Chilling Requirements in Chill Portions for California Crops

Part 3 in the series – What can we learn from the low chill winter of 2013-2014

In my last post, Counting Chill Better – Using the Chill Portions Model, I discussed how the chill portions model (also known as the Dynamic Model) is different from the chill hours model. Now that you can figure out how much chill you got in chill portions, how much do you need? Below is a table of the estimated chilling requirements of most of California’s major tree crops, and a few particular important cultivars. Most of the requirements listed below are based on scientific research. A few (*) are estimates based on how much chill we got last winter and how the crops responded in the spring and at harvest. If you are interested in the requirements of cultivars not listed here (especially for apricots, cherries, peaches and nectarines) or want to know the scientific reference information for a particular estimate, check out the more thorough version of this list at the UC Davis Fruit & Nut Center site.

Crop Chill Portions Requirement
Almonds 22-32
        ‘Nonpareil’ 23
Apples (‘Golden Delicious’) 50
Apricots 30-69
Cherry 30-70
        ‘Brooks’ 37


Peaches 8-75
Pistachios 54-65
        ‘Kerman’* 54-58
        ‘Peters’* 60-65
Prune (‘Improved French’)* 55-60
Walnut 38-72
        ‘Chandler’* 45-50
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7 thoughts on “Chilling Requirements in Chill Portions for California Crops

  1. I saw the chill portion number for Non pareil as 23 and range for almonds as 22 – 32. My question is what kind of yield did an orchard with a chill portion number of 23 have as compared to an orchard that had a chill portion number of 32 or higher. I have heard of orchards with chill portions in the 50s ending up with yields of over 3500 pounds/ac. I would appreciate your thoughts.

    1. Great question, Adam. There’s no research that has looked at this directly, but I’m inclined to think more chill could harm you more than help you. We know that broadly speaking the more chill the buds get, the more compact the bloom. The shorter the bloom window, the tougher it is to get all those flowers pollinated. Most of the studies that estimate chilling requirements are based on bloom success or bloom timing. Researchers cut a dormant branch and put it in a growth chamber to replicate spring temperatures. If it blooms in 2 weeks, they say it has met it’s chilling requirement. Then they go count how much chill accumulated in the field before cutting the branch. I did some research that looked back at county crop report data. I found that on a county basis, yields can be above average in California with chill as low as 22 chill portions (counting Nov 1- Dec 31). Whether doubling that chill could take you from average to stellar yields – there’s no research on that. If anyone reading this wants to send me 30+ years of block-specific yield data, I could take a crack at it. But there’s obviously a lot between bloom and harvest that can interfere with high yields, so it’s hard to say.

  2. Pingback: ‘Spring’ already? | Fruits and Votes

  3. Question: if I was to apply a rest breaking agent on Non-Pareil and Monterey varieties, how many chill portions would you target as an application date? I am working on a block of almonds in Kern Co., with the warm winters the Sonoras are blooming sooner than the Non-Pareils and Montererys with poor or any overlap. We have applied Surround to the Sonoras without measurable success to try to slow the bloom. So this year I thought we would try to speed up the N.P. and Montereys.

    1. Ron,
      I am trying to get an answer for you, but which rest breaking agent are you planning to apply?

      We have done limited work with rest breaking agents due to minimal effect in plant response. Generally, the strategy described above would only assist with pollinating the Sonoras as Sonora pollen is already in the hive for the Nonpareil bloom. In contrast, it may negatively effect the Nonpareils and Montereys due to an accelerated bloom. I would approach with caution.


  4. Hi,
    Could you say me what’s the chill portions requirement for grapes?
    and where can I found the requirements for each variety of grapes?


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