While cold weather may seem dangerous for fruit trees, freezing temperatures are important for high-quality fruit production. A fruit or nut tree needs a certain number of chill hours during the winter, as this induces the dormant period that enables it to set fruit during the spring.
Every plant species has different chilling requirements, and understanding what a plant needs is crucial in selecting fruit trees that will thrive in your area. You can do this by understanding the number of chill hours your region receives per year. This article will explain what chill hours are, why they’re important, and how to calculate them for your location.
What Are Chill Hours?
Chill hours refer to the total number of hours during the winter when temperatures fall within a specific range, typically between 32 and 45°F (0 and 7.2°C). This period of cool temperatures is crucial for many deciduous fruit trees and plants to enter a proper dormant phase. During dormancy, essential physiological processes occur, ensuring the plant’s fruit buds develop correctly and are protected from premature blooming.
Why Are Chill Hours Important for Fruit Trees?
For many fruit plants and trees, experiencing a dormant period with these lower temperatures is essential for optimal crop production. These dormant periods serve to rest the plant during winter and regulate key developmental stages, including budding.
Understanding the specific temperature requirements of your chosen fruit varieties is crucial, especially for commercial growers, where fruit yield directly impacts financial success. Insufficient chilling hours can disrupt these processes, leading to delayed or uneven flowering, poor fruit quality, poor fruit set, and lower yields.
Chill Units vs. Plant Hardiness Zones
While plant hardiness zones indicate the coldest temperatures a plant can survive, chill hours measure the number of hours within a specific cool temperature range required for optimal crop production. This distinction is vital: a plant may survive in a given hardiness zone but fail to produce a satisfactory yield if its chill hour needs are not met.
For maximum fruit production, you should select a fruit tree variety graded for your area and then look up the specific chilling requirements and average chill hours your area receives. First, use the USDA Plant Hardiness Zone Map to find your location and ensure that the plant will thrive in your area. Once you’ve narrowed down your options, use the USDA chill hours calculator to ensure your nut or fruit tree will get enough chill hours.
How Chilling Requirements Impact Fruit Crops
For commercial growers, selecting fruit plants that align with their location’s typical chill hour conditions is paramount. This ensures consistent and abundant yields, maximizing profitability. Even for home gardeners, choosing trees that conform to expected chill hours can lead to healthier, more productive plants and a more satisfying gardening experience.
How to Measure Chill Hours?
Chill hours (sometimes called chilling hours) are the number of hours below a certain threshold temperature that occur during the winter period. This threshold temperature is often counted as 45°F (approximately 7.2 °C). In some cases, the threshold is combined with a second, lower temperature, such as 32°F (0 °C), so that chilling hours are only calculated for hours where the temperature observed was between 32 and 45°F (0-7.2 °C).
Using Chill Hours
It’s crucial to understand the chilling requirements of different fruit or nut trees before you start planting, as this ensures you will receive a good crop every year. As it is generally impossible to change the local temperature or move a fruit tree, historical average chill hours can help choose the best variety of fruit plant for a particular location. Some crops require higher chill hours than others.
Apple trees usually have the highest chilling requirements of commercial fruit trees, so they are typically found in colder climates like Washington State. Citrus trees need fewer chill hours, as they are native to the Mediterranean region. Pecans only need about 300-500 chill hours per year, which is why you see this nut tree in warmer climates like Texas.
Using Hourly Weather Data to Calculate Historical Chill Hours
To calculate chill hours, we first need a reliable source of hourly historical weather data for the location of interest. For this, we can use the Weather Data Query Page. This provides hourly weather history and weather forecast data in a variety of formats, including CSV, Microsoft Excel, and as a weather API, if we want to integrate the data into an application.
In this example, we are going to use the page to download the hourly temperature for Yakima, Washington, USA, for the fall and winter from September 1st to March 1st. If this is your first time building a dataset using the Visual Crossing Query Builder, you can use the following article as a guide.
How to Build a Weather Dataset for Download, Bulk Import or Scheduling – Visual Crossing Weather
We then download the data as an Excel file so we can calculate the chill hours directly in Excel. Other applications, such as Google Sheets, databases, or other data tools, could also be used to create the calculation.
In our Excel workbook, there are many columns of weather elements. To calculate chill hours, we will use the hourly temperature column, ‘temp’.
