Fruit Tree Chilling Requirements - March 2, 2005
Jeff Schalau, County Director, Agent, Agriculture & Natural Resources
Arizona Cooperative Extension, Yavapai County
This time of year, many gardeners are contemplating which deciduous fruit tree variety they should purchase. New varieties are being released each year and the catalogs tempt gardeners with irresistible descriptions. Before choosing a fruit tree variety, you should know something about chilling requirements, how they are used to select appropriate fruit tree varieties.
Deciduous fruit trees need a certain amount of winter chilling to break down growth inhibitors in flower and vegetative buds. Many nurseries provide an estimate of the chilling requirement (also called chill hours) needed for the tree to be success in a given climate regime. Varieties vary by chilling requirement and varieties are recommended based on the "average" number of winter chill hours a given area receives over the years.
The classic definition of chilling requirement is the number of hours the temperature is below 45 degrees F and above 32 degrees F. More recently, it has been suggested that hours above 65 degrees F be subtracted from the previous calculation. While it sounds feasible at first, the National Weather Service does not routinely calculate and publish these statistics. To further complicate this issue, there are many horticulturists whom are in the process of rethinking how chilling requirements should be calculated. Given all this difficulty and disagreement, the fruit trees themselves are usually very forgiving. The real trick is to know that there is a chilling requirement and what is the appropriate “ballpark” figure for your area.
There are some observable symptoms to look for in fruit trees that have chilling hours incompatible with the local climate. Fruit trees with a lower chilling requirement than necessary frequently experience crop loss due to early bloom and a late spring frost damages the crop. In these situations, the chilling requirement was exceeded. Conversely, planting varieties with higher chilling requirements than required can result in uneven bloom and/or delayed foliation. In these cases, the chilling requirement was not met. Remember chilling requirements are based on averages and calculated averages are made up from extremes and each chilling requirement has some latitude.
After searching for new information on this inexact topic, I found no reliable new information for Arizona. So based on experience, I provide some ranges of chilling requirements for most areas of Yavapai County by elevation range. For elevations above 6,000 ft, look for varieties with chilling requirements above 1,000 hours. For elevations between 4,000 and 6,000 ft, look for varieties with chilling requirements between 750 and 1,000 hours. For elevations between 2,500 and 4,000 ft, look for varieties with chilling requirements between 500 and 750 hours. Remember, you probably have some flexibility here depending on your orchard site/microclimate.
Remember that stone fruits (peaches, apricots, cherries, plums, etc.) are typically earlier to bloom than pome-type fruits (apple, pear, quince) and will produce crops less frequently in areas prone to late spring freezes. Always buy high quality fruit trees from reliable sources. I have always preferred bare root trees over containers.
When purchasing a fruit tree, look at the labels or packaging to find the chilling requirement for that variety. You can ask the nurseryperson to assist you in locating this information. Additional information about fruit tree varieties and their chilling requirements can be accessed at the Dave Wilson Nursery web site: www.davewilson.com. This site has chilling requirements and descriptions of many varieties.
Naming of companies or products is neither meant to imply endorsement by the author nor criticism of similar companies or products not mentioned.
The University of Arizona Cooperative Extension has publications and information on gardening and pest management. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 ext. 14 or E-mail us at firstname.lastname@example.org and be sure to include your address and phone number. Find past Backyard Gardener columns or submit column ideas at the Backyard Gardener web site: http://cals.arizona.edu/yavapai/anr/hort/byg/.
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