Cold Enough for Your Blueberries?

Advice for the Home Gardener from theHelp Desk of the
UC Master Gardener Program of Contra Costa County 

Client's Request:  I'm going to plant blueberries in my garden, probably in large pots to maintain the low pH (acidity) soil needed. I also understand that blueberries need cold winter hours to produce good crops. Can you tell me how many cold winter hours are needed by blueberry variety and where I might find the information on what the typical cold hours are here in the County.

Help Desk Response:  Thank you for contacting the UC Master Gardener Program's Help Desk with your question. Winter cold hours needed for various fruiting plants and trees are defined as “chill hours”. Chill hours are monitored by UC Davis at 4 sites in the County at El Cerrito, Moraga, Concord, and Brentwood (see map right).

Blueberry bushes need a minimum number of chill hours by variety each winter to enhance their fruiting. Chill hours are defined as hours less than 45°F but above 32°F. If a blueberry bush doesn't experience enough chill hours in the winter, the flower buds might not open at all in spring, or they might open unevenly.  In either case, fruit production will be reduced.

We have found the best resource for calculating chill hours is the Chill Calculator on the UC Davis Fruit and Nut Research and Information Website at:
http://fruitsandnuts.ucdavis.edu/Weather_Services/chilling_accumulation_models/. 

UC Davis CIMIS Monitoring for Chill Hours

Depending on where you are in the County, your chill hours may be lower or higher than the nearest tracking station. For example, the closer to the River or Bay, you are (e.g. Martinez, Crockett, Richmond, El Cerrito), the milder the climate and the lower the number of chill hours. If you are further inland, the Brentwood, Concord, or Moraga stations would be more appropriate estimations of your chill hours with some locations with evben higher chill hours. Chill hours can also vary by year; the drought years saw much lower chill hours for most locations due to “mild” winter weather. Because of this, our “resident expert” on blueberries recommends that you may want to choose types that require lower chill hours for success, e.g., between 400 – 600 hours of chill time. You want to choose cultivars that have the best chance of long term success.

Below is a list of some “Low Chill” Southern Highbush Varieties that have been found to do well in our USDA Zone 9B. There are, of course, many additional cultivars available that you can choose from with similar chill hours to these:

Emerald:  250 Chill Hours, Zone 8-10
Jewel:  200 Chill Hours, Zone 8-10
Jubilee:  500 Chili Hours, Zone 5-9
Misty 300 Chill Hours, Zone 5-10
Southmoon 400 Chill Hours, Zone 6-9
Oneal 600 Chill Hours, Zone 5-9
Sunshine Blue 150 Chill Hours, Zone 5-10
Reveille - 500 to 600 chill hours. Zone 6-10

You may not be able to find some of these varieties in your local nurseries, but there may be others with similar potential. You may also be able to mail order the bushes, but my limited personal experience found that many out-of-state nurseries won't ship blueberries into California.
 
Here is a link to additional UC information about growing blueberries:
http://ucanr.edu/sites/gardenweb/berries/blueberries/

Finally, here is a link to some additional information about chill hours in our area posted in our blog almost a year ago along with links to earlier posts. We will be updating it when the “2016-17 chill hour season” ends February 28th. http://ccmg.ucanr.edu/HortCoCo/?blogstart=66&blogasset=12496

Good luck on your blueberries. Please let us know if you have any additional questions we can help you with.

Help Desk of the UC Master Gardener Program of Contra Costa County (SLH)

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Note: The  UC Master Gardeners Program 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/CCMGBlog/).