The accumulated hours of cold temperatures that many fruit trees and fruit-producing plants require during dormancy to flower and fruit properly.
Many fruiting plants need a period of winter cold before they can resume normal growth in spring. These cold temperatures help satisfy the plant's chill-hour requirement and signal that winter has passed.
After enough chill hours accumulate, dormant buds can develop normally into leaves and flowers. If a plant does not receive enough winter chill, flowering may be delayed, uneven, or greatly reduced.
Different varieties have different requirements. Some need only a few hundred chill hours, while others require much longer periods of winter cold.
Chill hours play a major role in fruit production. A variety that is well suited to your climate will generally flower and fruit more reliably than one with mismatched chilling requirements. Understanding chill hours helps gardeners choose appropriate fruit trees and berry-producing plants for their region, improving long-term harvest success.