Question: What are "Chill Hours" on peach trees and what do you look for? I live in Delta, Colorado and we currently have peach trees and we are looking to increase our orchard by 4 or 5 dozen trees with some mid August to late August ripening. Which would you recommend? Darla
Answer: Chill hours - It is not really funny, but as I was starting to answer this, the whole mid and eastern section of the country is in the deepest freeze I have ever seen. That is not what we are talking about here.
Many deciduous plants and trees require a certain amount of winter/dormant chilling to properly set fruit or even flower. In its simplest definition, chilling hours are the number of more or less continuous hours under 45 degrees F. during the dormant period. Of course nothing is that simple since some days the temperature raises high enough to offset chilling that has previously occurred.
There are several theories how to calculate chilling hours and they can be reviewed in more detail here:
California Chilling Hours by Location
Why is this important? Some examples: Every one's favorite Elberta Peach needs approximately 800 chilling hours. It will not fruit in the coastal Los Angeles or Orange County area which only gets 200-500 hours of chilling. But the newer, equally delicious (L.E. Cooke discovered) Santa Barbara Peach will fruit (approx. 300 chilling hours). Popular Bartlett Pears require approximately 800 chilling hours. At one time there were vast orchards of Bartlett Pears in the San Jose region of California. As San Jose and Silicon Valley grew in population (and cement/asphalt), the winter chill was rasied and now we recommend other varieties for the area since Bartlett will not fruit consistently.
We grow in the richest agricultural area of the world and sufficient chilling hours has a lot to do with it. Although our Central Valley of California is rarely in a deep freeze, we often get a winter "Tule" fog that keeps the afternoon temperatures lower than if the sun were out. It is not uncommon to get 1,000 to 1,300 hours of winter chilling, allowing us to grow nearly anything and we usually do not get the hard spring freeze that does damage to young growth on the trees.
How do we determine chill hours for a variety? One way is to watch where it fruits and compare it to the number of recorded chill hours for that location. We depend a lot upon passionate fruit hobbyists like the California Rare Fruit Growers (CRFG) to take new varieties and plant them in their yards across the climate zones and report back their successes and failures. Often the chill hours are set high (we know it fruits here) and slowly lowered over the years as more experience dictates where it will fruit. For example, many Apples are labeled high chill - because they are grown in Eastern Washington and upstate New York and that was the extent of their testing. It took hobbyists to plant Fuji Apples in warm winter climates and discover they will fruit in areas with under 400 hours of chilling - a far cry from the 1,100 hours originally assigned to it.
In some stone fruits, like peaches and nectarines, you can compare bloom dates and determine chilling similarities from that. That is not true across all varieties.
Now that we discussed chill hours, let's answer your other question. Where you live, you do not have to worry about having enough chill hours - you have plenty! Your area of concern is hardiness to survive winter cold and late frosts that affect early blooms. Trees that bloom later in the spring are desirable to avoid damage to blossoms due to frost.
We have created "Recommended Variety Lists" for many places based upon climate. Another name for this page might be "Best Tasting" for that climate since our recommendations come from our favorite tasting fruit.
Here is a recommended variety list developed for our foothills which you might like to browse:Foothills List
And another one for colder areas:Cold Areas List