Chilling requirement

The chilling requirement of a fruit is the minimum period of cold weather after which a fruit-bearing tree will blossom. It is often expressed in chill hours, which can be calculated in different ways, all of which essentially involve adding up the total amount of time in a winter spent at certain temperatures.

Some bulbs have chilling requirements to bloom, and some seeds have chilling requirements to sprout.

Biologically, the chilling requirement is a way of ensuring that vernalization occurs.

Chilling unit in agriculture is a metric of a plant's exposure to chilling temperatures. Chilling temperatures extend from freezing point to, depending on the model, 7 °C (45 °F) or even 16 °C (60 °F).Stone fruit trees and certain other plants of temperate climate develop next year's buds in the summer. In the autumn the buds go dormant, and the switch to proper, healthy dormancy is triggered by a certain minimum exposure to chilling temperatures. Lack of such exposure results in delayed and substandard foliation, flowering and fruiting. One chilling unit, in the simplest models, is equal to one hour's exposure to the chilling temperature; these units are summed up for a whole season. Advanced models assign different weights to different temperature bands.

According to Fishman, chilling in trees acts in two stages. The first is reversible: chilling helps to build up the precursor to dormancy, but the process can be easily reversed with a rise in temperature. After the level of precursor reaches a certain threshold, dormancy becomes irreversible and will not be affected by short-term warm temperature peaks.Apples have the highest chilling requirements of all fruit trees, followed by apricots and, lastly, peaches. Apple cultivars have a diverse range of permissible minimum chilling: most have been bred for temperate weather, but Gala and Fuji can be successfully grown in subtropical Bakersfield, California.

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