What are the chills hours? Why dormant fruits trees have chilling requirement?
The chilling requirement of dormant fruit trees gives the definition of chill hours. In the late summer/early winter, as the days become shorter and cooler, the deciduous fruit trees and buds secrete the growth inhibitor hormones that drive the tree to dormancy state and prevent any further growth to protect the tree and its reproductive system from the harsh winter. However, for the trees to be able to wake up and buds bloom from this state later in the spring, it should be exposed to a certain amount (hours) of low temperature between 35°F to 45°F (chill) during its dormancy in the winter so it break or reverse the effect of this sleeping hormone. This amount of hours/temperature is called the chill hours and it vary from fruit type to another and range from few hours if any to above 1000 hours. Below is the chill hour map from the research team at University of Maryland which shows the different climate areas in the US. Also we included in the bottom of this post a sample of chill hours requirements for some fruit trees adopted from University of California.
| Type of fruit | Approx. hours at <45°F needed to break dormancy | Equiv. time if continuously exposed to <45°F |
|---|---|---|
Almond
|
200–300
|
8–13 days
|
Apple
|
1200–1500
|
7–9 weeks
|
Apricot
|
700–1000
|
4–6 weeks
|
Cherry, sour
|
1200
|
7 weeks
|
Cherry, sweet
|
1100–1300
|
6–8 weeks
|
Chestnut
|
300–400
|
2–3 weeks
|
Fig
|
a few hours
|
–––
|
Filbert (Hazelnut)
|
1500
|
9 weeks
|
Kiwifruit
|
600–850
|
3.5–5 weeks
|
Olive
|
200–300
|
8–13 days
|
Peach/Nectarinea
|
650–850
|
4–5 weeks
|
Pear
|
1200–1500
|
7–9 weeks
|
Pecan
|
400–500
|
3–4 weeks
|
Persimmon
|
<100
|
4 days
|
Pistachio
|
1000
|
6 weeks
|
Plum, American
|
3600
|
5 months
|
Plum, European
|
800–1100
|
5–6 weeks
|
Plum, Japanese
|
700–1000
|
4–6 weeks
|
Pomegranate
|
200–300
|
8–13 days
|
Quince
|
300–400
|
2–3 weeks
|
Walnut, Persian
|
700 (Payne) – 1500 (Franquette)
|
4–9 weeks
|