range of environments is still needed.

A study in HortScience reported on an aspect critical for predicting peach bloom. According to scientists William R. Okie and Bryan Blackburn from the USDA–Agricultural Research Service, the relationship between chilling and heat requirement had been seldom studied in commercial peaches, yet is critical to modeling bloom in areas with mild winters. The team studied the effects of increasing chilling on peach floral budbreak by investigating peach growth in three unique environments: natural bloom in the orchard, weekly sets of cuttings forced in uniform conditions, and incremental supplemental chilling of cuttings followed by forcing.

"Our results suggested that the flower buds of a given cultivar vary substantially in chilling requirement," Okie said. "While younger, less mature buds may bloom later, the majority of flower buds on shorter twigs are set by midsummer in Georgia. The cause of their variability is unknown, but a range in chilling requirement can extend bloom, which increases the chances of fruit set in a moderate climate with spring frosts."

Most currently used chilling models consider dormancy break a sequential process: after appropriate chilling is received, a fixed amount of heat accumulation will produce bloom. The experiments showed that bloom time is a function of the chilling status of the bud (i.e., where it is on the exponential curve relating chill and heat, or how responsive it is to heat) as well as the amount and timing of heat accumulated in the orchard. "Once the critical chill has been reached, the relative proportions of chilling and heat that occur will influence how fast the bud proceeds to budbreak," Okie explained.

Okie and Blackburn noted that although their exponential curve concept is predictive with artificially chilled cuttings placed at a uniform forcing temperature, it does not address how the bud responds to heat interspersed with daily chilling, which in turn reduces the amount of further heat needed. "An understanding of how to calculate this heat accumulation may allow development of a more robust model for peach bloom that works across a wide range of environments and experimental treatments," they concluded.


The complete study and abstract are available on the ASHS HortScience electronic journal web site:

Founded in 1903, the American Society for Horticultural Science (ASHS) is the largest organization dedicated to advancing all facets of horticultural research, education, and application. More information at


Original Article:

Increasing Chilling Reduces Heat Requirement for Floral Budbreak in Peach
William R. Okie and Bryan Blackburn
HortScience 46:245–252. [Abstract] [Full Text] [PDF]

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