Winter Chill Update: December 2016

Written by Bob Beede, UCCE Kings County, Emeritus.

Editors note: This article was developed with high chill crops, particularly pistachio. Almonds aren’t as chill dependent, but the broad topics discussed are relevant to all perennial horticultural crops.

Those of you who are outside every day know that it has been pretty warm.  The only cold weather occurred during the first week of December in which many locations within the Central Valley experienced three to five days of temperatures ranging from 30-32⁰F.  The Colusa CIMIS station reported 26⁰F the morning of December 6.  Table 1 provides the chill portions for various sites throughout the Valley between September 1 and December 13 for the past four winters, as well as 2010 in which over 70 chill portions were accumulated by February 15.  This exceeds the 58-60 chill portions estimated to satisfy the rest requirement of the Kerman cultivar. The Peters male may have a chill portion requirement as great as 65.  The values in parentheses are the total chill portions accumulated by station and year.  2013 and 2014 were significantly warmer than 2010, in which dormancy was well satisfied throughout all areas of the state.  The 2015 data shows good chill accumulation was well on its way throughout the Central Valley in mid-December, and continued cold temperatures through January contributed to the record 2016 crop.  In contrast, 2014 was already showing deficient chill accumulation at several locations by mid December.  The Arvin/Edison and Coalinga stations might be considered the “canary in the coal mine” for early assessment of future low chill winters.  As you can plainly see, chill portion accumulation is not good thus far, and in some locations, less than the 2014 winter.

Table 1. Chill portion accumulation for various CIMIS stations statewide from 9/1-12/13 for selected years. Numbers in parentheses are the total chill portions accumulated at each station by year from 9/1- 2/15.

A localized weather station would provide much more valuable data than a poorly maintained CIMIS station which may be used as a scratching pole by a white faced steer!  This should be a top priority purchase for high chill crops.

Reviewing the research and observations of Dr. Julian Crane, Dr. Louise Ferguson, and Bob Beede,  it all clearly states that Kerman and Peters do not grow normally when they do not receive adequate winter rest.  Our research efforts suggest Kerman requires 750 hours below 45⁰ F, and Peters 850 hours in order to leaf out and bloom promptly in the spring.  One experiment suggested that Peters continued to benefit from cold temperatures up to 1200 hours below 45⁰F.  It was also reported in these studies that a minimum of 500 hours below 45⁰ F was needed to initiate much bud break from Peters.  University of California Circular 179, “Deciduous Orchards in California Winters”, by W.H. Chandler and D.S. Brown (1936), states that December and January are the two most critical months in California to satisfy the rest requirement.  During the 2013 and 2014 winters, the unusually warm temperatures in January did not provide its complement of chill hours.

The effect of high winter temperatures is thought to be two-fold; they negate the effect of chill hours already accumulated by altering the complex physiological processes occurring during dormancy, and they elevate the bud respiration rate which consumes the limited amount of carbohydrates critical for spring growth.  UC Davis Plant Sciences Associate Professor Maciej Zwieniecki (Dr. Z) has joined our pistachio industry research team to study this important aspect of tree biology.  Dr. Z suggests there may be a critical amount of carbohydrates and other growth substances needed to produce normal growth in the spring.  This may explain why oiled trees performed so poorly in 2015.  Oil is thought to enhance rest breaking by causing a slight stress to the tree which is not phytotoxic.  In the process of metabolizing the oil, the tree increases its respiration rate, which renders it more responsive to favorable spring temperatures for growth.  Thus, the high January temperatures and oil treatment possibly had a compounded effect on carbohydrate depletion from elevated respiration.  When it came time for bud break, the deficiencies in both chilling and available sugars created the perfect storm for poor leaf out and fruit set.  There could have also been detrimental effects to male and female flower development and receptivity.  Because of the current uncertainty of this winter’s weather pattern, oil application is NOT being suggested at this time due to the negative impact it had on the 2015 season.

To check chilling, go to the “Weather-Related Models and Services” section of the UC Fruits and Nuts Center. Select “chilling accumulation models” from the menu, and then “Cumulative Chilling Portions”.  This site allows you to see the chill portion accumulation for every CIMIS station in the state. Historical data is also available, which is helpful in estimating current accumulation relative to other years. Chill portions and chill hours are found on this webpage.  Keep in mind that these stations were designed to accurately estimate water use, NOT chill accumulation.  The data is collected in an open grass-covered area which may influence the temperatures compared to those within the orchard environment.  The absence of fog also causes temperature differences up to 20⁰F between ambient (air) and the buds.  Obviously, we are concerned with the bud temperatures, so it would be helpful to make note of those warm, fog-less winter days.

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