Cold Enough for Your Deciduous Fruit Trees?

Advice from the Help Desk of the
UC Master Gardeners of Contra Costa County

On October 26th, we posted a blog (“Your Fruit Trees Need Their Chill Time”) updating information on “chill hours” in Contra Costa County. Using the simplest prediction model to forecast blossom and fruit set success, University of California (UC) measures and summarizes the number of chill hours below 45^OF from November 1 to the following February 28/29^th. UC and others' studies have shown that the number of chill hours below 45^OF is important to properly bring deciduous fruit trees out of their dormancy to blossom and set fruit. The data in that earlier blog was from the winters of 2000-2001 to 2014-2015 utilizing the 5 UC CIMIS (California Irrigation Management Information System) weather stations located in or adjacent to the County. (We also included Alameda County's Pleasanton CIMIS station to provide for interpolation for San Ramon Valley.)

As explained in the earlier blog, without adequate chill hours and depending upon the fruit tree, blossoming can be erratic and fruit set can be significantly reduced. Our earlier blog provides lots more information on the importance of chill hours for your fruit trees. The previous blog also explains how you may be able to estimate the chill hours in your garden from the UC CIMIS weather stations and possibly even down to your neighborhood using Private Weather Station data available on the web (see the blog comments and the web's GetChill.net).

Most likely as a consequence of the ongoing drought, winter last year (2014-2015) was considered a “warm winter” compared to our “normal” winters. Chill hours were quite reduced for most of the County and many deciduous fruit trees had erratic blossoming and poor fruit set. (Note: the data for the Concord CIMIS station is somewhat counter to this. As of the posting of this blog, we don't have an answer to that and are still looking into the reported data consistency).

So, what about this winter? Is it or will it be cold enough for good fruit production by your deciduous fruit trees? We reviewed the CIMIS data for this winter from November 1^st through Saturday, December 26^th. (What else would a Master Gardener be doing the day after Christmas?) The table below is the results of that analysis.

Station & CIMIS #	Chill Hours <45OF
	Nov 1st - Dec 26th		Average Nov 1st to Dec 26th	Seasonal Average (Total)
	2015	2014	2010-2014	2010-2014
Brentwood #47	339	82	319	837
Concord #170	485	725?	490	1102
El Cerrito #213	186	13	89	191
Moraga #178	630	265	443	1160
Pleasanton #191	489	164	419	1030

Except for the above-noted inconsistency of the Concord CIMIS station, the data would indicate that half way through the chill hour “period” we are seeing a typical “cold winter” with a good prognosis that if the “norm” holds, we should see adequate chill hours. However, there are many factors at play in that forecast. For example, another warm winter will keep the chill hours down as will clear weather that will also bring warm daytime temperatures but countered with cold night temperatures.

You can also check GetChill.net and possibly see what your nearby PWS chill hours are. Use of the PWS data comes with a caveat though that the data is often not as complete as the UC CIMIS stations.

Are you a gambling gardener? Is it going to be a cold or warm winter? Are we going to get enough chill hours to have a successful fruit set… let alone enough water?

Help Desk of the UC Master Gardeners of Contra Costa County (SIM)

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Note: The  UC Master Gardeners of Contra Costa's Help Desk is available year-round to answer your gardening questions.  Except for a few holidays, we're open every week, Monday through Thursday for walk-ins from 9:00 am to Noon at 75 Santa Barbara Road, 2d Floor, Pleasant Hill, CA  94523. We can also be reached via telephone:  (925) 646-6586, email: ccmg@ucanr.edu, or on the web at http://ccmg.ucanr.edu/Ask_Us/ MGCC Blogs can be found at http://ccmg.ucanr.edu/HortCoCo/ You can also subscribe to the Blog (//ucanr.edu/blogs/blogcore/blogroll.cfm).