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Home HortScience Pistachio Trees Chill Out in Australia
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VICTORIA, AUSTRALIA—Fruit and nut trees that thrive in temperate or subtropical regions rely on cool winter temperatures to produce flowers after winter dormancy. The right amount of nature's "winter chill" results in homogeneous and simultaneous flowering. When fruit and nut crops are grown outside their traditional growing regions, however, growers need additional information about the amount of chill required and that which is naturally available. To measure these critical conditions, researchers have developed "chilling models" that convert temperature records into a "coldness" metric. The models help growers select appropriate fruit and nut species and cultivars for the local climate and remediate unfavorable conditions through management practices. Though chilling models are useful, they are not completely accurate. 

Scientists are studying chill requirements in hopes of advancing Australia's emergent pistachio industry. Pistachio trees were first planted in southeastern Australia in the early 1980s; nut bearing began in the early 1990s, and full yield production was realized in the early 2000s. The region, classified as a "warm grassland" climate, often lacks the necessary winter chilling temperatures needed to produce consistent crops. To remediate these less-than-desirable conditions, growers can apply winter oil to trees—the oil application helps to promote even and timely budbreak. Timing is essential for this practice to be effective; growers in Australia must make the decision to apply oil before late August. Therefore, growers need a prediction of chill accumulation by mid-August to decide whether oil applications will be necessary.

"The pistachio industry in Australia requires a reliable model to determine the accumulation of chill to assist in orchard management decisions," said Jianlu Zhang of Australia’s Pistachio Growers’ Association. Zhang and colleague Cathy Taylor from Victoria's Department of Primary Industries, Australia, designed a research study to compare chilling models. "Currently used models treat the effect of hourly temperatures on chilling accumulation, in mathematical terms, as a time-homogeneous stationery process. In other words, they assume that 2 hours at the same temperature recorded at different times in a long-term observation contribute equally to the breaking of dormancy," explained Zhang and Taylor, whose study of chilling models was published in HortScience.

Zhang and Taylor evaluated three chill models using greenhouse work, historical data analysis, and field validation. The Chilling Hour, Utah, and Dynamic Models were studied to determine chilling requirement for 'Sirora' pistachio trees in southeastern Australia. The researchers tested and compared the models’ ability to calculate chill accumulation and to predict the necessity for winter oil application after mild winters.

"The Dynamic Model produced the best determination for fulfillment of chilling requirement with 59 chill portions. The required number of growing degree-hours above 4.4 °C from chill fulfillment to 50% bloom was 9633," Zhang noted. "Five years of practical application shows that 57 portions accumulated by mid-August works well. Although occasionally this model will underestimate the final chill portions, growers have still accepted it as a management tool that provides security in years when sufficient chill may not accumulate."

The project was funded by Horticulture Australia Ltd. using voluntary contributions from industry and matched funds from the Australian Government.


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:

The Dynamic Model Provides the Best Description of the Chill Process on 'Sirora' Pistachio Trees in Australia
Jianlu Zhang and Cathy Taylor
HortScience 46:420–425. [Abstract] [Full Text] [PDF]

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