Why Do Plants Need Chill Hours? (Plus How Many)


If you have fruit trees in your yard, you may have heard that chill hours are necessary for some plants.  You might be wondering what chill hours are, and why plants need them at all.  I was wondering the same thing, so I did some research to find out.

So, why do plants need chill hours?  Plants need chill hours to act as a sort of natural clock to tell them when winter is over.  That way, they can wait to start producing flowers until bees and other pollinators are active.  They can also avoid loss of flowers and fruit due to late spring frosts and freezes.

Of course, the chill hours that a plant needs depends on the species and the variety.  Different climates will vary in how many chill hours they provide.  Let’s dig into some of the details about chill hours and plant dormancy.

Why Do Plants Need Chill Hours?

Plants need chill hours for their own protection and reproduction.  Chill hours tell plants when winter is over, so that they can start producing flowers to prepare for pollination.

blueberry bush
Some plants, such as blueberry bushes, need a certain number of chill hours each winter in order to break dormancy in the spring.

It may seem inconvenient that you cannot grow certain plants and trees in some areas due to chill hour requirements.  However, if a plant produces flowers too early, there is a danger of damage due to late spring frosts or freezing.

 According to Washington State University, “Cold acts as a sort of reset switch that enables some plants to enter dormancy, to rest and recharge before producing next year’s harvest.”

For more information, check out this article on chilling hours from Washington State University.

Many plants need a certain number of chill hours in the winter to produce flowers the following spring.  Flowering too early risks damage to flowers or complete loss of fruit due to late spring frosts or freezing.

Flowering at the wrong time also means that pollination will not occur, since bees are not active in the winter.  For this reason, plants evolved to coordinate their flowering with times when bees and other pollinators are active.

bee on blueberry flower
If a plant produces flowers before bees are active, then pollination may never occur.

A lack of chill hours can cause fruit trees to produce fewer leaves, produce flowers later than normal, and set fewer fruits.  The tree may also weaken, which increases the chance that the tree will succumb to pests and diseases.

In short, plants track chill hours as a type of clock, to tell them when winter has passed.  Contrary to popular belief, a plant does not track hours below freezing!

Instead, plants track hours between 32 and 45 degrees Fahrenheit (0 and 7.2 degrees Celsius) to help them decide when to break dormancy and start producing flowers.

Types of Dormancy

There are actually two different types of dormancy, both of which protect plants from cold damage: endo-dormancy and eco-dormancy.

In endo-dormancy, there is something within the plant that is preventing growth.  A plant in endo-dormancy will not grow, even when temperatures are warm.

cherry blossoms
Cherry trees and other fruit trees cannot produce fruit until they break out of both endo-dormancy and eco-dormancy.

Endo-dormancy is an advantage, because it prevents a plant from producing flowers during a brief period of warm temperatures in the middle of winter.  In other words, endo-dormancy keeps the plant from being “tricked” into producing flowers when more freezing temperatures are still on the way.

In eco-dormancy, the external conditions prevent the plant from growing.  Usually, the condition of cold temperatures cause eco-dormancy and prevent a plant from growing.

Endo-dormancy occurs first, usually starting in the fall when shorter days and colder temperatures begin.  As mentioned earlier, plants in endo-dormancy track chill hours as a sort of clock to tell them when winter is over.

After a plant experiences enough chill hours, it will break out of endo-dormancy.  After that, warm temperatures will cause the plant to break out of eco-dormancy.

If a plant gets enough chill hours too early in the winter and then experiences a temporary warm spell, then the plant will start producing flowers.  If temperatures fall too low after that, damage will occur, including loss of flowers or fruits (possibly all of them!).

For more information, check out this article on dormancy and chill hours from the Michigan State University Extension.

What Is A Chill Hour?

A chill hour is an hour where the temperature is between 32 and 45 degrees Fahrenheit (0 and 7.2 degrees Celsius).

The chilling requirement of a plant is simply the number of chill hours the plant needs in the winter before it breaks dormancy.

According to Wikipedia, “the chilling requirement of a fruit is the minimum period of cold weather after which a fruit-bearing tree will blossom.”

For more information, check out this article on chilling requirements from Wikipedia.

