Chilling Requirements

Pistachio Chilling Requirements – PGAI Updates (see below)

What are Chill requirements?

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.

Courtesy: ASHS Press Releases :American Society for Horticultural Science

Full Paper: The Dynamic Model

PowerPoint Presentation:  The Dynamic Model_Presentation

QLD Gov/HIAL Online Chill Calculator


This site accesses daily weather station data for the period 1968 to the current year for 600 locations across Australia. This data is used to make calculations for the accumulation of winter chill and growing degree days/hours.

Because the chill and growing degree day calculations are based on weather data from the Long Paddock SILO, the calculations can only be updated when the weather data becomes available. The weather data from yesterday is usually available after about midday (AEST). The Long Paddock SILO uses data form the Bureau of Meteorology and patches this to create complete daily data sets. Stations that no longer record temperature data are synthetically in-filled with gridded data derived from statistical analysis.

All calculations are performed using the chillR package of Luedeling et al. (2013) & Luedeling (2017).  The website has been developed by the QLD Government in conjunction with Hort Innovation.

Link to the Website Here:  Online Chill Calculator

 Pistachio Chill Bulletins by year including all regions.

2021 September 20th  Regional Chill Records – download here: Chill Newsletter No4_20210920

2021 August 3rd Regional Chill Records – download here: Chill Newsletter No 3 August 2021

2021 July 29th Regional Chill Records – download here:Chill Newsletter No 2 July 2021

2021 June 1st Regional Chill Records – download here: Chill Newsletter No 1 June 2021

2020 August 31st Regional Chill Records – download here: Chill Newsletter No 4_31st August 2020

2020 August 14th Regional Chill Records – download here: Chill Newsletter No 3_14th August 2020

2020 July Regional Chill Records – download here: Chill Newsletter No 2_31st July 2020

2020 June Regional Chill Records – download here: Chill Newsletter No 1_28th June 2020

2018 Regional Chill Records – download here:  PGAI Chill Newsletter No 1_5th July 2018

2017 Regional Chill Records – download here: PGA Chill hour Newsletter No 3 16th August 2017

2016 Regional Chill Records – download here:

2015 Regional Chill Records – download here: Chill Bulletin 2015 FINAL

2014 Regional Chill Records – download here: Chill Bulletin 2014 FINAL

2013 Regional Chill Records – download here: Chill Bulletin 2013 FINAL

2012 Regional Chill Records – download here: Chill Bulletin 2012 FINAL

2011 Regional Chill Records – download here: Chill Bulletin 2011 FINAL