Episode Details

What are Fruit Tree Chill Hours? Do They Matter?

In today’s newsletter podcast (Above. Originally aired in Episode 105 of Garden Basics), Tom Spellman of Dave Wilson Nursery explains the term “chill hours” and how cold winter temperatures helps production on many deciduous fruit trees, especially apples. Chill hours refer primarily to low temperatures (usually between 32 and 45 degrees Fahrenheit) during a fruit tree’s dormant months in the winter.

But Tom Spellman is throwing a monkey wrench into the hard rules about growing fruit trees that require lots of winter chilling hours (more than 800 hours) in areas of the country that don’t get that cold in the winter. And he is having a lot of success in his test orchard in Southern California, where accumulated winter chill hours seldom top 100, but still has nighttime cooling in the 40’s and 50’s. Tom explains his “high chill trees in a low chill test area” in today’s newsletter podcast (above). The key, Tom points out, is “All Gardening is Local” (hmm…that sounds familiar!).

Most chill hour requirements listed for deciduous fruit trees were originally established where the tree has been growing - and producing - for decades. The only way to see if a particular fruit tree that supposedly requires winter chill can succeed in your area, is to try to grow it in your locale. And as Tom points out in an article he wrote for the Dave Wilson website, “Fruit varieties can be productive where chill hours are less than recommended if there is sufficient cool weather (45-55°F) during the dormant period.”

1. In fall, deciduous fruit trees lose their leaves and enter a dormant state in order to survive winter. To end dormancy, bloom and set fruit they require a certain amount of winter cold (their “chilling requirement”) followed by a certain amount of heat. Chilling requirements vary widely among varieties.

2. Fruit varieties with chilling requirements much lower than received at the planting location may end dormancy and bloom too early, subjecting tree, bloom and fruit to freeze damage. Conversely, varieties with chilling requirements much higher than received will suffer delayed, weak leafing and blooming and will not fruit satisfactorily.

3. To quantify fruit tree chilling Dave Wilson Nursery uses the most widely used method, the Hours Below 45°F model. One hour below 45°F during the dormant period (autumn leaf fall to spring bud break) equals one hour of chilling or one “chill hour”.

4. Research indicates fruit tree chilling also occurs above 45°F (to about 55°F), is most effective at about 35-50°F, and does not occur below about 30°F. Chilling temperatures are most effective in early dormancy and during that time accumulated chilling can be negated by temperatures above 60°F. Such findings help explain the response of specific varieties to different climates, i.e. to different patterns of cold and warm temperatures.

5. Generally, fruit growers have most success with varieties that have stated chilling requirements similar to the chilling typically received at the planting location: “high chill” varieties (800-1,000+ hours) for cold winter climates, “low chill” varieties (500 hours and less) for warm winter climates.

6. Published chilling requirements for a fruit variety can be the minimum chilling required to produce a satisfactory home garden crop up to a higher requirement for the consistent, maximum crop sought by commercial growers. Note that many apple varieties can set fruit with far less chilling than recommended, color and quality depending on climate.

7. Fruit varieties can be productive where chill hours are less