Katherine Pope8 CommentsJanuary 22, 2015Horticulture

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.

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Katherine Pope7 CommentsJanuary 13, 2015Horticulture

Counting Chill Better – Using the Chill Portions Model

Part 2 of 3 in the series – What can we learn from the low chill winter of 2013-2014 In my last post, Is Last Year’s Warm Winter the New Normal?, I discussed low chill winters like last year’s coming more often in the near future. The first step to preparing for those warmer winters is counting chill better – moving from counting in chill hours to counting in chill portions. What makes the chill portions model (also called the Dynamic Model) better? There are three basic difference between the chill hours model and the chill portions model. Chill hours counts any hour between 32°-45° F as the same. Chill portions gives different chill values to different temperatures. No more wondering about the value of ‘warm’ chill hours. Temperatures between 43°-47° F have the most chill value. The chill value on either side of that range are lower, dropping to no value at 32° F and 54° F. Chill hours only counts up to 45° F. Chill portions count up to 54° F. This makes chill portions better able to approximate how the trees we grow, most of which evolved in fairly mild climates, count chill. Chill hours does not subtract for warm hours. Chill portions can. The math is tricky, but the concept is simple: Chill portion accumulation is a two-step process. First, a ‘chill intermediate’ is accumulated, but can be subtracted from if cold hours are followed by warm hours. Second, once the chill intermediate accumulates to the certain threshold, it is converted into a ‘chill portion’ and the chill intermediate count starts over from zero. The chill portion cannot be undone by later warm temperatures.

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Katherine Pope4 CommentsNovember 8, 2014Horticulture

Is Last Year’s Warm Winter the New Normal?

Katherine Pope, UCCE Farm Advisor Sacramento, Solano and Yolo Counties Part 1 of 3 in the series – What can we learn from the low chill winter of 2013-2014 With harvest wrapped up, it’s a good time to take stock of the impacts of the warm winter of 2013-2014. Average chill was down 25% in the Central Valley, falling behind in January and never catching up. Orchards in many crops showed classic symptoms of low chill – delayed and extended bloom, poor pollinizer overlap and weak leaf-out. Prolonged bloom likely resulted in some cherries, pistachios and prunes experiencing warmer bloom temperatures, which decreased yields for many. Drought-related water stress likely contributed to some of the yield, size and quality issues we saw at harvest. But low chill was almost certainly responsible for a great deal of the unusual tree behavior, low yields and poor quality. So what can we learn from this tough year moving forward?

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