The chill hour map is a result of research done at the University of Maryland. Use the map to estimate the chill hours for your area. Chill hours are roughly the number of hours between the temperatures of 32-45 degrees fahrenheit. Winter hours above 60 degrees are subtracted from the totals.
The idea is that a deciduous plant goes dormant in the cold winter to protect itself from the cold. The plant needs to stay dormant while the weather is freezing and then know how soon after it gets above freezing it can safely start growing. It must do it late enough so it doesn't get frozen back by a late frost but early enough so it can get a full season of growth and fruiting in before it must go dormant for the next year. The plant has a process, refined over millenia of evolution that tells it when to start growing in the spring and that process accounts for the amount of above freezing temperature (chilling hours) it needs.
Of course when we play with mother nature and grow plants in climates where they are not native, we run into lots of problems and this is one of them.
The following is an estimated list of chill hours needed by most of the varieties offered by Raintree Nursery. Most chill hour lists give such a broad range for each fruit as to not be of much use. At the risk of being sometimes incorrect, we have summarized the data to give an estimate that will be useful. Unless you wish to experiment, we recommend making sure you have several hundred more chilling hours than are listed to increase your odds of success. Many low chill varieties take less chill hours than listed below. For instance our low chill apples Anna and Dorset Golden need only 100-200 chill hours. Our low chill blueberries Sunshine Blue needs 150 and Misty 300. While there are low chill kiwi, raspberry, peach, apricot and almond varieties, these varieties are not offered by Raintree Nursery. These estimates are taken from the books "Temperate Zone Pomology", "Your Edible Landscape Naturally", "Growing Fruits and Nuts in the South," and from the Dave Wilson fruit guide. For instance Temperate Zone Pomology states that the chilling hours for apples is 700-1800. However growers in California have had success growing apples with much fewer chill hours. In Southern California, Kevin Hauser at Kuffel Creek Apple Nursery has shown that many apple varieties which were thought to need 800 or more hours thrive at about 400 hours and very high summer temperatures in the inland valleys of Southern California. Among the varieties he has had success with are Rubinette, Honeycrisp, Fuji, Pristine, William's Pride, Bramley, Arkansas Black and Red Boskoop.
TYPE OF FRUIT CHILL HOURS
Apple 400-1000 (Low chill varieties are less)
Japanese Pear 400-500
Blueberry (Northern) 800
European Pear 600-800
European plum 800-900
Japanese Plum 300-500
Plum Cot 400