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The characteristic of “chilling hours” is important for tree fruit. This week’s column is taken directly from a University of Illinois publication that nicely explains the concept of chill hours and why fruit trees at higher elevations or colder microclimates (such as north facing slopes) need higher chill requirements.
Plants will break dormancy at different times each spring. Besides the warming weather, “chilling hours” influence how soon we see bud swell and blooms. Normally, with the onset of fall weather, deciduous fruits and nuts have stopped actively growing and have begun to drop their leaves in preparation for a long winter rest. This rest period, termed endodormancy, is necessary to winter survival for not only fruits and nuts, but for many trees, shrubs and bulbs. Endodormancy is a type of dormancy where the buds are dormant because of an internal plant inhibitor system that prevents growth even under ideal external growing conditions.
Once a plant has entered endodormancy, it will not grow again until it has received enough cold to overcome the dormancy. However, the best temperature for chilling is not the extreme cold. Effective chilling temperatures to terminate dormancy are generally between 32 to 45-degrees Fahrenheit, and approximate hours of chilling required to break dormancy will vary according to species and cultivar.
When the chilling requirement has been met, rest (endodormancy) ends, and the plant is ready to grow, regardless of time of year. The continued dormancy of the plant then depends on another form of dormancy called quiescence or ecodormancy. With this type of dormancy, buds are dormant as a result of external conditions unfavorable to growth, i.e., as long as the temperature is cold, the plant will not grow. For this reason, tree fruit varieties with low chill hour ratings are more likely to break dormancy early with early warm weather we often experience in February or early March.
Early warm weather can overcome ecodormancy, allowing premature growth that can be damaged by subsequent cold and frosts.
When purchasing fruit plants it is important to select a variety with a chill requirement appropriate to our region. Tree fruits developed for Mediterranean and sub-tropical climates can be problematic for some of our elevations and north-facing slopes. Tree fruit varieties with a low chilling requirement are more susceptible to early bud break and subsequent frost/freeze injury.
For more information on tree fruit, small fruit, and brambles suitable to Rockbridge County, contact the Rockbridge Extension office.
RAMGA Seed Event This Saturday
Take note of the Rockbridge Area Master Gardener free seed event taking place this Saturday, Jan. 20, featured in this week’s News Gazette in the Lifestyle section.
