Chill Hours for Fruit Trees - Calculation using BOM Australia

Many of our Bare Root Fruit Trees that we sell in June and July will not give you fruit unless you get the right Chill Hours in Winter.

For Example the Angel Peach Tree has two different varieties. A High Chill and a Low Chill Variety. If you live in Cairns and buy the High Chill Angel Peach you will not get any fruit.

Definition of Chill Hours for Fruit Trees
Time spent below approximately 7 degrees.

High Chill Fruit Trees are 650+ Hours
Medium Chill Fruit Trees are 450-650 Hours
Low Chill Fruit Trees are 150-450 Hours

Silly Question: Do I go outside every day of the year with a stopwatch and a thermometer?

Answer: You could, but it is obviously a very funny waste of time

Calculating Chill Hours at your Address in Australia
Just to make it clear straight up plants are alive and as such there is no exact science for the amount of chill hours a certain fruit tree needs, it is always going to be approximate.

In Australia we are privileged to get the website for the Bureau of Meteorology and this allows us to find the Average temperature of the coldest month. We then use this temperature to Estimate the chill hours.

Step 1
Go to the the BOMS Climate Data Online

Step 2
Data about: Temperature
Type of Data: Monthly
Select: Mean Minimum Temperature
Weather Station: [Type your own Suburb here]
Nearest Bureau: Closest to your address
Click: Get Data

Step 3: This will give you a view of the Monthly Mean Minimum Temperature for your suburb.

Step 4: Scroll down the page to the last year of information and select the Lowest Monthly Mean Temperature which is usually in July but sometimes June.

Step 6: While you are looking at this number take note of the previous year minimums. Perhaps this year was an exception and you might like to use another years minimum. So in this case I am using 7.2 degrees.

Step 7: Now we want to find the Mean Maximum Temperature that matches this figure from Step 6  So I need to remember 2011 and the month of July. ( yours might be June )

Go back to the BOMS Climate Data Online ( Likely already opened ) But this time select:
Data about: Temperature
Type of Data: Monthly
Select: Mean Maximum Temperature
Weather Station: [Type your own Suburb here]
Nearest Bureau: Closest to your address
Click: Get Data

Step 8: Scroll Down to the latest year and select the temperature for the corresponding month to step 6.

In my example it is 21.1 degrees.

Step 9: Now we want to add the Mean Minimum Temperature and the Mean Maximum Temperature together and divide by 2 to get the average. In this case (7.2+21.1)/2=14.15

Step 10
Now we try and fit this number into the Chill Hours Table below. The Chill hours in this table are commonly used to describe Fruit Tree Chill Hours and you will see them across the Daleys Fruit Tree Website in descriptions, across the web and fruit tree reference books.

Chill Hours           Average Temperature for coldest month
0                               19.7 degrees (No Chill)
300                           15.5 degrees (Low Chill)
330                           15.3 degrees (Low Chill)
450                           14 degrees (Medium Chill)
500                           13.6 degrees (Medium Chill)
520                           13.1 degrees (Medium Chill)
600                           12.7 degrees (Medium to High Chill)
800                           10.2 degrees (High Chill)
1100                         7.9 degrees (High Chill)
1250                         0 degrees (Very High Chill)

So for the case used which was 14.15 degrees we can approximate this to 330-450 Chill Hours or Low Chill. If a plant that requires Medium Chill was purchased it might produce fruit some years when the winter is colder but not others when the winter is warmer.

Things to Consider
Remember how we said you could take a stopwatch and a thermometer outside? Well here is your chance. The weather station that you chose in step 2 is very unlikely to be exactly the same as your backyard orchard. So what you can do is every morning just before sunrise record your backyard orchards temperature. Then go to the BOMs Climate Data Online and fill out the form for Daily Data by selecting:

Data about: Temperature
Type of Data: Daily
Select: Minimum Temperature
Weather Station: [Type your own Suburb here]
Nearest Bureau: Closest to your address
Click: Get Data

Now if the reading you got from the thermometer in you backyard orchard is lower then the BOMs minimum temperature you can assume that you will get slightly more chill hours then the calculation you got by using the above steps. This means that you "might" be able to get fruit with a higher chill variety.

Conclusion and Final Note
If you are reading this it is likely that you are a fruit lover who wants to test the boundaries of what is possible. At Daleys we have come to accept that people in Victoria (Australia's High Chill Country) want to grow Tropical Fruit Trees and people in Rochampton (Australia's Low Chill Country) want to grow high chill temperate fruit trees even though we don't like your chances of getting fruit. However some of you have many examples where you have succeeded. The Daleys Fruit Tree Forum has many of these stories.

If you are in a Low Chill environment and want to grow fruits such as Blueberries, Apples, Plums and even Cherries then we often have varieties that are Low Chill. For example the Acerola Cherry Tree crops in Low Chill Climates.

Growing High Chill Fruit Trees in a Low Chill Climate
Now if you know that you don't get the right chill hours and are still super keen to grow high chill plants my only other suggestion is to grow your plants in pots and get yourself a super big freezer to put them in during winter. JOKING!!!!!!!!!!