Chilling Hours are Important for Dormancy and Blooming

Chilling Hours Reference Map based on University of Maryland research.

Chilling hours are important, but many people are unduly concerned.

Chilling hours is a complex and confusing issue. Because so many customers have asked about chilling hours, Grandpa has compiled some charts from the best sources that he has to help customers concerned about chilling hours. Many varieties have not been researched sufficiently as to their specific chilling hour requirements. Where Grandpa lives in the “banana belt” of Southwest Michigan, accumulating sufficient chilling hours never has seemed to be a problem, so Grandpa has never been too concerned, but too many questions have come his way, and here is his spin on the subject.

Unless you live in a region where it is VERY WARM all year around, such as Southern California, Florida, the Desert Southwest, or the Gulf Coast regions, you may likely get enough chilling hours in most years for many varieties to properly break dormancy and bloom.

Enough chilling hours are required during the dormant season to satisfy the plants need for dormancy. Insufficient chilling hours may delay bloom or even the breaking of dormancy and growth of the tree in the spring. Chilling hours are typically calculated at an optimum temperature of 40 degrees F. Over 50 degrees F NO chilling hours are accumulated as the tree is attempting to break dormancy and may be losing some of its cold hardiness at that time. Also, if the temperature is under 32 degrees F NO chilling hours are accumulated because the plant is fully dormant with no cellular activity.

The total number of hours during the winter that the plant accumulates hours in that 32-50 degree range will determine whether it has accumulated enough chilling hours. Because accumulated chilling is such a variable thing, the requirements are not hard firm numbers, but rather soft areas and ranges.

Micro-climates and regional variations will also affect chilling hours, as well as hardiness zone issues. For the most part it is more important to make sure that you have selected varieties that are sufficiently hardy for your zone. Higher altitudes and elevations often are colder and will accumulate more chilling hours than lower elevations, such as valleys or sunny slopes, and their hardiness zone may differ slightly also.

Even so, many varieties vary in their response to chilling hours and there seems to be little pattern. Therefore, Grandpa takes them with a "grain of salt!" Unless you are in a region with very low chilling hours, you should not get too worried about this issue. This is typically around Zone 9.
Hardiness zones and chilling hours also seem to show little relationship. This is likely due to the genes that control winter hardiness and cold tolerance not being the same or linked to the genes that control when a plant will come out of dormancy and bloom (chilling hour requirement).

In regards to chilling hours and hardiness, Grandpa foresees only two possible problems:

If you grow a LOW-chill hour varieties in a high-chill hour climates, your variety will reach its chilling hour requirement early during the dormant season, and thus if an untimely warm spell comes along (example "January Thaw") or if spring comes excessively early (like last year!) then that variety may be likely to break dormancy too early. Once dormancy is broken, frost and cold tolerance decreases and so the chance for losing some of the crop is much higher.

If you grow a HIGH-chill hour variety in a low-chill hour zone where the variety may not accumulate sufficient chilling hours each year, you may likely see difficulties some years in the tree breaking dormancy or blossoming.

For the most part, if you live in or near an area that has 600 or more chilling hours, you will likely be able to grow almost all varieties that we offer. If you live in a zone with less than 400-600 hours of chilling in a typical year, then you may want to examine varieties that are in the lower chilling hour requirement range.

It helps to also talk with experienced gardeners and/or orchardists in your area and find out which varieties do best for them. Local experience is a very good guide.
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