Fruit & Nut Research and Information Center
Fruit & Nut Research and Information Center
Fruit & Nut Research and Information Center
University of California
Fruit & Nut Research and Information Center

Using the Chill Predictor in Prunes

About Chilling & Dormancy | Using the Chilling Predictor | GO TO THE MODEL

Franz Niederholzer, UC Farm Advisor, Sutter/Yuba Counties

Bloom at the wrong time, due to extreme weather at bloom or miss-matched pollenizer flowering, can significantly reduce the harvested crop and grower income. Advancing bloom is a tool that may help fruit and nut growers in California spread risk from hot or freezing temperatures at bloom or poorly timed pollenizer flowering.

Hot or freezing temperatures at bloom can damage a prune crop, significantly reducing grower income. Advancing prune bloom is a tool that can help prune growers in California spread risk from hot or freezing temperatures at bloom.

A heavy oil application (4 – 5 gallons of narrow range horticultural oil/acre) at the right timing will advance prune bloom by several days in most years. The traditional dormant spray timing to advance bloom is late December to mid-January. Oil can be applied alone, or combined with a pesticide for peach twig borer and aphid control. Recent research shows that timing a dormant oil spray after a certain amount of chilling let’s you fine tune your dormant spray timing.

The most consistent prune bloom advance occurs when oil is sprayed after 30 – 40 chilling portions accumulate. This target (30 – 40 chilling portions) usually falls in the traditional dormant spray window for advancing bloom — late December to mid-January. The 30 – 40 CP target usually lasts 2 weeks in an average Sacramento Valley winter. This chilling predictor will help you plan a dormant season oil spray. It will not guarantee excellent bloom advance or a great crop. That depends on the temperature after application. But it is your best bet to consistently advance bloom year in and year out.

Top of page

Steps in Using the Chill Predictor:
  • Use the Chill Predictor starting December 1, when, on average, cool weather and steady chilling begins.
  • Select the CIMIS weather station that best represents weather in the orchard.
  • Select the target chilling level that you want: 30 CP, 35 CP, 40 CP, etc.
  • The Chilling Predictor will tell you:
    1. The earliest date that your CP target will be reached based on perfect chilling weather. This date will be earlier than your target actually will be, but it lets you know how much guaranteed cushion you have before your target CP date.
    2. The earliest date that your CP target will be reached based on the average chilling accumulation from Dec 1 to Jan 31 in the last 20 years. It could be a little earlier, it could be a little later.
  • Knowing earliest dates for your CP target could help you plan spraying, pruning, etc.
  • Recheck the Chilling Predictor as the predicted date gets closer.

Please Note:

  • Don’t apply a dormant spray containing a pesticide within 48 hours of a predicted rain when orchard soil is saturated. Do everything in your power to keep pesticides out of surface water.
  • Weather after dormant spray will affect bloom advance. If it is a warm winter, bloom can be advanced as much as 2 weeks. If it is a cool winter, bloom advance may be 2–4 days.
  • Advancing bloom with oil doesn’t guarantee a good prune crop. It lets you spread your risk from heat or freezing temperatures at bloom.
  • Avoid tree damage from "oil burn". Don’t spray high rates of oil when soil or trees are dry.
  • Plan to get bees earlier than you would for trees that are not treated with oil.

Why Use the Chill Predictor After December 1st?

  • The coldest months of the year in the Sacramento Valley are December and January.
  • Chilling accumulation in December and January is constant. It is a straight line through this period.
  • In November, chilling accumulation per day is generally less and more variable than in December. If you use the Chilling Predictor before December 1, you will overestimate the earliest day to spray. That is, the model will predict an earliest time to spray that is later than if you waited until December 1 to run the model. The whole purpose of the Chilling Predictor is to provide you with a “no earlier than” spray target date. By December 1, chilling accumulation is generally at a constant rate and then the earliest estimated spray day can accurately be predicted (see figure below).

Average Chilling Portions accumulation over time using 15 years data from Nicolaus CIMIS weather station
Average Chilling Portions accumulation over time using 15 years data from Nicolaus CIMIS weather station

Top of page

Webmaster Email: fruitsandnuts@ucdavis.edu