As mentioned earlier, plants track chill hours, but not freezing hours, to determine when winter is over so that they can safely break dormancy.

At What Temperature Do Plants Go Dormant?

Plants will go dormant at different temperatures depending on the species and variety.

For instance, blackberries will go dormant at -10 degrees Fahrenheit (-23 degrees Celsius) for thorny varieties, and 0 degrees Fahrenheit (-18 degrees Celsius) for thornless varieties.

Blackberries will go dormant at 0 degrees Fahrenheit for thornless varieties.

For more information on plant dormancy, check out this article from the Purdue University Extension.

Do Dormant Trees Need Sunlight?

No, dormant trees do not need sunlight.  A dormant tree is not growing or producing leaves, flowers, or fruit, so it does not need sunlight for photosynthesis (energy production).

sunlight through forest
If sunlight causes tree bark to become too warm in the winter, damage from freezing can result.

However, a gradual decrease in sunlight during the day (due to shorter day length in the fall) is one signal to trees that they should enter dormancy.

Keep in mind that too much sunlight in the winter can cause damage and death to trees.  For example, direct or reflected sunlight can cause temperatures in the 70 degree Fahrenheit range on the bark of trees, even when air temperatures are freezing.

If the tree’s tissues become too warm and water moves into them, they will be vulnerable to freezing.  To prevent this, shade the trunk of the tree with a burlap wrap.

For more information, check out this article from the Michigan State University Extension on how trees survive the winter.

Do Dormant Trees Need Water?

As mentioned above, dormant trees do not grow or produce leaves, flowers, or fruit, so they do not need to produce energy (which requires water).  Thus, you should not need to water trees in the winter when they are dormant.

garden hose
Do not water trees if temperatures are too cold, or if there is snow on the ground.

However, you should not withhold water from trees that have not yet entered dormancy, since this can damage the root system or the entire tree.

Instead, water trees into the fall season, as long as air and soil temperatures are over 40 degrees Fahrenheit (4 degrees Celsius) and there is no snow on the ground.

For more information, check out this article on fall and winter watering from the Colorado State University Extension.

Chill Hour Requirements for Fruit Trees

Many plants in the far north or south have low requirements for chilling hours.  In the south, this is due to short winters.  In the north, this is due to the fact that temperatures rapidly decline to freezing, so there are not many hours when the temperature is between 32 and 45 degrees Fahrenheit.

The chill hour requirements depend on species and also on variety.  For instance, blueberry bushes may only require 400 to 600 chill hours.

On the other hand, many fruit trees need 1000 or more chill hours.  There are even some low-chill varieties that may only need 300 chill hours or less.

Some varieties of apple trees need 1000 or more chill hours to break dormancy.

When choosing fruit trees for your climate, it is important to choose trees based on your hardiness zone.  You can find your USDA Plant Hardiness Zone with this map.

However, it is also important to ensure proper chill hours for any trees you plant.  Otherwise, the trees may never break dormancy, and they will be unable to produce flowers or fruit.

You can find a chill hours map on this page from the University of Maryland Extension.

Below is a table giving approximate chill hour requirements for some common fruit trees and berry bushes.

Fruit Tree or
Berry Bush
Chill Hours
Apple800 to 1100
European Pear800 to 1100
Asian Pear600 to 900
Peach400 to 1050
Nectarine400 to 1050
Japanese Plum400 to 750
Cherry1000 or more
Blackberry50 to 800
Highbush Blueberry900 to 1000
Southern Highbush
Blueberry
150 to 500
Rabbiteye Blueberry400 to 700

The information in the table above comes from the Alabama Extension website.

Of course, this table is only a guideline.  For more detailed information, check the catalog or website of a nursery that sells the fruit trees or bushes you are interested in growing.

For more information, check out this article on chill hours from the Missouri State University Extension.

Conclusion

By now, you have a much better idea of why plants need chill hours.  You also know what chill hours are, and how they vary depending on the climate you live in.

I hope you found this article helpful – if so, please share it with someone who can use the information.  If you have any questions or advice about chill hours, please leave a comment below.

jonathon.david.madore

Hi, I'm Jonathon. I’m the gardening guy (not guru!) who is encouraging everyone to spend more time in the garden. I try to help solve common gardening problems so that you can get the best harvest every year!

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