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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, what is that? Isn’t it enough to know your USDA plant hardiness zone? That’s about cold temperatures—so 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 UC Davis 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 chill hours each winter to regulate their growth.
If a tree doesn’t experience enough chill hours in the winter the flower buds might not open at all in spring, or they might open unevenly. In addition, the production of leaves may also be delayed.
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 video, Fruit Trees – A Selection Guide, 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’ chill requirements to your local chill hours—use the menus at our fruit tree page and find trees sorted by “Chill Hours Needed”.
Here are some guides to low chill (less than 300 hours) fruit trees:
The majority of the fruit and nut trees require higher chill hours. The selection is huge, so no matter where you live, there is a perfect fruit tree to choose from. Choose wisely and 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.
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.
Jan 28th, 2012 at 7:11 am
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
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.
Larry Hume Says:
Nov 15th, 2014 at 9:52 am
I used to live in Corona ,Ca. This is a city known for its citrus at 400 chill hours or less a winter, I had 4 varieties of cherry that produced fruit, Royal Rainer, Lapins, with Minnie Royal and Royal Lee being by far my most dependable. Producing heavy crops of very tasty sweet cherries, starting the 2nd year, after being planted, in the 15 gallon size.
Dec 20th, 2014 at 5:18 pm
I have a dwarf apple and orange and fig and lime as well as a lemon. I have all potted and pulled them inside the orange looks dead all the leaves fell off and the apple is blooming only on one grafted branch the fig and lemon and lime have leaves and continue to mature how do I know if the orange is still alive I still keep it lightly watered.
Suzanne Fellows Says:
Dec 22nd, 2014 at 12:23 pm
What kind of apple do you have? Most apples will do just fine outside, unless it is a low chill apple and you live someplace really cold. Citrus will typically drop their leaves if stressed out, such as repotting. You can do a scratch test to see if it is still alive. If it is green under the bark then it is alive. I would allow the soil to dry out before watering again. You might want to watch our video on growing citrus indoors. Here is the linke, http://www.groworganic.com/organic-gardening/videos/indoor-citrus.
Dec 28th, 2014 at 10:41 am
What is the best variety of small fruit, tart-to-sweet, dwarf to semi-dwarf apple tree for me to plant in zone 6? Are rose bushes good pollinating partners for it?
Suzanne at Peaceful Valley Says:
Dec 29th, 2014 at 1:13 pm
Roses will not pollinate apple trees. Here is a link for apple trees that will grow in zone 6, http://www.groworganic.com/seasonal-items/fruit-trees/apple-trees.html?usda_zone=448 Each .description will talk about the fruit quality and whether or not it needs a pollinizer. I really like the Pink Lady apple. Very crisp sweet to tart flavor and starts producing after a couple of years. This one may work for you. Hope this helps.
Linda Siska Says:
Mar 28th, 2015 at 9:27 am
I live here in Grass Valley, close to downtown. I could not find any chill hours listed for Nevada County—do you know what they are for our area? I’ve also seen this area sometimes listed as USDA zone 7 and sometimes listed as zone 8, so I’m not sure who to believe.
Suzanne at Peaceful Valley Says:
Mar 28th, 2015 at 12:39 pm
According to the USDA Zone map with a zip code of 95945, you are in zone 9a (follow the link in the article above). For chill hours I am guessing that you are 700-800 chill hours. But for more accurate readings, you might want to check with your the local master gardeners. They may actually have someone in the area that has mad the recordings.