The relatively warm weather we’ve been having has prompted several grape growers to ask whether the San Joaquin Valley will accumulate enough chilling hours, the number of hours with temperatures below 50 degrees, this winter. The short answer is 'yes.'

In fact, the San Joaquin Valley typically meets the chilling requirements for grapes by the end of December.

Why are chilling hours important?

As grape canes mature, their buds enter a type of dormancy in which their growth is suppressed despite otherwise favorable conditions.

Repeated exposure to cold temperatures dissipates this form of dormancy and once the chilling hour requirement is satisfied, it is only low temperatures which prevent bud burst. The number of chilling hours required by grapes varies among the different varieties, but most grapes only need about 150 chilling hours. This is much less than that required by other temperate fruits such as cherries or peaches which may need up to 800 hours of chilling.

How to calculate chilling hours

In California, we typically calculate chilling hours from Nov. 1, through the end of February. Cumulative chilling hours are calculated by either summing the hours below 45 degrees or those between 32 degrees and 45 degrees. The hours can be recorded with your own weather station or by using one of several Web sites which calculate hours from weather stations nearest your vineyard. One convenient Web site for California’s growers is the Fruit and Nut Research Information Web site. It draws on data from the CIMIS weather station network to calculate the chilling hours for several locations in California.

You may want to consult the Web sites listed below for additional information on chilling hours. Clearly insufficient chilling hours are unlikely to limit growth next spring, but a lack of precipitation could be a problem. Currently, the San Joaquin Valley has only received 1.0 (as of Nov. 26) inches of rain, slightly over half of our normal season to date of 1.79 inches. The dry 2008 spring and hot summer have left our soils dry and few precipitation events to date have not improved the situation. If 2008 continues to be dry, growers may experience grapevine damage caused by freezing temperatures much like that seen during the 2006-2007 winter and spring. If you have not already done so, a post-harvest irrigation is recommended.

Additional information on chilling requirements at:

— Fruit and Nut Research Information: http://fruitsandnuts.ucdavis.edu/

— Weather_Services/UC IPM: http://ucipm.ucdavis.edu/WEATHER/wxretrieve.html

— CA Irrigation Management Information Systems (CIMIS): http://wwwcimis.water.ca.gov/cimis/welcome.jsp