Chill Hours

Chill Hours, also known as a vernalization requirement is the theory of the minimum period of cold weather after which a fruit-bearing tree will blossom.  Theoretically if a deciduous tree doesn’t experience enough cold or “chill hours” in the winter the flower buds might not open at all in spring, or they might open unevenly giving no or poor fruit crop. In addition, the production of leaves may also be delayed.

When you see a number like 800 for chill hours it usually means the number of accumulated hours below 45 degrees in that winter or dormant season. How a deciduous fruit tree actually accumulates winter chilling is more complex than represented by the easy-to-use 45°F model. Research indicates fruit tree chilling 1) does not occur below about 30-34°F, 2) occurs also above 45°F to about 55°F, 3) is accumulated most effectively in the 35-50°F range, 4) is accumulated most effectively early in the dormant period, and 5) in early dormancy can be reversed by temperatures above 60°F. Chilling calculation methods such as the 32-45°F model, Utah model, Low Chilling model, Mean Temperature model and Dynamic model incorporate some or all of these findings. To date, all models tend to give significantly different results for different climates.

Often looking at the chill hour “requirement” on a variety of tree on 3 different websites or catalogs will give 3 different numbers. This is primarily due to the fact that the needs are a guesstimate and never have been tested in a controlled environment. Add to this that most commercial orchardists believe if you are within 200 hours of the chill requirement it is good enough for a successful crop. If you have a low chill tree that meets it’s requirement early it may come out too soon and be damaged by freeze. Rarely and issue in hot climate areas. A high chill variety may never reach it’s requirement and come out slow, uneven and not bloom.  Chill hours are only counted after the onset of dormancy, this is important to remember for later.

The promotion of chill hours was primarily for the benefit of commercial operations. Most apple orchards are sprayed up to 14 times before harvest. They can be sprayed for additional calcium, to encourage blossom drop for thinning, for various fungi and diseases and for different pests. The commercial grower wants his fruit to all bloom at the same time so he sprays for each item only on one trip and harvests on 1 or 2 trips. Every additional trip or mixed type spraying loses his profit. But is this what the home grower wants? Do you want to have 2-3 bushels of apples off your tree all ripe and ready at one time or do you prefer the harvest spread over say 4-6 weeks?

What does all this mean to home growers in warm climates? Maybe not a lot. Many higher chill apple varieties are being grown in S. Cal in areas with 150 avg chill hours per winter, in the lower Gulf States and Florida.  Many tropical countries, some with zero chill hours, are defying the accepted logic and growing commercially viable crops. So how can this be? The apple genome has been mapped and found to be about 3 times the size of the human one. The genetic diversity is huge and we know very little in sum total about all it’s facets. Some thoughts are: A) That stated chill hours are wrong. B) The concept is wrong or incomplete. C) There is an unknown or unresearched factor at play. D) Genetics from heirloom varieties are important to being able to thrive in warm climates. Maybe all or some of the above.

Some more recent research indicates that chill hours only applies to how hardy the fruit bud becomes and that actually dormancy is the key factor. Remember chill hours do not accumulate while the tree is photosynthesizing, only when it is dormant. We often think of the two going hand in hand but that is not the case. Dormancy involves a totally different process triggering and relocating different hormones in the tree than chill hours does, often more related to day length. Many fruit crops, for example grapes and pomegranates, are induced into dormancy in the tropics through leaf removal and water restriction. This is a common practice in India. Once a year they are put dormant and then later brought out by supplying water and allowing leaves to grow again. Sometimes a hormone based product called Dormex or a similar one is sprayed so they come out at one time.

We cannot say we have all the answers and know which is the cause or factor(s) for sure but we have developed opinions and strategies, mainly based upon dormancy as the key to produce crops of some apples in low chill environments.

Here are some tips and tricks and ideas we think will help make you more successful in apple growing in hot climates. But most of all, have fun and enjoy experimenting!