Correlation of winter chill hours with harvests from deciduous fruit trees

I am experimenting with inserting an Excel spreadsheet directly into my blog (see below, bottom of post). It works so-so. I can’t alter the width of the columns after I paste the image, so the winter chill hours (number of hours below 45 degrees) that each fruit variety requires are kind of quirky looking.

Avocado and citrus trees do not require chilling. Our avocado tree is about to burst forth in bloom.

Avocado and citrus trees do not require chilling. Our avocado tree is about to burst forth in bloom.

In this spreadsheet, I separated the deciduous fruit trees (top of spreadsheet), which require winter chilling to set fruit, from the citrus and avocado trees (bottom of spreadsheet), which require no chilling, and subtotaled them separately.

We had good winter chill in 2011-2013, moderate chill in 2014, and a pathetic 76 hours of chilling during the 2014-2015 winter. The harvest of fruit from the deciduous trees appears to correlate better with the amount of winter chill than the citrus harvest–no surprise. I am not expecting very much deciduous fruit in 2015 due to the low chill this winter. My August Pride peach has a pathetic 10 blossoms on it, and my Katy Apricot is blooming early with very few blossoms. The Asian pears, apples, nectarines, and Babcock peach are still dormant.

Our Florida Prince peach tree has nearly finished blooming, and is setting a good crop of peaches.

Our Florida Prince peach tree has nearly finished blooming, and is setting a good crop of peaches.

Most of my fruit trees were planted 2007-2014. Some (persimmon, pomegranate, Gala apple) are too young to bear fruit yet, and some have not yet reached their good bearing years. And some years the birds and night critters get to the fruit before I do. So this is an experiment in progress.

Bees are busy pollinating our lime trees, which are currently in bloom. The lemons and oranges aren't far behind the limes.

Bees are busy pollinating our lime trees, which are currently in bloom. The lemons and oranges aren’t far behind the limes.

I hope you can see the chill hours in the Required Chill Hour column.

My conclusion, based on very limited data and a lot of speculation, is that I can expect the warmer winters brought to us courtesy of global warming to have a negative effect on my harvest of deciduous fruit.

# of trees TREE Variety req chill hrs 2015 2014 2013 2012 2011 2010
  (actual chill hrs)   76 166 497 389 340 95
1 Apple, Fuji 200-400
2 Apple, Gala 200-500
1 Apple, Granny Smith 400 14 60
1 Apricot, Katy 200-300 65 8
1 Asian Pear, 20th Century 300-400
1 Asian Pear, Shenseiki 250-300 121 78
1 Asian Pear, 4 varieties grafted 250-400
1 Blackbery, thornless 100-500
2 Grape, Red Flame seedless 100+
1 Nectarine, Panamint 250 18 56
1 Nectarine, Snow Queen 250-300 37
1 Peach, August Pride 100-200 8 16
1 Peach, Babcock Improved 250-300 5 44
1 Peach Florida Prince 150 150 252 108
1 Peach, Garden Gold 450-500
2 Persimmon, Fuyu 100-200
1 Plum, Santa Rosa 300-500 80 9
1 Pomegranate, Wonderful 100-200
  SUBTOTAL Deciduous   0 271 557 301
1 Avocado, Littlecado 0 133 74 196.5
2 Lemon, Eureka 0 14 51.5
1 Lemon, Meyer 0 321 95 205
2 Lime, Bearss 0 127 385 106.5
1 Orange, Navel 0 460 329 544
1 Orange, Valencia 0 5
  SUBTOTAL Zero chill fruits   0 1041 897 1108.5  
Total oz TOTAL FRUIT in oz 0 1312 1454 1409.5
TOTAL LBS TOTAL FRUIT in lbs   0 82 91 88

About Lou Murray, Ph.D.

I'm a professional writer/photographer, avid gardener, and active environmentalist living in southern California. I am retired from writing a weekly newspaper column on environmental topics in the Huntington Beach Independent, but I am still teaching at the Orange County Conservation Corps. This blog chronicles my efforts to live a green life growing as much food as possible for my husband and myself on a 4,500 sq ft yard that is covered mainly by house, garage, driveway, and sidewalks.
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8 Responses to Correlation of winter chill hours with harvests from deciduous fruit trees

  1. daphnegould says:

    For us it is the cold and windy conditions which can kill buds that affects how much fruit we get. That and our late freezes. If the weather is perfect when the tree blooms, then we get a good fruit set, otherwise not.

  2. Ghislaine says:

    I’m in SoCal too (San Diego) and I’m seeing the same type of phenomenon, although I’d wager we got even fewer chill hours than you state. Anyway, I get the impression my more established trees are adapting better to those lousy winters, do you see a difference between your younger and older trees?

    • Ghislaine, sorry to be so late to reply. My trees are all still fairly young, and I see them bearing more as they get older. I just learned about winter chill hours, so it may be some time before I have any meaningful data on this. The UC Davis website should provide the chill hours nearer to you. The closest one to me is Irvine. That may be the only datapoint for Orange County, not sure. We are nearer the coast than Irvine, so I’m not sure how that affects the chill hours.

  3. Norma Chang says:

    Do all grapes require chilling hours or certain varieties? Hope you get fruits from some of your deciduous fruit trees.

  4. Margaret says:

    As Daphne said, our issue is usually late freezes but last year I think a lot of my buds were killed off during the winter by the abnormally cold temps. This winter seems to be no different and looks like we are heading towards a record breaking February with lows well below what we normally get. So even though it is for completely opposite reasons, looks like I may also be headed towards another fruitless (or very sparse) year.

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