What Are Chill Hours?

Chill hours are also referred to as CU or chill units, which deal with plant dormancy, which is prompted by longer nights and lower temperatures in the fall and early in the winter.

Some plants require chill hours in order to thrive. The changes that occur in the plant cells when the temperature drops and daylight lessens either allow or don't allow a plant to survive. Those that can survive chilly temperature and even need it have nature’s anti-freeze. Some trees and plants require specific amounts of cooler temperature. If they don't receive it, they will not flourish.

If you are in the business or hobby of growing fruit, you must be aware of chill requirements. Knowing the average chill hours that occur in an average winter allows a gardener to know which varieties of fruits will grow in a specific location. Blueberries, for example, have very specific chill requirements as do pears, plums, apples and peaches.

If a particular plant must be exposed to a certain number of chill hours but isn’t, it will not grow as it should, and it may never bloom or bear fruit. However, if a fruit is subjected to too many chill hours, this can cause the dormancy period to end too soon and cause the fruit to bloom too early. When this happens, a hard freeze can kill the flowers, and fruit will not set. Freezing weather can also damage the leaves.

The way a gardener finds out what the chill hour average is in his neck of the woods requires knowing that a chill hour is comparable to an hour of air temperature that is between 32 degrees Fahrenheit and 45 degrees Fahrenheit. During an average winter, the total number of chill units that are accumulated in a specific area of the country reveal the chill hours. Temperatures that are above 60 degrees Fahrenheit detract from chill units and temperatures below 32 degrees fahrenheit do not contribute to chill units. Consequently, you must subtract an hour for every hour that is above 60 degrees and for every hour that is below 32 degrees. Another method to figure out chill units maintains that an hour of cold temperatures between 32 degrees F and 45 degrees F satisfies one hour of a tree’s chilling needs.

The Utah Model for determining chill units says an hour of chill below 34 degrees F is worth virtually nothing but sixty minutes of temperatures between 35 and 36 degrees F amounts to one-half of a chill hour or 30 minutes. One chill hour can be chalked up when weather is in the 37 degree to 48 degree range and weather in the 49- to 54 degree range gets 30 minutes or one-half of a chill hour. No chill hours are gained from weather in the 55- to 60 degree range and temperatures above 60 are negative chill. You cannot apply the chill unit approach in warm areas where there is insufficient chill.

Chill units work in tandem with heat unit to determine the stage that a tree or plant will develop. If a tree is located in an area where there are only 200 chill units, then this tree will require more heat units to prompt bud development.