What are Chill Hours for Fruit Trees? Chart Included

Understanding a fruit tree’s life cycle and growing requirements is key to improving its yield. That may involve knowing some technical aspects, from soil quality, and nutrient needs, to light requirements.

Most deciduous trees undergo what we call chill hours – a very important stage preceding blooming and fruit-bearing. Before you choose fruit trees and berries for your climate, it is vital to understand their chill hours as this will affect their yield.

So…

What are chill hours, and how are they important for my fruit trees? 

The meaning of chill hours

Chill hours are the number of hours that a fruit tree needs to spend at a certain temperature range in order to break dormancy and flower and bear fruit during the growing season. The temperature range for chill hours is typically between 32°F and 45°F (0-7°C) over winter when the tree is undergoing dormancy.

The exact range can vary depending on the species and variety of fruit trees.

In some practices, we subtract the number of hours where temperatures during the winter exceed 60°F, which then reduces your total number of chill hours throughout winter.

For example, if you live in a region that experiences a few episodes of heat waves during winter, you’ll end up with fewer chill hours.

Importance of chill hours

Chill hours are essential for fruit trees because they need this dormancy and chill requirement to break buds and form fruit the following season. A telltale sign that fruit trees did not get enough chill hours is a lot of leaves but no fruit.

In summary, here’s why chill hours are important:

• They help to break dormancy and promote flowering. The right amount of chilling time is required for healthy bud growth. The tree accumulates chilling hours throughout the
• dormant season, which aids in breaking down inhibitory compounds and encourages healthy bud growth.
• They ensure fruit trees flower and produce fruit on time.
• Adequate chill hours always result in a bountiful yield, assuming other growing conditions are met.

Expert tip: If your primary concern is the quantity of fruit yield, I recommend choosing low-chill fruit trees (less than 300 hours) to maximize on the return on your investment. Remember, inputs like labor, compost, fertilizer, etc., are costly and can be even higher when growing chill-needy trees.

Effect on fruit trees

Daniel J. Leonard, a Calhoun County Extension Director and Agriculture, Horticulture & Natural Resources Agent notes that “too few chilling hours equals poor to no fruit the following year.  Too little cold can be just as harmful as too much!” ^[2]

Failure to receive adequate chill hours, most fruit trees can fail to produce optimal blooms and good fruit sets. In other words, poor fruit quality may result.

Apples, blackberries, and peaches, for example, depend on cool weather in the winter to promote proper bud growth in the spring when we expect them to blossom and yield fruit soon after.

In fact, timing your planting is very important. The blooms of your fruit trees should ideally avoid the final spring frost that’s known to damage budding fruit. While it is difficult to be accurate with this timing, aiming for a close match is highly recommended.^[3]

Types of chill hours

There are three main models for calculating chill hours:

Hours Below 45°F model

This model requires continuous temperature monitoring during the dormant period of the fruit tree and specifically focuses on temperatures below 45°F (7.2°C). This threshold represents the critical temperature range for chill accumulation in most fruit trees. Temperatures above 45°F do not contribute to chill hours.

To calculate chill hours, the model sums up the number of hours that fall below 45°F during the entire dormant period. For example, if the temperature is below 45°F for 10 consecutive hours, that would add 10 chill hours to the cumulative total.

Utah model

The Utah model quantifies Chlling Units (CU) and uses different ranges of temperatures as a contribution to the total chill hours for dormancy completion.

According to this model, a chilling unit is equivalent to one hour registered to have temperatures ranging between 2.5 and 9.1°C. This range is said to be effective for dormancy completion.

Here’s the concept data for calculating chilling units according to the Utah model:

1 hour below 34°F = 0.0 chill unit; 1 hour 35 – 36°F = 0.5 chill units; 1 hour 37 – 48°F = 1.0 chill units; 1 hour 49 – 54°F = 0.5 chill units; 1 hour 55 – 60°F = 0.0 chill units; 1 hour 61 – 65°F = -0.5 chill units and 1 hour >65°F = -1.0 chill units 2.^[4]

Dynamic model

The dynamic model of calculating chill units was established in 2006. While it is fairly complex to use, it is considered to be the most accurate because it accounts for the duration of cold temperatures, as well as the number of hours that the temperature falls below certain thresholds.

