David Doll3 CommentsDecember 23, 2020Horticulture

Tracking Winter Chill in Fruit Growing Regions

Written by Franz Niederholzer1 and Kitren Glozer2 1UC Farm Advisor, Sutter/Yuba Counties, 2Associate Project Scientist, Department of Plant Sciences, UC Davis “Is it a good chilling year?”  That’s a common coffee house question this time of year in fruit and nut growing regions.  The amount of chilling a perennial crop accumulates in a given winter will influence bloom conditions – the most crucial time in a crop season.  Less chilling than a certain tree crop requires can lead to an extended bloom; too little altogether can result in bud death and drop in some crops like apricot and sweet cherry.  ‘Good chilling’ (more chilling than the minimum required) can produce a very short bloom season, or ‘snowball bloom’.  Chilling models can provide growers and their advisors with key information to time dormancy-breaking materials (hydrogen cyanamide, oil, etc.) to manipulate bloom to improve or maintain yield and quality in regions where local chilling is marginal for a certain crop.  Climate change may make achieving critical chilling more important in years to come.  In this article, we’ll briefly review chilling and chilling models.  Deciduous perennial crops break bud in the late winter or spring after a certain amount of cold weather (chilling) followed by a certain amount of warmer weather.  Think of this process as a relay race.  The chilling is the first leg of the race, and the heat accumulation is the second leg.  Bud break is the finish line.  The length of each leg of the race depends on crop and variety.  The weather during the race influences the runner’s speed.  Good chilling weather speeds up the chilling leg, warmer weather can slow it down or even stop it.  Cool weather on the warming leg slows the progress towards bloom.  Since most perennial crops are not native to North America,

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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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