Chilling requirement
cold weather requirement in agriculture
The chilling requirement of a fruit is the minimum period of cold weather after which a fruit-bearing tree will blossom productively. 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 units or chilling hours
A 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). 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.
Contrary to popular belief, lack of chilling hours does not usually kill a plant, nor prevent a dormant plant from eventually leafing out. However, it may add stress that can be one factor in a plant’s demise; as such, a previously injured plant may not survive long after a warm winter.
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