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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David Doll13 CommentsDecember 14, 2016Horticulture

Sun Reflecting Products for Increased Winter Chill? 

Written by Bob Beede (UCCE Kings County, Emeritus) and David Doll (UCCE Merced) Tests of winter applied kaolin clay or calcium carbonate-based materials intended to either reflect solar radiation or diffuse it continue.  Results from David Doll, UCCE Farm Advisor, Merced County, and Valley Orchard Management, showed an increase in chill portion accumulation and a 200 to 250 pound increase in CPC yield over untreated trees when kaolin-clay was applied prior to the 2015 season. More can be found here.  The data thus far suggests that spraying these materials to mitigate the negative effects of warm winter temperatures does not assure you of a normal crop, but it might prevent NO crop! This past winter, Carl Fanucchi and Bob Beede collaborated with ORCAL, the company which manufactures ultra-fine, dry ground calcium carbonate, which is marketed in liquid form as Mask® and Diffusion®. They performed UNREPLICATED screening trials in two locations; one in Buttonwillow, and the second east of Highway 99 on Pond Road.  The screening trials included single and double applications applied January 12 and February 12.  A December treatment was planned, but the field could not be accessed.  Flower bud temperatures were monitored in treated and untreated areas using tiny thermocouples inserted into the buds without causing their death.  The resulting data showed bud temperatures were reduced by as much as 100F, and the rate of heating during the morning hours was also slower.  Calculations indicate that the January treatment increased chill portion accumulation by about 13%, due to the lower bud temperatures.  Weekly rating of the treatments for bud break and bloom were also performed.  The treated trees emerged more evenly, and the second treatment of Diffusion applied in February delayed development by four to five days.  The single January treatment developed at about the same rate as the untreated. 

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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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David Doll2 CommentsApril 17, 2014Horticulture

Pros and Cons of Earlier, Warmer Spring

A few discussions this week around an “earlier” than normal year for insects and nut development. We are somewhere around 10-12 weeks post bloom in the Northern San Joaquin Valley. Nut developing is progressing as expected. We have experienced higher than normal temperatures during stage one of growth which may lead to sizing issues – especially in later blooming varieties (i.e. ‘Fritz’) – similar to what we saw in 2013. Endosperm development should begin or has begun in most ‘Nonpareil’ and pollinator orchards. I suspect that kernel fill will be earlier, leading to an earlier ripening/hull-split/harvest timing.

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David Doll5 CommentsJanuary 4, 2013Horticulture

Winter Chill Reduction from Climate Change

I came across these papers discussing the reduction of winter chill due to climate change within the central valley of California: 1. Luedeling E, Zhang M, Girvetz EH (2009) Climatic Changes Lead to Declining Winter Chill for Fruit and Nut Trees in California during 1950–2099. PLoS ONE 4(7): e6166.  2. Baldocchi and Wong (2008). Accumulated winter chill is decreasing in the fruit growing regions of California.  Climatic Change. March 2008, Volume 87, Issue 1 Supplement, pp 153-166.

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