Understanding importance of chill hours for fruit trees

Gail W. of Sonoma asks: I am looking to purchase a few fruit trees and have heard people at the rare fruit tree grower’s sale in Santa Rosa last January talk about fruit tree chilling requirements. What does that mean?

Most fruit trees lose their leaves in the fall and go dormant, meaning they stop growing and hibernate over the winter to get through the freezing temperatures. A dormant fruit tree will not start growing again, which includes flowering and setting fruit, until it has experienced a certain amount of cold temperature that is equal to its minimum “chilling requirement,” followed by a certain amount of warm weather.

Fruit tree chilling requirements can vary quite a bit from one variety to another. If a variety’s chilling requirement is considerably lower than the normal winter temperatures where it is planted, one nice warm spell might be all it takes to end its dormancy and have it bloom too early. If that happens, the flowers are vulnerable to freeze damage. If the chilling requirement is much higher than the average winter temperature, blooming will most certainly be delayed and erratic, with perhaps, little or no fruit set.

For a productive fruit tree, in most climates, a variety’s chilling requirement should approximately match the amount of chilling that area has. There are many apple varieties that will set fruit without getting the required chilling requirement. But, the fruit quality and color depend on cool nights in summer and in the fall.

There is a simple and widely used method for quantifying fruit tree chilling, used by Dave Wilson Nursery, one of the largest wholesale growers of deciduous fruit, nut and shade trees in the United States, located east of Modesto in the Sierra Nevada foothills, near the town of Hickman. It is the hours below 45 degrees - the Below 45°F model, which equates chilling to the number of hours at temperatures below 45 degrees occurring in the dormant period: autumn leaf fall to spring bud break. These hours are termed “chill hours.” As chilling is complex and difficult to measure precisely, published fruit tree chilling requirements are necessarily approximate or estimated.

Varieties that are better adapted to colder climates will usually have chilling requirements of 800 to 1,000 or more hours. “Low-chill” varieties are adapted to the warm-winter climates and are defined at Dave Wilson Nursery as requiring 500 or fewer chill hours. For coastal Southern California, low-chill varieties are considered to be those requiring less than 300 hours.

When choosing fruit tree varieties, refer to your favorite retail nurseries and the Sonoma County Master Gardeners (sonomamg.ucanr.edu) for local fruit growing information and ideas.

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Tom K. of Windsor asks: We do a lot of barbecuing with charcoal briquettes. I was wondering if we could toss the ashes into the garden or use as a mulch?

Do not put the ashes from your charcoal barbecue into the compost pile, garden, or incorporate it with soil for containers. Charcoal has ingredients added to it to help in the burning process, and they are ingredients you do not want added into your garden soil. Wood ashes from the fireplace are okay, but use in moderation.

Dana Lozano and Gwen Kilchherr are garden consultants. Send your gardening questions to The Garden Doctors, at pdgardendoctor@gmail.com. The Garden Doctors can answer questions only through their column, which appears twice a month in the newspaper and online at pressdemocrat.com.