Temperate Fruits Need Chill Hours
Many of the fruits available in the grocery store, such as apples, pears, peaches, plums, grapes, and berries, are adapted to climates in the middle latitudes of our planet. Middle latitudes are referred to as temperate zones, thus the term “temperate fruits.” Temperate fruits require dormant cold periods to complete their life cycle. The ability to become dormant during the cold also provides varying degrees of winter hardiness.
The cold weather requirement of dormant buds to grow, flower, and develop properly is commonly referred to as the chilling requirement. The required cold periods vary widely by species. Chill is measured in hours, units, or portions. There are several models for calculating chill hours. The most widely adapted method, (Between 45 and 32 Model) calculates the number of hours where temperatures are between 32◦ and 45◦F. Your success with growing fruit is greatly dependent on understanding the chill hour requirements.
So what is actually happening? Do plants have a calendar?
In the late fall and early winter, fruit trees go into dormancy and hormones within the plant suppress the bud until temperatures warm up and the days lengthen in the spring. The hormone that causes the plant to go into dormancy breaks down in the temperature range of chill hours. In an ideal winter, the hormone has enough chill hours to completely break down by the time spring comes around and the tree needs to begin blooming.
The weather in Central Texas can play havoc with this process. It’s not uncommon for unseasonable warm and cold temperatures in January and February. This can trick some trees into blooming early (and then freezing) or delay blossoms into late spring.
Find Your Chill Hours
Texas A&M AgriLife Extension has created the following Chill Hour map for Texas.
Travis County Is in the 600 hour region with a range of 450 to 750 hours. That’s a big variation, and the reason has to do with our local geography. A fruit tree needing 700 or more hours is not going to bloom correctly if your neighborhood has less than 500 hours. Or a tree rated for 450 hours may be damaged by frost in an area that typically has 750 chill hours.
There are several websites that can help you find the chill hours for your specific location. Use this information to match the tree’s chill hour requirements. This tool was used to calculate chill hours for the Travis County area: https://getchill.azurewebsites.net/. You’ll need the weather station number nearest you for the search. Chill hours are measured between November 1 and the end of February.
As you can see from the map, there is quite a bit of variance between the stations. The trees you select if you live near station KEDC (Austin Executive Airport outside of Pflugerville) and KAUS (Austin-Bergstrom Airport outside of Austin) can make a difference in the amount of fruit you hope to harvest.
Fruit Without Cold Requirements
There are some subtropical fruits that don’t require cold but have slight frost tolerance, such as citrus, figs, olives, persimmons, pomegranates, and loquats.
Strategies for Tree Placement
Once you know the chill hour requirements for your tree, you can choose the correct planting site to help facilitate the dormancy period. Here in Austin, cold winter winds generally arrive from the northwest. Warmer winds typically come from the Gulf in a southeast direction. This means that you can use structures or wind breaks to manipulate micro-climates to your favor. If you have a tree with a higher chill requirement, plant it on the northwest side of your property where it can get the full blast of the cold. Subtropical trees like loquats will do better on the southeast side of your property. Using your house, other trees, or hedges as a windbreaks will provide additional shelter from icy blasts.
Additional Resources for Chilling Requirements
Additional Fruit & Nut Resources
Fruit Varieties for Travis County
Aggie Horticulture Fruit and Nut Resources
Fruit and Nuts for Austin Overview
Homeowners’ Guide to Pests of Peaches, Plums and Pecans
Small Acreage Fruit & Nut Crop Guides
Fruit & Nut Disease Control Products for Texas
Growing Pierces’s Disease Resistant Grapes in Central, South and East Texas – includes great tips on pruning.
Texas Patio Citrus
Aggie Citrus Program
Texas Native Pecan
Evaluating Pecan Problems