As the days shorten and the temperatures fall, many bulbs, perennials and trees, especially fruit trees, enter a dormant or rest period. The external sign that dormancy is occurring in trees is the onset of changes in leaf color and later leaf fall. This is evidence that photosynthesis and visible growth are progressively being shut down. Once trees enter dormancy each year, they are better able to withstand the cold winds, frost and freezing temperatures associated with winter.
The amount of cold required by a plant to resume normal growth following the cold of winter is commonly referred to as its chilling requirement. Plant and tree species vary widely in the amount of chill necessary to break dormancy. Each type of tree has a unique requirement to reach a given level of bud, flower and fruit development. Some varieties of apple require as many as 1,800 hours while other cultivars can break dormancy with less than 400 hours of chill. An index to determine the amount of chill a plant receives is based on monitoring the number of hours the temperature dips below 45 degrees Fahrenheit.
The resting period is broken satisfactorily if or when the chilling requirement is met and temperatures begin to climb above 40 degrees. Normal growth, flowering and fruiting resume in the spring as the level of growth regulating promoters in flower and vegetative buds slowly increases and the level of growth regulating inhibitors decreases.
If a plant or tree does not receive the required number of cumulative chilling hours to suspend dormancy, the fruit crop produced that year will be small, and the fruit possibly misshapen and reduced in size. Symptoms, such as sparse growth, delayed bloom development, flowering that never results in full-sized fruit and reduced fruit quality are symptomatic of insufficient chilling.
Rising temperatures caused by climate change may reduce winter chill enough to cause serious problems for fruit and nut tree yields, especially those grown in the warmer growing areas in California. Researchers at the University of California at Davis and the University of Washington analyzed data on hourly and daily temperatures from 1950 and 2000 in California, and projected trends for later in the 21st century. Their findings provide evidence that California’s winter chill has declined substantially. In fact, by 2100, orchards in California are expected to experience less than 500 chill hours per winter.
Many species are only marginally suited to the environment in which they are grown, and even small changes in temperature patterns may become a problem. Farmers in California are especially worried about the effect climate change will have on pistachios and many commonly grown varieties of plums and peaches.
Inasmuch as Marin County has a Mediterranean climate, and thus relatively moderate winter temperatures, low chill cultivars of apples, pears, pomegranates, quince, nectarines, Japanese plums, persimmons and peaches — which require a minimum amount of winter cold (300 to 500 hours) to produce a good crop — perform best here. For information on chilling requirements of specific plants, go to homeorchard.ucdavis.edu and click on fruit and nuts.
Farmers and gardeners in most California counties have access to official data on chill hours through the Pomology Weather Service at the UC Davis Fruit & Nut Research Information Center. This service records chill hours. You can also contact the UC Marin Master Gardeners or farm advisors to access a local source of chill hour information.
In recent years, low chill varieties (less than 300 hours) of quince, fig, persimmon, almond, olive, blueberries and pecan have been developed. The best advice is to ascertain the typical number of chill hours for your garden setting and select plants with lower chill hours (i.e. figs instead of plums) and/or cultivars that need fewer chill hours.
To prepare for the impacts of climate change, more effort is needed to breed cultivars with lower chilling requirements, to develop tools to cope with insufficient winter chill, and to better understand the temperature responses of trees and plants.