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Over the Garden Fence

Chilling Hours Help Break Spring Dormancy

Our plants will break dormancy at different times each spring. This depends on, as you can guess, the kind of weather we have. Besides the warming weather, “chilling hours” influence how soon we see bud swell and blooms.

Quoting Extension Educator Elizabeth Wahle, Fruit Specialist, “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 45O F (it is generally agreed), and approximate hours of chilling required to break dormancy will vary according to species and cultivar.”  

Plants well-adapted to Illinois will not have any trouble getting those chilling hours in our typical spring. However, Wahle goes on to explain why in some years breaking dormancy is troubling.

“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, late winter thaws and warm early spring weather in Illinois can be especially disastrous to fruit growers. Early warm weather can overcome ecodormancy, allowing premature growth that can be damaged by subsequent cold and frosts. When purchasing fruit plants, particularly peaches, it is important to avoid low-chill requirement cultivars that have been selected for Mediterranean and sub-tropical climates. Chilling requirement for these cultivars are met very quickly in the Midwest, making them all the more susceptible to early bud break and frost/freeze injury.”

From this we can expect plants that have different exposures in our yard to break dormancy at different times, even though they are the same plant. The chart below explains why peaches and apricots are always early and at a higher risk of frost and freeze damage each year compared to apples, which are considered very winter hardy.

Plant Type                             Approximate Hours of Chilling Required to Break Dormancy (<45oF) ___________________________________________________ Apple                                                     800 to 1750 Raspberry                                              800 to 1700 Pear and sour cherry                          600 to 1500 Currant and gooseberry                     800 to 1650 Walnut                                                   400 to 1550 Plum (Japanese)                                   600 to 1600 Pecan                                                     650 to 1550 Cherry (sweet)                                      500 to 1450 Blueberry (highbush)                          800 to 1250 Peach                                                     375 to 1200 Apricot                                                   300 to 1000 Blackberry                                             350 to 600 Grape                                                     50 to 400 (adequate growth)                                                                 400 to 1650 (better and faster growth) Strawberry                                            50 to 300

By better understanding chilling hours for all the plants in our yard, we can have a better understanding of the phrase, “right plant in the right place in the right soil.”

About the author: Richard Hentschel’s expertise extends across several subject areas with specialties in lawn care, fruit tree production, woody ornamentals, and home and community gardening. During his 45-year career in horticulture and agriculture, Hentschel became a well-known and respected expert for commercial and homeowner audiences, industry organizations, and media. He retired from University of Illinois Extension in April 2022 with nearly 30 years of service as a Horticulture Specialist and Educator in northern Illinois.