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The LEAF Network is a community-based organization with the mission to link people with the benefits of edible trees and support edible trees with people’s stewardship.

Chill Hour Requirements

Click Here for a PDF of Chill Hour Requirements


Chill hour requirements are the cumulative number of hours that wintering trees must spend between the temperatures of 32°F and 45°F to stimulate proper bud growth, fruit set and fruit development after the winter dormant period. Edible trees that have chill hour requirements are shown at LEARN - Selected Edible Trees that Grow in Arizona. Specific chill hours for selected edible trees and for low chill varieties are shown in the table Chill Hour Requirements below. See the Edible Tree Directory for the range of chill hours needed for the full range of Arizona edible tree species. For additional information, see CHOOSE - Cold Air and Chill.

Many fruit and nut trees that grow in temperate climates (climates that are moderate rather than extremely cold or hot) have chill hour requirements. Most temperate fruits, such as figs, pomegranates, olives, apples, pears, peaches, and plums require 200 to 2000 chill hours in the winter. Fruit and nut trees requiring long chill hours are suited to the higher elevations of Arizona (High Plateau/Mountain and High Mountain areas), while fruits with low chill requirements (600 hours or below) grow best in the desert zones (Low to High Desert regions). In general, the later a fruit or nut tree typically blooms, the more chill hours it requires.

Determining tree chill requirements is not an exact science, so different sources have different ranges of chill hours. Use the resources on this page to determine the chill hours available in your area of Arizona and choose trees accordingly. Many types of fruit and nut trees have low-chill varieties that can be planted in desert areas. Ask experts at local nurseries and Cooperative Extension offices for their recommendations for appropriate tree varieties for your areas. Experiment to see what grows best at your site.

Dormant pomegranate in winter is accumulating chill hours.

To calculate the Chill Hours available for your Arizona region, follow the steps below:

  • Step 1. Go to the Weather Underground website to see meteorology stations across the US. Find Arizona and click on the weather station nearest you. Record the code for that weather station.
  • Step 2. Go to the website Get Chill Hours . Enter the Weather Underground weather station code for your area. The website will calculate the number of chill hours in your area.
  • Step 3. Using the number of chill hours typical of your site, visit nurseries and websites that focus on fruit and nut varieties with different chill hour requirements to select appropriate trees.

FOR EXAMPLE:
In Skull Valley, AZ find the nearest Weather Underground Station at https://www.wunderground.com/us/az/skull-valley?MR=1. The weather station code is: MYHSA3. Enter this code into the http://getchill.net/website, and select “calculate chill.” Result for the “Below 45 Model” is 1194 chill hours. Result for the “Between 45 and 32 Model” is 1063 chill hours.

CLICK TO DOWNLOAD A TABLE PDF

Copyright 2017

LEAF is under the fiduciary stewardship of the Arizona Community Tree Council, a 501(c)3 non-profit organization.

PO Box 65122, Phoenix, Arizona 85082-512





 

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