Type of chill hour	Temperature range	Accuracy
Hours below 45°F model	32°F to 45°F	Simple
Utah model	32°F to 47°F	More accurate than hours below 45°F model
Dynamic model	32°F to 47°F	Most accurate type of chill hour model

Chill Hours and Fruit Tree Varieties

Chilling requirements vary widely depending on the species and variety of fruit trees or berries. Some trees have a high chill hour requirement – typically those that grow in the cooler climates found in the north, while others need fewer chill hours – typically those in the southern regions with warmer climates.

As a general rule, choose a low-chill variety if you live in a warm climate, but if you live in a cold climate, plant a high-chill variety.

Examples of fruit trees with the lowest chill hour requirements are figs, quince, olives, chestnuts, persimmons, almonds, and pomegranates.

Some types of fruit trees have undergone crossbreeding to improve their adaptations to chill hours. Examples are varieties of peaches, apples, plums, and cherries.

• High-chill fruit trees are those trees that require more than 800 chill hours.
• Low-chill fruit trees are those that require under 300 chill hours

Here’s a table showing the average chilling-hour requirements for different fruit trees and shrubs:

Fruit tree/shrub	Required chilling hours
Apples	200–1000
Grape	100–600
Fig	100–200
Persimmon	200–400
Blueberry	150–700
Pomegranate	100–200
Citrus	0–100
Pecan	300–500
Blackberry	200–600
Plum	400–700
Peach	200–800
Pear	400–900
Strawberry	200–400

Peach trees

Unlike apple trees, peach trees require less cooling. Usually, they require 300 to 600 cooling hours. ‘Redhaven’, ‘Elberta’, and ‘June Gold’, for instance, call for between 400 and 500 hours, 550 to 600 hours, respectively.

Peach trees can encounter delayed bud break, inconsistent flowering, and decreased fruit set if these conditions are not satisfied.

Cherry trees

Cherry trees require different amounts of chilling. While sour cherry breeds have lower demands, approximately 400 to 700 hours, sweet cherry varieties have higher requirements, roughly 700 to 1,200 hours. The ‘Bing’ sweet cherry, for instance, needs 700–800 hours, whilst the ‘Montmorency’ sour cherry needs 400–500 hours. Inadequate cooling can result in decreased fruit sets and poorer yields, which can affect cherry growers’ financial returns.

Plum trees

Plum plants have mild chilling requirements that range from 500 to 900 hours. ‘Santa Rosa’ (400-500 hours), ‘Methley’ (500-600 hours), as well as ‘Stanley’ (800-900 hours) are other examples. If plum trees lack enough cooling hours, aberrant fruit development can occur, resulting in deformed or biologically disordered fruits with lower quality and market value.

Pear trees

Pear trees, like apples, often require more chilling time. They typically require 600 to 1,000 cooling hours. For example, ‘Bartlett’ necessitates 600-800 hours, ‘Anjou’ necessitates 800-1,000 hours, while ‘Bosc’ necessitates 800-1,000 hours. If pear trees do not get enough cooling hours, it can cause delayed bud break, inconsistent flowering, and decreased fruit set, reducing the total output and economic sustainability of pear orchards.

Converting chill hours to days

1 chill hour is equivalent to 1 hour of temperature between 32°F (0°C) and 45°F (7.2°C).
So, 1000 chill hours = 1000 hours of such temperatures.
To convert hours to days:

Hours ➗ 24

= 24 ➗1000

= 41.67

So, 1000 chill hours is equivalent to approximately 41.67 days.

Here’s a calculator you can use. Simply enter the chill hours of your fruit tree and hit the ‘Calculate Button.

FAQ

Choosing the right variety for your climate

A rule of thumb is to choose fruit trees that require the lowest amount of chill hours for your USDA growing zone. Growing a low-chill tree in a zone with high natural chill hours means the tree will break its dormancy too soon.

The prevailing frost will damage the buds, leading to poor yield.

Gardener’s note: I recommend planting a few different varieties of fruit trees with a range of chilling hours to guarantee some harvest because we have observed chilling hours vary from year to year in the same regions. It is normal to experience fewer or more chill in a year compared to the previous year.

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