Chilling Hours

 

Chilling, or exposure to cold, is necessary in many flower and fruit bearing plants, so that they will produce flowers and/or fruit season after season. Plants vary in their need for chilling, and the cold temperatures to which they need exposure also vary. For example, peonies need to be in soil at or below 32 degrees F for a time before they will flower. Tulip bulbs and hyacinth also need a cold treatment in order to flower. Some day lilies are temperature (chilling) dependent. I have such a variety in my landscape. When Santa Maria has a cold winter, they flower in the spring. When there is a mild winter, they only produce a crop of beautiful green leaves.

 

Such is also the case with fruit trees. If the tree does not get adequate chilling, it will usually bloom over a long period, and then most or all of the fruit may abort and all you have is a shade tree for that season.

 

The physiological process of chilling and what happens inside the plant is still not fully understood. However, for real life purposes scientists have established that chilling hours in fruit trees begin to accumulate when temperatures go below 45 degrees F. Chilling hours continue to build while temperatures are between 45 degrees F and 32 degrees F. No chilling accumulates below freezing (32 degrees F). Also the chilling hours remain constant even though temperatures get above 45 degrees F, but chilling hours are lost if the temperature gets above 60 degrees F. So, you add one chilling hour for every hour that temperatures are between 45 and 32 degrees during the day-night temperature cycle. However, if the temperature gets above 60 degrees F during the day, one-half hour of chilling is lost from the total accumulated. Chilling hours calculations are only kept between November 1 through March 30. This is the usual dormant season for California.

 

The above temperature perimeters make the calculation of chilling hours a complex process, and accurate temperature records are needed to determine each season’s chilling hours. Here on the Central Coast we have many days when temperatures are above 60 degrees F, even though our nights are cold. As a result chilling can be erratic, and the total accumulated is not as high as we might expect.

 

Official weather stations in Santa Barbara County, where temperatures are tracked to use for chilling hour calculations, are Cuyama – 1378*, Goleta Foothills – 0, Santa Barbara – 186, Santa Ynez – 1039, and Sisquoc – 698. My horticulture advisor colleague tells me that Santa Maria accumulates about 480 hours, and San Luis Obispo – 227 hours.

 

When you purchase fruit trees (peach, nectarine, apricot, apple, pear, plum, i.e., any stone or pome fruit), select varieties at or below the chilling hour requirements that accumulate in your area.