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Chill hours: what are they, how do I count them, and why do my fruit trees care?
Oct 26, 2011 - Charlotte from Peaceful Valley
WHAT’S A CHILL HOUR?
You’re happily choosing your bare root fruit trees from our catalog when you suddenly notice extra numbers in the tree descriptions.
Number of chill hours? Isn’t it enough to know your USDA plant hardiness zone? That’s about cold temperatures—why do you need another number?
The USDA zone tells you the coldest temperatures in your area. Broadly speaking, the chill hours tell you how long the cold temperatures last.
The traditional definition of a chill hour is any hour under 45F.
But wait, there’s more. Academics have competing theories on what “chill” means. Some say the chill only counts if the temperature is between 45F and 34F. Some differ over chill calculations for the Utah model, and let’s not forget the new Dynamic model…. If this debate sounds like your cup of iced tea, follow those links.
HOW TO COUNT CHILL HOURS
Here’s the best way to count chill hours: get someone else to do it!
Luckily, there are institutions already tracking this information.
Farmers and gardeners in most California counties have access to official data on chill hours through the Pomology Weather Service at the Fruit & Nut Research Information Center. This service records chill hours—so you don’t have to.
If you’re not covered by this weather service, contact your local Master Gardeners and Farm Advisors to find out your local source of chill hour information.
CHILL HOURS ARE IMPORTANT TO FRUIT TREES
As you know, trees cannot walk away if they don’t like the weather—they have to stay and suffer through it. Nut and fruit trees (except for citrus) need a specific number of cold hours each winter to regulate their growth.
If a tree doesn’t experience enough cold in the winter the flower buds might not open at all in spring, or they might open unevenly.
Okay, you’re thinking, how about just planting trees with low chill requirements? That way they’re sure to get enough cold weather. Sorry, that won’t work. In our new video Tricia explains that a low-chill tree in a high-chill area would break dormancy too soon and be damaged, or even killed, by the cold weather.
So be sure to match your new bare root trees to your local chill hours—use the left menu at our fruit tree page and find trees sorted by chill hours. Look forward to an orchard that lives happily ever after.
For complete orchard information see the book California gardeners rave about, The Home Orchard, written by experts from the University of California.
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