With the unseasonable weather we are having, there is one subject that I get asked a lot about lately: “Are the warm temperatures going to hurt the trees? Will they start blooming?” Well the answer is no. But that opens up a complicated and fascinating subject.
When a fruit tree finishes its growing season, it slowly enters the process of dormancy as winter approaches. Temperatures play a large role in this. With the warm fall we had this past year this process was delayed for weeks. But once the trees reached the dormant stage, that’s when the chilling begins.
Fruit trees need a certain number of “chilling hours” in order to rest and prepare for the next season. A chilling hour is defined in different ways. Some of the calculating models say that any temps below 45°F count, but others, oddly enough say that temps below 30°F do not count as chilling hours. Still other models begin at 50°F and below and count any hours above 60°F as subtracting from chilling hours. Confused yet? So am I! (and the trees probably are too!) But it is generally accepted to use temperatures between 32°F and 45°F as satisfying the chilling hour standard. And that is the degree range that is tracked by adding up the hours each day during the winter.
Different fruits, and even different varieties within a fruit type, have different requirements for chilling hours. Most apples need 700-1000 chilling hours to grow and produce properly. There are exceptions such as Gala, Fuji and Granny Smith which need only 400 -600 hours. That is why those varieties are commonly grown in California. Most peaches need 750-950 hours although again there are some as low as 200 hours. Cherries need 1000+ chilling hours–ever notice that most are grown in the northern U.S.A.?
So what happens if a fruit tree doesn’t receive enough chilling hours in the winter? Well the tree may produce buds with very weak and uneven blossoms, or it may bloom later and over a wide time period. In severe cases the trees may not bloom at all and have a difficult time producing leaves!
Fortunately here in Michigan we have pretty consistent accumulations of chilling hours year after year. The varieties of fruits that we grow are nicely suited to the chilling hour numbers that we see. If you have ever seen a climate zone map such as those put out by nurseries and seed companies, chilling hours are one factor that goes into determining the the different zones. Want to calculate the chilling hours in your neck of the woods? Here is a place to do that.
So our fruit trees need some time to just chill out in order to recharge for another growing season. Probably a lesson in there for all of us!
Have a fruitful week!