Fruit Chill Hours

About Chill Hours

While a tree is dormant in the winter, it has more tolerance to cold temperatures. Trees of this type that lose their leaves in winter are called deciduous fruits.

It would be bad news for a tree to decide to wake up and flower in the middle of winter. Its buds could freeze, and it would likely be damaged. In order for a tree to know when to wake up it essentially "measures" how much cold it has been exposed to. Once that amount of cold temperature has accumulated, when warm weather arrives the tree can begin flowering or leafing out. If a warm spell occurs before the cold requirement has been met, a tree should not yet flower.

The best results for a home orchard happen when trees are selected that line up with the local climate. For example trees that only require a small amount of chilling may flower prematurely and lose their crop to a freeze in our climate.

The Eastern Sierra, at least in terms of chilling requirements, is a cold climate. You should not select low-chill varieties when possible or risk losing your crop to frost.

Horticulturists usually measure chilling requirements in units called "chill hours" that reflect how much cold trees have been exposed to in their dormant season.

There are several methods to record chill hours or chilling requirements. The most common one that nurseries which sell to homeowners will be using is a count of the hours where the temperature is below 45°F. This is the simplest method.

Other methods that more accurately reflect plant physiology exist, but this one is easy and widespread. If you'd like to get an overview of other methods, this summary from Dave Wilson Nursery is good.

The University of California has a calculator that will allow you to find chill hours using 4 methods for many weather stations in California. For most home orchardists in our area this is likely more thought that you need to give the topic. 

You'll be fine if you remember these 3 things for our area*:

1. Aim for fruits that have more than 600 chill hours listed if that's an issue;
2. We always have plenty of chill hours for just about everything you'd want to plant here;
3. What's more important is to look for varieties that are listed as adapted to cold climates or late flowering.

* If you're not in the low desert, that is. That's a whole different thing! Contact us for advice on raising fruits there.

Owens Valley Chill Hours

Using the hours below 45°F method (which is common but maybe not the best way to do this) you can use these chill values as a starting point to plan with.

Fish Springs to Benton, Including Bishop: 2,200-2,500 hours

Round Valley: 2,300-2,700 hours

Aberdeen to Manzanar: 1,900-2,500 hours

Lone Pine to Olancha: 1,800-2,200 hours

Wilkerson: Probably has about as many chill hours as Bishop, but because it is less prone to late frosts than the valley floor, you have a little more flexibility on what to plant in terms of trees requiring less chilling.

We've not been able to locate quality, long term recommendations for actual chill hours for the Owens Valley in our files. This is likely because we haven't been a commercial growing area for a century, and that we are cold enough in winter that it really isn't an issue for trees that grow here to break dormancy.

If anything, we worry more about plants that only have a small chilling requirement because they tend to flower too early. An example would be almonds which are about 200-400 hours using this method.

What we do have is access to UC's chill calculator. Looking at that data and taking into account when trees break dormancy here, we've estimated the values above.

But again, we get plenty of chilling here. Not much reason to worry about cold!