Plant Chill Hours: Why Are Chill Hours Important

Apple Tree Covered In Snow
(Image credit: TatyanaMishchenko)

You might see the term “chill hours” when looking at fruit trees online or notice it on a plant tag when shopping for them. If you’re giving serious consideration to starting a fruit tree in your yard or even planting a small orchard, you may have looked up the term. There you were confronted by another unfamiliar term-- vernalization -- and often a complicated description.

If you want to grow some fruit trees and need some simple information about plant chill hours and why they’re important, continue reading. We’ll attempt to break it down here in simple terms that are easy enough for anyone to understand.

What are Chill Hours?

Chilling hours are basically the hours between the temperatures of 34 and 45 degrees F. (1-7 C.) in autumn that will reach the tree. These are calculated for when the fruit tree is preparing itself to enter dormancy for winter. Hours when temperatures normally reach 60 degrees F. (15 C.) are not included and not counted as chill hours.

Many fruit trees require a time of exposure to temps that are low, but above freezing. These temperatures are required for the trees to perform as we expect them to, like producing flowers that become fruit.

Why are Chill Hours Important?

The proper minimum of chill hours is necessary for flowers and subsequent fruit to form on the tree. They tell the energy within the tree when to break dormancy and when to change from vegetative growth to reproductive. Hence, the apple tree blooms at the appropriate time and the fruit follows the flowers.

Trees that do not get the proper chilling hours may develop flowers at the wrong time or none at all. As you know, no flowers mean no fruit. Flowers that develop too early may be damaged or killed by frost or freeze. Improper flowering may create a reduced fruit set and reduced fruit quality.

Vernalization is another term for this process. Various trees have different chilling hour requirements. Nuts and most fruit trees need a required number of chill hours. Citrus and some other fruit trees don’t have a chill hour requirement, but most do. Trees with low chill hour requirements are available.

If you need to know how many chill hours a new tree needs, you can refer to the tag in the pot or you can research and go a little further. Most places that sell fruit trees purchase them wholesale by the USDA hardiness zone where the store is located. If you are not in the same zone or just want confirmation, there are places to look and calculators are available online. You can also contact your county extension office, which is always a good source for information.

Becca Badgett

Becca Badgett was a regular contributor to Gardening Know How for ten years. Co-author of the book How to Grow an EMERGENCY Garden, Becca specializes in succulent and cactus gardening.