Chill Hour Requirements for Fruit Trees and Shrubs
You may have read or heard about chill hours and what the term actually means when it comes to fruit trees.
Often times, chill hour requirements for different fruit trees are included in the product description attached to the tree.
But why are they important and how do you know if you have enough? Let’s go over these and other related questions in more detail.
Why Are Chill Hours Important?
It’s been long known that many fruit trees require exposure to at least some amount of cold temperatures during the winter in order to bear fruit the following season.
Fruit trees that drop their leaves and go dormant in the winter use accumulated chill hours to help them determine when to break dormancy.
Without sufficient chill hours, fruit trees may not produce any fruit at all and even if they do, the quality and quantity of fruit may suffer.
Identifying fruit tree cultivars with low chill requirements is especially important for areas with mild winters.
How Are Chill Hours Defined Exactly?
While the overall concept of chill hours is rather straightforward, there is no consensus about how to define and measure them. There are actually multiple models that try to account for required chill hours.
The two most common models go by either (i) the number of hours under 45°F or (ii) the number of hours between 32°F and 45°F.
That said, research has shown that the way a fruit tree goes about accumulating chill hours is more complex.
Significantly, there is no cut-off temperature that stays constant in all environments, and chill hour accumulation can be more or less effective depending on the temperature range. Some chill hours may still be accumulated above 45°F and there may be little or no chill hours at freezing temperatures below 32°F.
In practice, the two chill hour models are unlikely to contradict each other because climates with lots of hours below 32°F will usually also have more than enough hours between 32°F and 45°F.
Do All Fruit Trees Require Chill Hours?
Fruit trees chill with chill hour requirements include many deciduous fruit trees such as peaches, apples, almonds, pears, nectarines, plums, apricots, cherries, figs and pomegranates.
Importantly, chill hour requirements can vary even within the same fruit tree family. For example, one type of apple tree may require 800 chill hours while another apple tree cultivar might only need need 200 chill hours.
At the same time, it’s important to point out that there are also many fruit trees without a chill hour requirement. In particular, many tropical and subtropical trees (e.g. oranges, avocados, passion fruit, mangoes, bananas, etc.) never go dormant and do not require any chill hours. On the contrary, they may be vulnerable to low temperatures.
What Are Low Chill Hours vs High Chill Hours?
Delineating low chill hours vs. high chill hours is somewhat arbitrary. Typically, however, fruit trees that require under 300 chill hours would be considered low chills whereas fruit trees that require more than 800 chill hours would be considered high chill. Everything else in between would be considered an average level of chill hours.
How Do I Find Out the Amount of Chill Hours in My Area?
There is no central database that will list chill hours for all regions of the United States. Your best bet is to consult with your local nursery or local colleges. Some limited information may also be made available online.
Does Climate Change Affect Chill Hours?
Yes, climate change affects chills hours. As winters become milder in many areas of the United States, accumulated chill hours drop.
This is particularly problematic for transitional agricultural zones such as the Central Valley of California. Orchards in such areas will eventually have to switch from high-chill cultivars to fruit trees that require a lower number of chill hours.