Fruit Trees & Chill Hours

Fruit trees require cold weather, known as chill time, to help generate fruit.

In order for trees to produce fruit, they must undergo a period in which cold weather induces dormancy. This die-back of the tree in winter helps them store up energy for the fruit producing season in the spring and summer. In general, fruit trees need temperatures to drop below 45 degrees F for a set period of time, with each fruit and each variety of fruit typically needing a different amount of so-called "chilling hours."

Benefits of Cold Weather

A general way to measure chill hours is to tally up the amount of hours between early November and mid-February in which the temperature stays between 32 degrees F and 45 degrees F. Once calculated, you can determine whether an apple tree that has a chilling requirement of, for example, 1,000 chill hours is suitable for your region. The final number may help determine that your yard is more suitable for a tree needing fewer chilling hours.

Topography Plays a Role

Chill hours can vary within a region depending on the topography and the position of the tree in a yard. Temperatures at the top of a hill or at the bottom of a valley can vary to the point where a fruit tree needing a set amount of chill time may do well in one area, but fail to produce fruit in an area not too far away. A tree located in a sunny spot on the east side of a house, where the morning sun shines first, can have a different chilling time than a tree in a different part of the yard.

Long Chill Time for Apples

Many varieties of apple trees have longer chill hours than other fruit trees. The popular Gala apple, for example, requires up to 600 chill hours. This explains why states like Washington, Michigan and Wisconsin are popular apple-growing regions.

Medium Chill Time for Pears and Apricots

Pears and apricots need fewer chilling hours. Kieffer Pears, a variety used for canning and cooking, require about 350 chill hours. Gold Kist, a variety of apricot that tolerates warmer winter climates, needs 300 chill hours.

Shorter Chill Time for Peaches and Plums

Peach and plum trees generally require fewer chilling hours than some of their other fruit tree counterparts. The Babcock variety of peach needs about 250 hours of chill time, as does the Beauty variety of plums.

References

• California Agriculture, March-April 1984: A quick method of estimating chill hours
• University of Arizona College of Agriculture: Deciduous Fruit & Nuts for the Low Desert