What Are Chill Hours and Why do My Fruit Trees Care?

Fruit Tree Chill Hours

As gardeners and orchard enthusiasts, we are often concerned with the temperature conditions in our growing regions. We consult USDA hardiness zones to understand the coldest temperatures our areas can experience, but there's another crucial factor to consider: chill hours. Chill hours are a critical component in the successful cultivation of many fruit and nut trees. Let's review what chill hours are, where to find them for your area, why they are essential, and the risks associated with trees that receive insufficient chill hours, particularly in cold climates.

What Are Chill Hours?

Chill hours are a measurement of the cumulative amount of time that a fruit or nut tree spends exposed to temperatures below a specific threshold, usually 45°F (7.2°C), during the winter months. These hours are counted only when the tree is dormant, meaning it is not actively growing or flowering. This chilling period is crucial for the tree's overall health and productivity during the subsequent growing season.

How to Find Your Chill Hours

The Mississippi State University Extension has developed a helpful resource to determine chill hours in your area (click here or use the button below). The cumulative annual chill hours for a specific region are obtained by summing up the individual chill hours recorded. The calculator provides average monthly data based on various weather stations and can be used for an approximate average at your growing location. 

Mississippi State University Extension

Why Are Chill Hours Important?

Chill hours serve several critical functions for fruit and nut trees. Let's explore why they are indispensable for these plants:

  1. Breaking Dormancy: Dormancy is a natural state that allows trees to conserve energy and protect themselves from harsh winter conditions. During dormancy, trees suspend their growth processes, including budding and flowering, to survive the cold. Chill hours play a significant role in breaking this dormancy. When a tree accumulates enough chill hours, it receives the signal that it is safe to exit dormancy and begin its growth cycle.

  2. Flower Bud Development: One of the primary reasons chill hours matter is their influence on flower bud development. Adequate chill hours promote uniform and healthy flower bud formation. Without sufficient chill hours, trees may struggle to produce flowers, and if they do, the buds might open unevenly or not at all in the spring.

  3. Leaf Production: Chill hours also impact leaf production. When a tree doesn't receive enough chill hours, it may delay the production of leaves. This delayed growth can affect the tree's overall ability to photosynthesize, which is essential for energy production and fruit development.

  4. Fruit Set and Quality: For fruit trees, proper chill hours are directly linked to fruit set and quality. Inadequate chill hours can result in poor fruit development, reduced fruit size, and decreased fruit yield. The fruit may also lack the desired flavor and sweetness.

  5. Pest and Disease Resistance: Dormancy, regulated by chill hours, helps trees become more resilient against pests and diseases. When trees break dormancy prematurely due to insufficient chill hours, they are more vulnerable to potential threats, as their natural defenses are not fully prepared.

Risk of Low Chill Hours in Cold Climates

Now, you might be thinking, "Why not just plant trees with low chill requirements in cold climates? Wouldn't that guarantee enough cold weather for the trees?" While it might seem like a viable solution, it's essential to understand that not all fruit and nut tree varieties have the same chill hour requirements. Moreover, some trees may have a chilling requirement that is simply too low to thrive in a particular region's climate.

  1. Unsuitable Varieties: In cold climates, planting fruit trees with low chill hour requirements might seem like a practical approach. However, this strategy may limit the variety of trees you can grow. Some of the most popular fruit and nut trees, such as apples, peaches, cherries, and almonds, often require a substantial number of chill hours to thrive. Choosing low-chill varieties might mean sacrificing your favorite fruit or nut tree types.

  2. Insufficient Chilling: Even if you select low-chill varieties, there's no guarantee that they will receive enough chilling hours in extremely cold climates. Climate variability, especially in recent years due to climate change, can disrupt the consistency of chill hours. Trees that rely on a limited number of chill hours may break dormancy too early and suffer the consequences.

  3. Environmental Factors: While chill hours are a crucial factor in tree dormancy, they are not the sole determinant. Other environmental conditions, such as temperature fluctuations, humidity, and frost events, can also affect dormancy and tree health. Low chill hour trees in cold climates may still be susceptible to premature budding if these factors are not adequately addressed.

