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Chill Hours

More and more people are starting to question the idea behind “Chill Hour Requirement”.  Very little scientific research has been done in controlled lab settings on this.  Those that have been done usually rely on a few newer commercial cultivars.


So how do we really know what an apple tree’s chill requirements are, if any?  Many varieties have been developed in places such as New York or Minnesota that have an over-abundance of the normal chill hours.  Most of it must come from guessing.  Assuming what the parent’s requirements were and therefore suggesting the progeny would be the same is often done. One problem with that approach is that who really knows what the parent tree’s requirements were and if they were a guess also?  The apple genome has been mapped and shown to be 3 times the size of the human’s.  That’s a lot of genetic diversity and potential. Who is to say it will have the characteristics of the parent?


More and more people are growing apples in areas where it was assumed they would not do well because of lack of chill hours.  Now there are many different formulas and methods to calculate chill hours. The chilling process in trees is not completely understood and it is difficult to produce a model that perfectly explains the accumulation of winter chill. So we have the Accumulated model, The Utah model, The Dynamic model, The Richardson’s model, all in an attempt to explain what is required for varieties to fruit properly.  Also some research points to chill hours only affecting the hardiness of the fruit bud and tree, while dormancy is the key factor in having them come out and flower correctly.


While chill units accumulate normally when a tree is dormant in winter, the two things are not the same. Trees can be induced to go dormant in summer also by certain stressors, drought and stripping leaves among them.  In fact the two things, dormancy and chill hours, are not so much related but may be intertwined in some fashion we do not yet know or understand.


What we do know is that some apple varieties that have supposedly have higher chill hour requirements are actually performing well in low chill environments. Riverside in Southern California, some of the southern parts of the Gulf states and Florida here in the US. Apples are being produced in Israel and Middle East Countries, Australia, South Africa, Tanzania, Uganda, Kenya and more.

One of the effects of lack of chill hours is that the tree will flower over a long period of time and then ripen over a spread out period. Definitely not good for commercial growers from a labor standpoint, but it could actually be beneficial to a homeowner.  Would you rather get all your apples in a 2 week period, feast or famine style or have them over a 2 month period?

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