What's a Chill Hour?

You're happily choosing your blueberries from our catalog or website when you suddenly notice extra numbers in the 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 45°F. But wait, there's more. Academics have competing theories on what "chill" means. Some say the chill only counts if the temperature is between 45°F and 34°F. 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 Master Gardeners and Farm Advisors to find out your local source of chill hour information.

Chill Hours are Important to Blueberries

As you know, blueberry bushes cannot walk away if they don't like the weather -- they have to stay and suffer through it. Blueberries need a specific number of cold hours each winter to regulate their growth. If a blueberry bush 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 blueberries with low chill requirements? That way they're sure to get enough cold weather. A low-chill blueberry in a high-chill area would break dormancy too soon and be damaged, or even killed, by the cold weather.

More Blueberry Information

Will you be planting your blueberry bushes in rows for a you-pick farm? You'll appreciate Grow the Best Blueberries, an info-packed booklet from the Storey Wisdom series. Are you hoping to plant blueberries as a hedge, or mixed in as an edible part of your landscape? Find creative ways to use blueberries in The Fruit Gardener's Bible. Be sure to match your new blueberries to your local chill hours -- use the left menu at our blueberry bush page (clicking on Filters) and then click on Chill Hours. Look forward to blueberries that live happily ever after.