In our video, Fruit Trees – A Selection Guide, we explain that a low-chill tree in a high-chill area would break dormancy too soon and be damaged, or even killed, by the cold weather.

In Summary

Understanding chill hours and their significance is essential for successful fruit and nut tree cultivation. These hours are not just arbitrary numbers but a critical aspect of a tree's life cycle. Adequate chill hours ensure that trees break dormancy at the right time, leading to healthy growth, abundant fruit production, and resistance against pests and diseases.

While it may be tempting to opt for low-chill varieties in cold climates, it's essential to consider the specific requirements of your chosen fruit and nut trees. Sometimes, the best strategy is to select varieties that are well-suited to your region's chill hour availability, and when necessary, implement protective measures to safeguard your trees from adverse winter conditions. By understanding the importance of chill hours and making informed choices, you can enjoy a bountiful harvest of delicious fruits and nuts in your home orchard, regardless of your climate zone.

Here are some guides to low chill (less than 300 hours) fruit trees:

  • Apples – Anna, Low Chill multi-graft, Dorsett and Sundowner
  • Apricots – Gold Kist or Katy
  • Plums – Methley, Burgundy, Satsuma or Mariposa
  • Pluot – Dapple Supreme
  • Cherry – Royal Lee, Minnie Royal and Royal Crimson
  • Peach – Red Baron, Low Chill multi-graft, Saturn, Babcock, Sauzee Swirl, Mid-Pride and Eva's Pride
  • Nectarine – Spice Zee Nectaplum, Double Delight or Snow Queen
  • Pears – most require over 300 chill hours. Asian pears require the lowest chill hours of all pears.
  • Figs, Pomegranates, Quince, Persimmons – all require 300 or less chill hours
  • Special Hybrids – Spice Zee Nectaplum or Flavor Delight Aprium

The majority of the fruit and nut trees require higher chill hours. The selection is huge, so no matter where you live, there is a perfect fruit tree to choose from. Choose wisely and look forward to an orchard that lives happily ever after. For complete orchard information see the book California gardeners rave about, The Home Orchard, written by experts from the University of California.

You can find information on specific rootstock for our trees in our Fruit and Nut Tree Characteristics database.

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12 comments

I came across chill hours first about 18 months ago after our move from the snow belt to SC. I found no real official tally of the hours anywhere locally so started a spreadsheet to do so myself. I used the ‘anything under 45’ calculation because we don’t go below freezing very often. Last year was fine – I came up with 850 hours which was fine for most of my trees. This year I have seen some limitations with straight hours. We had cold at the end of November, then record warmth in December before returning to a cold January. Consequently I restarted the calculation in January. Some of my low chill trees thought Dec warmth was spring so started flowering – not many flowers but still some. In January alone I have tallied over 300 hours. However, the trees don’t use a spreadsheet and I think the number of chill days might need to be a part of the calculation but haven’t seen that anywhere.
Kate

Kate Copsey

Terry, I would bring the tree in before it gets below freezing. I would leave it inside until you are ready to move it out in the spring. I would not move it in and out in the spring, just wait until your temps are above freezing.

Suzanne

Hi, So I have a Black Mission Fig tree and live in zone 6b what if I let it outside in it’s container until it got down to the lowest temperature allowed before killing it. Then brought it indoors dormant and place in a sunny window until spring? Then acclimate it to the outdoors once danger of frost is over or move it daily?

Terry Bluestone

Sharon, chill is usually reported in hours and not weeks. There are charts available online or through your local ag extension that will get you approximate chill hours for your area.

Suzanne

How does “weeks” of chill translate into hours of chill? I am trying to figure out my bulb requirement of 12 weeks. In my area, I get about 6 hours per day at night during the winter. Daytime temps are usually in the 50”s and 60”s. I average about a 1000 hours of chill a year. Love to hear from you all.

Sharon

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