Chill hours at Highlander station, Vista CA

I’ve started this thread for two purposes … perhaps more will arise. (1) a recording of chill hours for those who live nearby, and (2) evidence for those who think USDA cold hardiness zones are the only thing that matters for fruit production.

I have a serious weather station on my property. So far this winter (Nov. 1 2017 to Jan. 22 2018) I’ve recorded the following chill.

Chill Methods:

  1. Hours below 45F = 41.0
  2. Hours between 32F and 45F = 41.0
  3. Utah method = -391.5

Utah calculations:
hours below 34F 0
hours between 34F and 36F 0
hours between 36F and 48F 115
hours between 49F and 54F 327
hours between 55F and 60F 495
hours between 61F and 65F 324
hours above 65F 508


I think (as expected) we have a lot more chill hours up here:
Below 45 Model: 540 chill hours
Between 45 and 32 Model: 513 chill hours
Utah Model: 575 chill units

I think the only thing I can say definitively about chill hours in SoCal is that none of the models work. I’m somewhat close to the Irvine field station, which has recorded 50 chill hours so far this year and has been under 100 in 3 of the last 4 years. Yet I still get healthy stone fruits rated 400 hours or so year after year. There just seem to be other inputs beyond chill that the scientists don’t understand. Unfortunately, with no stone fruit commercially produced down here, no one is likely to do the research to find out more.


That’s because it’s not just about chill hrs. Length of the dormant season is just as important for cherry, apricot, and to some extent pluots. Peaches and nectarines are mostly dependent on chill hrs. A short winter works for them if they get enough chilling.

I’m sure that’s not all involved but it’s part of the answer.

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And yet I have bumper crops of them every year.

I didn’t elaborate that very well. High chill peaches are pretty dependent on chilling. They need their hrs or they don’t leaf out well or have normal fruit. But once they get their chilling they are ready for a rapid warmup. The high chill cherries and apricots need their chilling and then a period of time with mild temperatures in order for the flowers to develop properly. They need time, ie a long dormant season.

The medium and low chill stone fruits are another matter. The dependency on a long dormant season has been bred out of them as far as I can tell. But I have less experience in the low chill arena because I’ve been able to manufacture chilling with my greenhouse setup.


SE PDX 1100 - 1500 hours to date, depending on model.

Just before Christmas, before winter really, we had 10 consecutive days with every hour between 32 and 45. That adds up fast. Now at mid-winter, we just had 5 consecutive days with zero chill hours.

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Below freezing for 5 days straight?

Or above 45F for 5 days straight.

The last three days have been 40-51F and constant cloud cover. That’s still very good chilling by any method other than the below 45. In Vista it would be cause for a party. Cloudy weather is better chilling than the same temperature with clear skies. The buds heat up by day in sunny weather and cool off at night below ambient.

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Very true, and likely a dimension missing from dormancy models.

Great idea. Nice work Richard!

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Above 45 degrees 5 days straight. Average historical low for the dates is 36.

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Chill Methods, 11/2017 through 2/2018
Last update: 03/01/18

  1. Hours below 45F = 112.0
  2. Hours between 32F and 45F = 112.0
  3. Utah method = -409.5 (negative)