Joe, a reader of this column, asked a pertinent question recently. “Will we have enough chill hours for our fruit trees this year?”
Dr. Esmaeil Fallahi, professor, research and extension pomologist at the University of Idaho Extension station in Parma, said he thought there’d be no problem in meeting the chill requirements of plants we grow here this year, but we could face damage later if frosts occur at the wrong stage of blossom-fruit development.
We’ve had a wonderfully mild winter this year, but the kinds of fruit we grow do require a certain number of chill hours before they’ll bloom, and after pollination, set fruit. Chill hours are defined by hours below 45 degrees F., but above 32 degrees. Some insist those chill hours must be above 34 degrees F. Temperatures of 60 degrees and higher (some say 55 degrees) subtract from the chill hours. I don’t keep track of chill hours, and I suspect professional orchardists in this area don’t either because we all rely on the University of Idaho agricultural extension department to keep track. So far we haven’t had to worry, but with climate change or warming we may have to be concerned. Usually growers in the southern tier of states and California are concerned.
The onset of vernalization, or dormancy, is triggered by a plant hormone, then fruit-bearing vines, trees and canes begin to rest. In late winter, when the weather warms, after a proper number of chill hours has occurred, the buds that were started the previous growing season swell and then break into flowers or foliage. If too few chill hours have occurred over the winter, the buds will intermittently break into flower or foliage, but not all at once as they’re supposed to. Later blooming fruit flowers are more susceptible to diseases such as fireblight, and may develop a poor set of fruit — small or deformed. The simultaneous flowering guarantees an abundance of pollen and full-formed fruit.
If the required chill hours haven’t yet been met and the weather turns unseasonably warm for a day or so, the buds won’t break into flower or foliage, because chill hour requirements protect. Chill hour requirements vary by variety or cultivar, and there are some that have what are regarded as “low chill hour” requirements, but once a vine, cane or tree is planted, chill hours for that variety are necessary for fruiting. It’s not something that can be changed, so if you think our winters will be warmer and warmer, select fruit-bearing trees, canes, vines or shrubs that have low chill hours, yet will survive zone 6 winters. Generally, most apples and cherries require 600 chill hours, but some apple varieties may only require 300 hours, for instance, so are suitable for growing in warmer climates than ours have been.
Some varieties of raspberries require as many as 800 hours of chill before flowering and producing fruit. I heard of a woman in Texas years ago who craved raspberries, so she ran up a huge bill with the iceman, who supplied her raspberry canes with ice. Her wealthy husband finally realized the cause of the high cost and canceled the iceman’s visits.
If you’re worried about your own fruit-producing canes, vines, shrubs or trees, search online for the fruitname, variety, chill hours — for example: peach, Elberta, chill hours.
The North Carolina State University Extension has a list of chilling hours for peach varieties: https://content.ces.ncsu.edu/chilling-requirements-of-selected-peach-varieties.
For apples, see https://www.grandpasorchard.com/_ccLib/attachments/pages/GO-Web-Chill+Hours+Chart-APPLE+(2013).
I’ll report the Idaho chill hours accumulated over the winter as soon as I receive the University of Idaho report.
Send garden questions to firstname.lastname@example.org or Gardening, The Statesman, P.O. Box 40, Boise, ID 83707.
Tip when starting seed indoors
If you’re starting seeds indoors, prevent damping off fungus by either bottom watering or spritzing with chamomile tea (room temperature). I use three tea bags in a 2-quart pot of water. This has worked well, but once the seedlings are transplanted outdoors, I must water with a watering can until the garden water is turned on.