To calculate the chill hours, we will count all the hours that were between 32 and 45°F using an Excel formula:
=COUNTIF(‘Weather Data’!C:C,”<45″)-COUNTIF(‘Weather Data’!C:C,”<32″)
In this case, we count all the hours less than 45°F and then subtract from that number all the hours below 32°F. If you wanted to include hours below 32°F, you could simply use the formula:
=COUNTIF(‘Weather Data’!C:C,”<45″)
This gives us the number of chill hours that have occurred in Yakima, Washington, as 1,358 hours.
As you can see, calculating chill hours is very straightforward. If you want to calculate the typical chill hours for a location, we could extend the above weather data to pull all the hourly data for the past ten years and calculate the number of chill hours per year.
FAQs about Chill Hours
What is the difference between “chill hours” and “chill units”?
While often used interchangeably, “chill hours” typically refer to the number of hours within a specific temperature range (e.g., 32-45°F). For example, if three hours in a day were below this threshold, then the plant would have accumulated three chill hours.
On the other hand, the Utah model uses “chill units,” which offer a more specific measure of chilling hours. It assigns a point value based on certain temperature ranges. These are as follows:
- 1 hour below 34°F: 0.0 units
- 1 hour between 35 – 36°F: 0.5 units
- 1 hour between 36.01 – 48°F: 1.0 unit
- 1 hour between 48.01 – 54°F: 0.5 unit
- 1 hour between 54.01 – 60°F: 0.0 unit
- 1 hour between 60.01 – 65°F: -0.5 unit
- 1 hour above 65.01°F: -1.0 unit
Temperatures below or slightly above 32°F are not considered effective chill hours because they are simply too cold. However, this model isn’t necessarily effective for every growing region. Generally, it is best suited for places with mild winters that are broken up by warmer days. California is a good example of a place where the Utah model may be helpful.
Are there different models for calculating chill hours?
Yes, there are several models. The simplest counts hours below 45°F (7.2°C). Another counts hours between 32°F and 45°F (0-7.2°C), as temperatures below freezing may cause harm to a low-chill tree. We used the 32F-45F model in our example.
However, some additional models are more useful for professional fruit farmers. The Utah model assigns different chill-unit values to hours within a broader temperature range (34-60°F or 1-15°C). Meanwhile, the Dynamic Model considers the two-step process of chilling accumulation and negation, using a unit called chill portions.
In general, the Dynamic Model offers better predictions in areas with variable winter temperatures, such as a high-chill area that occasionally experiences winter temperature spikes.
Why is the 45°F (7.2°C) threshold important in calculating chill hours?
Forty-five degrees and lower is the temperature range at which dormant buds begin to accumulate the chilling necessary to break dormancy and initiate budding. The range between 32°F and 45°F is considered the most effective for breaking dormancy in many fruit trees.
What happens if fruit trees don’t get enough chill hours?
Chill hours encourage fruit trees to go into dormancy. They will produce a type of “antifreeze” that prevents damage to their cells, and metabolic activity slows down. This allows them to conserve energy during freezing temperatures so that they have enough energy to bud and flower during spring.
Chilling hours are similar to how humans need a certain amount of sleep to function properly. A tree must rest during the winter, and it can only do so if temperatures remain below a certain threshold for a certain number of hours.
A fruit tree that has not met its chilling requirements will not break dormancy, which means it can’t break bud. Even with other positive growing factors like sufficient nutrients and pollination, they will experience reduced fruit set or may not produce any fruit at all.
Contrary to popular belief, insufficient chilling may not outright kill a tree, especially if its chilling requirements are usually met year-to-year. They may still produce some leaves and flowers. However, insufficient chill hours may lead to delayed or uneven blooming, lower yields, reduced fruit set, and poor fruit quality.
Fruit trees that get fewer chill hours than they need are more susceptible to certain diseases, just like a lack of sleep can reduce a human’s immune system function. One example is fire blight, a disease caused by the Erwinia amylovora bacteria. This disease impacts fruit trees from the rose family, which includes apples, crab apples, and pears.
How can I find the specific chill hour requirements for different fruit varieties?
There is a wide range of fruit tree chilling requirements. The Mississippi State University Extension Center offers a general list of how many chill hours each variety will need.
However, it’s important to note that a particular cultivar may have different chilling requirements than others of the same species. For example, Asian pears require about 300-500 hours of chilling time, while Bartlett pears need around 800 hours.
