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Chilling Requirements in Strawberries

Published on: October 16, 2008

Cold conditioning, or chill requirement, is an essential part of growing and producing strawberries in California. The chilling requirement is defined as being the cumulative period, usually measured in number of hours below a certain temperature, which is needed to produce the internal changes in the strawberry plant that result in the normal sequence of growth following winter dormancy. In strawberries, hours accumulated of temperatures between 28 and 45 degrees Fahrenheit are considered to be effective and are counted as towards chill requirement.

Chill requirement in strawberry is made up of two essential parts. One part is what the plant accumulates in the field before being harvested, and the other is accumulation of chill after harvest and the plant is in storage. There is a difference between the two. In-field chill takes place when the plant is still in the soil, out in the open and still has all its leaves. Supplemental chill takes place after harvest of the plant and occurs in a constant near freezing temperature, in the dark and the plant has none to very few leaves left.

Since the grower has little control over the accumulation of chill in the field, he or she should focus on the management of supplemental chill. Generally, growers should know that a high degree of supplemental chill in strawberry results in more vigor, meaning more vegetative growth, and less fruiting. A lack of supplemental chill will result in a plant which is less vigorous and with more tendency to fruit.

The catch is that growers need to strive to strike a balance between vigor and vegetative growth and fruiting. Too little vigor will result in a plant of reduced fitness which does not have a capacity to bear a great number of fruit, whereas a plant with large amounts of vegetative growth will have its fruiting ability compromised, since it is dedicating too many resources in the production of leaves, crowns and runners.

Additionally, accumulation of chill in the strawberry transplant makes it stronger and better able to survive the stress of harvest and subsequent transplant. Harvest of transplants means their leaves are cut off, they are ripped from the ground, tumbled in a drum and exposed to drying out in the open air. So, while in theory it is possible to grow good productive plants from transplants that are short of the recommended chill requirement, in practice, namely a commercial operation, plants lacking in supplemental chill requirement will not do well and some will probably not survive transplanting.

For University of California strawberry varieties, it is imperative that growers follow storage chill recommendations. These recommendations are based on years of research and should not be taken lightly.

Below are listed the storage chill recommendations for several popular UC varieties.

From UC publication 3351 “Integrated Pest Management in Strawberries”

Finally, growers should know that to a small extent, plant vigor stemming from an excess or lack of supplemental chill requirement can be controlled by early season flower management. Removing flowers early in the season, such as in January on the Central Coast, allows the plant to continue to develop vegetatively, and would make up for a small deficiency in chill. Removing flowers later or not removing them at all slows the plant vegetative growth and would put the brakes on, so to speak, on a plant with an excess of chill requirement.