As the days shorten and temperatures fall below 50 degrees, many bulbs, perennials and trees, especially fruit trees, enter a dormant or rest period. Leaf fall is the external sign that dormancy has occurred. As the leaves fall, photosynthesis and visible growth are progressively shut down. After trees become dormant in the fall, they are much better able to withstand cold weather.Ê
The amount of cold needed by a plant to resume normal growth following winter is commonly referred to as its chilling requirement. A fair index as to the amount of chill a plant receives is gained by monitoring the number of hours the temperature dips below 45 degrees.
Plant and tree species vary widely in the amount of chill necessary to break dormancy. Each type of fruit tree, for instance, 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 require less than 400 hours.
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.Ê
Dormancy protects the vegetative and fruit buds from the potentially damaging effects of the winter cold. As long as there have been sufficient chilling temperatures during the winter, flower and leaf buds develop normally. If a plant does not receive the required number of chilling hours to suspend dormancy, the fruit crop produced that year will be small, and the fruit itself possibly misshapen and reduced in size. Symptoms, such as sparse leaf growth, delayed bloom development, flowering that never results in full-sized fruit and reduced fruit quality are symptomatic of insufficient chilling.Ê
Marin County fits into Sunset zones 15, 16 and 17. The winter low temperatures in these three zones typically range from 28 to 21 degrees, 32 to 19 degrees and 36 to 23 degrees, respectively. However, according to Pam Peirce in the revised edition of her book, "Golden Gate Gardening," the chill factor in our central California coastal region ranges from fewer than 100 to 900 hours. Most coastal areas get up to 400 chilling hours. Areas more than a few miles from the ocean receive 400 to 900 hours.
If your garden is blocked by hills, shaded in winter, in a cold pocket or frequently shrouded by winter fog, it may get additional winter chill. Notably, temperatures of 70 degrees or above for four or more hours actually negates any chilling the plant received during the previous 24 to 36 hours.
As Marin has a Mediterranean climate, and thus relatively moderate winter temperatures, the cultivars of apples, pears, peaches, nectarines, plums and cherries - which require a minimum amount of winter cold to produce a good crop - often do the best here. Consequently, an ideal fruit variety in Marin is one that possesses a chilling-hour requirement that is satisfactory for the microclimate in your garden. (For more information on chilling requirements of specific plants, visit http://home orchard.ucdavis.edu/ general-tree.html)
In July, researchers at the University of California at Davis and the University of Washington published a study that gives winter chill projections for all of California. The authors expressed concern that as the global climate warms, California will no longer be suitable for growing many fruit and nut crops unless new tree cultivars with reduced chilling requirements and new management strategies for breaking dormancy are developed for years of insufficient winter chill.
Researchers analyzed data on hourly and daily temperatures from 1950 and 2000, and projected trends for later in the 21st century for 18 climate scenarios. Their findings provide evidence that California's winter chill has declined substantially.
Based on the data, the authors predict that winter chill will have declined from the 1950 baseline near 1,000 hours by as much as 60 percent by the middle of the century and by up to 80 percent by the end of the century. By the year 2000, winter chill had already declined to the point that only 4 percent of the Central Valley was still suitable for growing apples, cherries and pears, all of which have a high demand for winter chill. In general, orchards are experiencing a loss in winter chill that ranges between 50 and 500 hours a decade.
In recent years, low-chill varieties (less than 300 hours) of quince, fig, persimmon, almond, olive, chestnut and pecan have been developed. Thus, gardeners should learn the number of chill hours typical for their garden setting and select plants with lower chill hour requirements (such as, figs instead of plums) and/or cultivars of plants that need fewer chill hours.