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Chill Hours: What Are They, How do I Count Them, and Why do My Fruit Trees Care?

By on November 14, 2014

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:

  • Apples – Anna, Low Chill multi-graft, Dorsett and Sundowner
  • Apricots – Gold Kist or Katy
  • Plums – Methley, Burgundy Japanese or Mariposa
  • Pluot – Dapple Supreme
  • Cherry – Royal Lee or Minnie Royal
  • Peach – Red Baron, Low Chill multi-graft, Saturn, Babcock and Eva’s Pride
  • Nectarine – Spice Zee Nectaplum, Double Delight or Snow Queen
  • Pears – most require over 300 chill hours. Asian pears require the lowest chill hours of all pears.
  • Figs, Pomegranates, Quince, Persimmons – all require 300 or less chill hours

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.

  Comments (15)

A

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.

Posted by Anonymous on Oct. 28, 2011 at 5:42:33 PM

H

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

Posted by hope on Jan. 28, 2012 at 6:11:03 AM

C

Thank you for your garden news!

Posted by Charlotte, Peaceful Valley on Dec. 21, 2012 at 6:22:32 PM

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

Posted by GrowOrganic.com on Jan. 27, 2013 at 11:56:25 PM

T

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.

Posted by Tom in San Diego on Feb. 05, 2014 at 11:57:01 AM

L

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.

Posted by Larry Hume on Nov. 15, 2014 at 8:52:55 AM

C

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.

Posted by Carrie on Dec. 20, 2014 at 4:18:05 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.

Posted by Suzanne at Peaceful Valley on Dec. 22, 2014 at 11:23:32 AM

M

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?

Posted by Monica on Dec. 28, 2014 at 9:41:42 AM

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.

Posted by Suzanne at Peaceful Valley on Dec. 29, 2014 at 12:13:35 PM

L

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. grin

Posted by Linda Siska on Mar. 28, 2015 at 9:27:54 AM

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.

Posted by Suzanne at Peaceful Valley on Mar. 28, 2015 at 12:39:19 PM

D

I live in Tucson AZ, and buy most of my fruit trees from groworganic.com simply because they are one of the few sellers that make it EASY to find low-chill options. I also do most of my gardening by container…my yard soil might as well be cement, plus I rent my lot. I bought the 2 low-chill options of cherry trees as web-based, then promptly put them in the biggest containers I could afford at the time. They produced cherries like crazy. I had given them to my brother as a birthday present and he babied them. Unfortunately he kept them in the same container for far too long, I think. He has now bought property and last year transplanted into dirt, with supplements. He got blossoms, No cherries. I gave him some suggestions, he’s looking at pruning videos for cherry trees trying to save his trees. Outwardly they look healthy. I told him to keep doing the right thing, let their roots spread out. But just as an FYI, this is the best place to buy trees.

Posted by Donna on Feb. 05, 2016 at 11:49:50 PM

K

I am planting the cold hardy plants requiring min 200 chill hours in an Indian tropical location. The winter temperatures follow a trend of Min 7deg C and Max 27 deg C from December 15th to February 15th. The temperatures fall in the nights between 10pm to 7am to 7 -12deg C. Will that we sufficient to fulfill the chill hour requirements of the plants like the Apple, Plum, Pear, Apricot, parssimon, Honeyberry, Cherry and Chest nut. Pl advise.

Posted by Kishan Sood on Jun. 15, 2016 at 2:48:43 PM

Kishan, It will really depend on the chill requirements of the varieties of fruit trees that you want to plant. Typically apples, pear and cherries require more chill hours but there are varieties that do well in warmer regions. I would advise you to find out the chill requirements for the varieties that you want to plant and then decide if they will work for your area.

Posted by Suzanne at GrowOrganic.com on Jun. 17, 2016 at 9:07:16 AM

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