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CHILL HOURS – – – IN PLAIN ENGLISH
Calculating Chill Hour Requirements is very complicated. It is best to call the greenhouse to check the number of Chill Hours that usually occur in your location. Areas of the country where the climate is either very hot or extremely cold will not have much success trying to figure this out.
Without getting too technical about it:
In the Fall when leaves drop from the trees and you can stop mowing the lawn, you know your trees or plants are ready to start entering Dormancy. Chill Hours must be acquired in Late Fall through Early Winter. This induces Dormancy, and hardens them in before the shock of a full blown Winter sets in.
Anything between freezing temperature (32 degrees) and 45 degrees are what generally counts toward Chilling Hours. One hour below 45 degrees equals one Chill Hour.
If the temperature drops much below freezing, you will no longer accumulate Chill Hours. The plant or tree will be literally frozen stiff, and just like your own circulation, things just stop flowing.
The soil gets rock hard in the Wintertime. When tree limbs try to unthaw during a Winter warmup while the roots are still frozen solid it is a recipe for a disappointing fruit crop, if any at all.
RELATED ARTICLE: IMPORTANCE OF CHILL REQUIREMENTS
If Dormancy is interrupted in the middle of Winter by a warm spell (temperatures above 55 or 60 degrees) things start to go awry.
Whenever we have a “January Thaw” and I notice people wearing shorts to the supermarket, I know it will interfere with the accumulated Chill Hours of my fruit trees.
The same is true in March, when the old saying about, “In Like A Lamb And Out Like A Lion” sometimes interrupts a tree’s Dormancy cycle.
When these mid-Winter warmer temperatures happen, the number of acquired Chill Hours starts to reverse (you subtract from them.) Two steps forward and one step back, in other words.
Some plants and trees are easily fooled by these warmer temperatures, and will begin to produce flower buds and leaves. Nevertheless, when freezing temperatures return, this will “nip” them. This will bring your chances of a good fruit harvest to a halt.
RELATED ARTICLE: CHOOSING THE PERFECT FRUIT TREE
NOTE: I have not planted apricot trees. They usually bloom so early in the year that the frost nips the forming buds and no crop will result. This is sad, for apricot jam spread thickly on toast and a bowl of oatmeal is a favorite breakfast of mine.
THE “RIDDLE” OF GRAFTING ROOTSTOCK
Some grafting rootstock can tolerate much more winter cold than others. And some might actually require a longer period of wintertime chill.
I suspect the reason that calculating Chill Hours is not an exact science is due to the rootstock itself. If the fruit tree that the rootstock originally came from has a different time requirement for Chill Hours than the fruit tree that is being grafted onto it, this might be part of the problem.
Chill Hours must be followed by a certain amount of heat to break the period of Dormancy. Due to this, part of the cold hardiness of a tree derives from a higher heat requirement for ending dormancy.
In other words, once that tree goes dormant, it waits till warm weather is here to stay before producing buds. Selecting tree varieties that naturally bloom late is ideal for the colder regions of the country.
HAPPY PLANTING! Susan
For more in-depth information on GROWING YOUR OWN FRUIT