Chilling Requirement

During winter, temperate perennial fruit and nut crops enter a dormant state in which growth pauses. Dormancy is released once the plant has accumulated sufficient exposure to cool, but not necessarily freezing, temperatures — its chilling requirement.

Dormancy

A period of paused growth in perennial plants, commonly through winter in temperate climates.

Chilling requirement

The accumulated cold exposure a plant needs before dormancy is released and normal growth can resume.

Chill accumulation model

A method, such as chill hours or chill portions, used to estimate accumulated winter cold exposure.

• Apple — cultivars vary considerably in their chilling requirement
• Blueberry — chilling needs differ notably between highbush and other blueberry types and cultivars
• Grape — dormancy and chilling response vary by cultivar and region
• Olive — chilling needs are relevant mainly for olives grown in cooler-winter regions

Specific chill-hour or chill-unit figures are not given here, since requirements vary widely by species and cultivar and are typically determined through local variety trials.

Multiple chill-accumulation models are used in practice, and they differ in their assumptions about which temperature ranges contribute to chilling and how much each range contributes.

• Delayed leaf-out in spring
• Prolonged or uneven bloom period
• Reduced fruit set
• Potential yield or quality impact in severe cases

The main approach to managing chilling risk is selecting cultivars whose chilling requirement matches the site's typical winter climate. In some systems, chemical dormancy-breaking treatments are used, and growers may monitor accumulated chill through the season using a locally appropriate model.