As fruit and nut trees go into fall, shorter days and cooler temperatures stimulate a hormone which triggers the trees to go into dormancy and stop growing. Cold temperature breaks down that hormone and when the tree experiences enough cold temperatures, dormancy is broken and the tree starts to grow again, by flowering and developing leaves. The period of cold temperatures needed to break dormancy is the cumulative chill hours, temperatures between 32- 45°F from November to February. What happens if your tree does not get enough chill hours? The tree will produce leaves later, blossoms may not open or just drop and therefore your fruit tree will not produce fruit. One the other hand why not just plant trees that have a much lower chill requirement? If you plant low chill trees in a high chill area your trees will break dormancy too early and the blossoms will be killed by cold temperatures and you have the same result, no fruit. Chill hours needed are not an exact number, it is usually a range. If you don’t know the chill hours in your area, consult your local Farm Advisor, Master Gardener or even a local nursery may be able to help. We have an article on chill hours, however, the links included to find your chill hours are only for California counties. If you live outside of California, you will need to search the internet to find sources, weather stations or county Ag Offices that can assist you. There is another site you can use, but it will take a little bit of work since the chill hours can only be looked up 10 days at a time. But once you get your chill hours calculated you are done. Might want to look at chill hours for several periods, since it does fluctuate. Just start from November 1 and go through February of the following year.