For more specific details about chill hours in a chosen fruit tree variety, check with your local nursery, agricultural extension office, or reputable online resources specializing in fruit tree cultivation. They can provide specific information on the chill hour needs of different varieties.
Can climate change affect chill hour accumulation?
Climate change is a global process that is impacting weather patterns differently around the world. Whereas some areas are getting hotter and drier, others are experiencing more extreme cold spells. This means that chilling periods are shifting significantly and may impact which fruit trees thrive in certain areas.
For example, Maricopa County, Arizona, currently gets around 300 to 400 chilling hours per year, making it suitable for pecans and strawberries. As temperatures warm, Arizona farmers may need to switch to plants with the lowest chill hours, like citrus fruits.
Warm temperatures may make it very hard to maintain fruit production for certain species, like apples, as they have high chilling requirements. Honeycrisp apples have one of the highest numbers of chill hours, with at least 800 hours spent below 45 degrees Fahrenheit. This one variety accounts for up to 12% of Washington State’s apple crop, but it may be in danger due to climate change.
However, colder weather can also pose a problem for some fruit trees, especially low-chill varieties. If you’re using the Utah chilling model, you’ll note that temperatures below 34°F do not accumulate any chill units. A region that is experiencing historically low temperatures on a regular basis may require additional safeguarding to protect the plants from the extreme weather, and some plants with lower chill hour requirements may not produce fruit anymore.
How accurate is the chill hour calculation using historical weather data?
Using historical weather data provides a good estimate of chill hour accumulation. However, microclimates and local variations can affect actual temperatures. For the most accurate results, it’s best to use data from your nearest weather station.
The specific model you are using can also impact the accuracy of your calculations. The simple calculations we provided above are good for determining the typical chilling hours. Professional fruit farmers may consider using the dynamic or Utah chilling temperature models for greater accuracy.
Generally speaking, the dynamic chill model is the best choice for areas with more variable winter temperatures, as it considers the impact of different cold hours on fruit growth.
What other factors, besides chill hours, affect fruit tree dormancy and fruit production?
While chill hours are important, several other factors influence crop production, including the following.
- Plant Age: A younger tree will have lower chilling requirements and will bud more readily than an older tree, though this may not always lead to fruit set.
- Photoperiod: This is how long the days are. More sunlight, plus fewer cold hours, encourages the tree to break bud. This is why sudden cold temperatures in spring can damage fruit production.
- Soil Conditions: Insufficient nutrients in the growing space can lead to poor fruit quality. Poor drainage or low aeration can also make it difficult for the tree to get enough nutrients.
- Crowding: Putting too many plants in an area means that each does not get adequate soil nutrients, root space, or sunlight.
- Humidity: Just like animals, fruit trees will have a preferred humidity range. Too much moisture means that the plant can’t evaporate water or draw nutrients from the soil, which prevents fruit growth.
- Proper Pollination: A tree must be pollinated by its local pollinator species to set fruit.
- Regular Pruning: Careful pruning allows fruit trees to concentrate growth in certain limbs and produce flowers.
Can I use forecast data to anticipate future chill hours?
Yes, while our example focused on historical data, you can use forecast data to anticipate future chill hour accumulation. Using the Visual Crossing API, you can access both current and historical agricultural weather data, which can help you predict fruit tree chilling hours.
Visual Crossing blends data from multiple sources, including standard models and ensemble models. We will then determine a degree of confidence based on how much the different models agree with one another. High variability means lower confidence, while low variability provides a higher degree of confidence.
While Visual Crossing’s API relies on highly accurate sources, forecasting is always uncertain. Taking the average of historical yearly chill hours for your area can help you determine which fruit trees will thrive.
Why is it important to understand chill hours before planting fruit trees?
It is essential to select fruit trees whose chilling requirements match your local climate. Fruit trees are a serious investment and often do not produce fruit for several years, so you want to ensure the plant will develop properly.
Chilling requirements vary widely even within the same species. For example, the Spice Zee Nectaplum needs only around 200-300 hours of chilling temperatures, while the Angeleno plum needs between 400-450 cold hours.
Generally speaking, you should pick fruit trees whose chilling requirements match or are slightly lower than the average chill hours for your region. This way, you will still enjoy a good crop even if you have a warm winter. Low-chill varieties may need additional winter protection if you have cold weather.