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Chill hours: what are they, how do I count them, and why do my fruit trees care?

Oct 26, 2011 -
   
  Chill hours: what are they, how do I count them, and why do my fruit trees care?
Luscious peaches like these start with the right number of wintertime chill hours.
 
   

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.


Categories: Nut Trees, Fruit Trees, Apple Trees, Pomegranate Trees, Pluot Trees, Plum Trees, Persimmon Trees, Pear Trees, Peach Trees, Nectarine Trees, Multi-Graft Trees, Mulberry Trees, Jujube Trees, Fig Trees, Cherry Trees, Apricot Trees, Quince Trees, Edible Landscaping, Organic Gardening 101


Anonymous Says:
Oct 28th, 2011 at 5:42 pm

My Dwarf Minnie Royal Cherry and Dwarf Royal Lee Cherry I received last January both sprouted leaves by the end of January in FL. At the base of the leaf stems there were tiny red dots that I presume may be cherries when the trees mature. They needed lots of water in the dry season, but have both grown several feet over the summer and it is now near the end of October. I also have two plum trees that both have flowers now, far out of season. Over the winter, my young apple trees never lost most of their leaves, and instead the leaves froze at night and then thawed out during the day and they are still on the trees and green, just older. The low chill apples, Bartlett Pears and a Golden Delicious Apple grew taller but all grew hardly any new leaves. They all seem to be making their own adjustments as much as possible to the low chill,very hot/dry, and hot/wet climate.

hope Says:
Jan 28th, 2012 at 7:11 am

Hi,
I live in West Central Florida (Chassahowitzka).  Did the cherry trees produce fruit for you?  If so, awesome.  I’d love to find a cherry variety that will produce here too!  Please let me know if you’ve had success, and which varieties worked best for you.  (Also, if you’re an organic grower in our area, would love to trade local heirloom seeds with you!)
Thanks,
Hope

Charlotte, Peaceful Valley Says:
Dec 21st, 2012 at 7:22 pm

Thank you for your garden news!

Charlotte from Peaceful Valley Says:
Jan 28th, 2013 at 12:56 am

hope,

We have 6 varieties of cherry trees that grow in USDA zone 10, including the 2 that Anonymous is growing. Take a look http://www.groworganic.com/seasonal-items/fruit-trees/cherry-trees.html?usda_zone=452

Tom in San Diego Says:
Feb 5th, 2014 at 12:57 pm

Those red dots are glands on the leaf, not cherries.

Minnie Royal and Royal Lee should produce fruit in central Florida.

The biggest problem I have heard of is that the two have problems blooming in sync. At least when the trees are young.

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