Explanation of Chill Hours for Fruit Tree Planting

Peach trees require a certain number of chill hours for flowers and fruit to develop properly.

During winter dormancy, physiological changes occur in deciduous trees that prevent them from budding too soon. Fruit trees such as peaches, apples and pears must experience a certain number of chill hours or chill units to prevent them from breaking bud dormancy too quickly.

Definition

Although theories differ, the classic definition of a chill unit is an hour of air temperature between 32 and 45 degrees Fahrenheit. The chill hours for an area are based on the total number of chill units accumulated during an average winter.

Tree Selection

When planting fruit trees, gardeners must select varieties with chill requirements compatible to the growing area's average chill units. Chill requirements vary between types of trees and their cultivars. For example, certain peach trees developed for Southern climates need less than 300 chill hours while some Northern varieties require more than 800 chill hours. According to the University of Georgia, trees with low chill requirements can suffer frost damage if a temporary warm spell causes them to prematurely break dormancy.

Insufficient Chilling

Chill hours are important for normal leaf and flower development. Symptoms of insufficient chilling include delayed bud and bloom development, improper fruit formation and small, misshapen fruit.

References

• The Garden Academy: Understanding Chill Hours
• University of Georgia, College of Agricultural &Environmental Sciences: Peach Tree Physiology