What Are Chill Hour Requirements?

Have you ever heard the term chill hours but weren’t quite sure what it meant and if it was important for you to understand? Well if you are growing fruit trees or certain vegetables it’s probably something you need to know about.

photo of a budding apple tree

What Are Chill Hours?

Chill hours or chilling units is the measurement of a plant’s exposure in hours to chilling temperatures ranging from 32°F to 45°F. Many trees and vegetables require a certain number of chill hours to develop properly and be productive.

Tree Chilling Hours Requirement Chart

CropChill Hours
Apples200 to1000
Peach200 to 800
Plum400 to 700
Pear400 to 1100
Pomegranate100 to 200
Persimmon200 to 400
Fig100 to 200
Pecan300 to 500
Citrus0 to 100
Almond250 to 500
Chestnut400 to 500
Olive200 to 300
Walnut500 to 700

Other Plant Chilling Hour Requirements Chart

CropChilling Hours
Blackberry200 to 600
Blueberry150 to 1000
Grape100 to 600
Strawberry200 to 400
Kiwi300 to 800

Many biennial plants such as carrots, cabbage, and celery also require chilling hours to flower the second year to produce seeds.

Some Seeds Also Require Chill Hours

When it comes to chill hours for seeds the process is known as cold stratification. Seeds of many trees, shrubs, and perennials require this process before they will germinate.

This process can be simulated by soaking the seeds for 1 to 2 hours, thoroughly draining the seeds, spreading them out on a paper towel, wrapping them in another paper towel, placing in a ziplock bag, and putting them in the refrigerator for the required amount of time (usually about a month).

How Do I Find The Chill Hours For My Area

The Midwestern Regional Climate Center Vegetation Impact Program has put together some good interactive maps that will help you determine the approximate chill hours for your area.

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