What are chill hours?

If you have ever wanted to grow fruit trees on your property, you have likely seen references to a gardening term “chill hours”. Find out what it is and why it is important for you to consider when planting your trees.

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In general “chill hours” on various fruit trees and shrubs refers to the number of hours the temperature is below 45F. This period of cold is really critical for many species to set fruit. There is some debate about whether this is just the temperatures below 45F or between 45F and 32F. Either way, the higher your USDA hardiness zone, the less chill hours you tend to have.

The way it works is the temperature indicates to the tree when it is spring and thus time to put out flowers. Sometimes there is a late cold snap which can cause flowers or early fruits to freeze and die. There are a few things you can do to protect your trees but not a lot. A big freeze or bad weather like hail will hurt them.

It is a delicate balance between getting too little and too many chill hours. If a low chill variety gets too many chill hours, they will break dormancy too early and the new fruiting buds will freeze and break off. If a high chill variety does not get enough chill hours they will not break dormancy like normal and not set fruit.

How many chill hours do I need?

The answer is it depends. Due to the concentrated efforts of botanists and horticulturists, there are high, medium and low chill varieties of many deciduous fruit trees. High chill varieties can require 900 or more hours. Common varieties are Gala and Fuji apples. There are other fruits such as cherries, peaches, plums, nectarines, apples and pears all require some amount of chill hours.

However, because of selective breeding, many different fruits can be grown from Florida to Maine. It really depends on the varieties. Definitely check your local extension office or a local nursery for more information. They can tell you good varieties for your region and have advice on protecting them if you have a particularly chilly year.

What happens if it is a warm year?

Generally, if there are not enough chill hours, which is definitely a range, then the fruit trees will not be able to set fruit correctly. The tree will not go into or come out of dormancy or will do so at the wrong time. When a tree puts out flowers at the wrong time the flowers might be too early for pollinators and not have much fruit that year. Or the flowers freeze and die in a late freeze.

Warm weather for a high chill variety means that the tree will not receive the correct signal about when to flower. In these years, there is little you can do. This is why most people err on the side or having a lower chill variety in a potentially colder area. The risk there is that a late freeze will kill or damage the flowers. But with a light frost or a short period of freezing or your particular microclimate, you might be able to save the flowers.

How do I count chill hours?

It is actually harder than you might think. The problem when there is a warm spell, this negates any previous days of chill hours. For example, if the temperature raises above 45F for a few hours during the warmest part of the day, even if for a bit, this can change the signal to the tree. However, there are some guidelines and ways to figure this out for your area.

This is the best one I have found. But in general if you just keep an eye on a calendar and start counting any days where the temperature does not rise above 45F then you might have an idea about how many chill hours your tree has gotten. Or you can just check out this map and estimate the number of chill hours for your area. This will help you pick which fruit trees are best for your area and will be the most likely to succeed.

How do you pick the right plants for your chill hours?

The best place to look is either your local extension office or a local nursery. However, depending on where you live, you can definitely search for high or low chill varieties that might be best suited for your area. One key here is do not simply rely on your USDA hardiness zone. This is because of the fact that northern latitudes may have more chill hours but are in a higher zone because of the ocean buffer. A good example of this is the Pacific Northwest where the USDA zone might be 7-9 but the chill hours are very high. This is because many of the areas are below 45F for a lot longer than a similar zone in say Louisiana or Texas.

Nature Hills has an excellent post on low chill varieties of different fruit trees which can be helpful if you live in the south. Dave Wilson has a pretty good post on varieties that are above 500+ chill hours. The great thing is there are many different options even if you live in some extreme environments. Then you can have your very own fresh garden grown fruit!