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Calculate your Chill Hours and Grow the best Stone Fruit

Updated on May 7, 2015
Great fruit comes from great flowers - the right chill hours and plenty of pollination opportunities will help
Great fruit comes from great flowers - the right chill hours and plenty of pollination opportunities will help

Chill Hours is a calculation of how much time your fruit trees spend below 7ºC (44.5ºF) per year. Knowing how many chill hours you get in your location each year will help you to choose the right sorts of fruit trees that will succeed in your garden.

In late summer and autumn, as the days become shorter and cooler, deciduous fruit trees such as stone fruit stop growing, start storing energy, the buds go dormant, they lose their leaves and enter a state of healthy dormancy which protects them from the freezing temperatures of winter. This dormancy is triggered by a certain minimum exposure to chilling temperatures. Lack of such exposure results in delayed and substandard foliation, flowering and fruiting .For example, cherries, nectarines, plums need different chill hours to fruit vigourously.

Once the plant is dormant, a deciduous fruit tree will not resume normal growth, including flowering and fruit set, until it has experienced an amount of cold equal to its minimum “chilling requirement” followed by a certain amount of heat.

Fruit tree varieties all have different chilling requirements and range widely from one variety to another. If a fruit tree is grown where winter cold is insufficient to satisfy the variety’s chilling requirement, blooming and foliation will be delayed and erratic and fruit set and quality will be poor.

Peaches can be easy to grow if you get the right variety for your area
Peaches can be easy to grow if you get the right variety for your area

Chilling that exceeds a fruit tree’s minimum requirement can lead to a stronger bloom and, potentially a heavier crop. A disadvantage of heavier crops is they require more thinning to get the best fruit quality and size. If you are a home fruit grower you may actually prefer moderate crops and less thinning work up front.

A fruit variety’s chilling requirement is a key determinant of where it will consistently produce satisfactory crops of fruit. So, how do we measure chilling?

To calculate the chill hours at your place, if you are in Australia you can use the BOM website (www.bom.gov.au).

Select:

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

When the table shows, it will include Monthly Mean Minimum Temperature. 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 or August depending on where you live.

Go back to the Climate Data Online and 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.

Then find the data for the same year and month you used for the Mean Minimum Temperature.

If you are in the USA, you can find this information by going to the National Climatic Data Center, click here. Select data for Maximum Temperature of your location and Minimum Temperature of your location for the coldest month of the year.

Add these two numbers together and divide them by 2, to give you the average temperature in the coldest month. Then using the table below you can find your chill hours:

Average Temperature for Coldest Month
Chill Level
Chill Hours
19.7ºC/67.5ºF
No Chill
0
15.5ºC/60ºF
Low Chill
300
15.3ºC/59.5ºF
Low Chill
330
14ºC/57ºF
Medium chill
450
13.6ºC/56.5ºF
Medium chill
500
12.7ºC/54.9ºF
Medium to High Chill
600
10.2ºC/50ºF
High Chill
800
7.9ºC/46ºF
High Chill
1100
0ºC/32ºF
Very High Chill
1250

It’s always a good idea, when considering your purchase to talk to a local nursery person. They will know they chill hours of local areas and are in the best position to advise you what will grow well in your part of the world.

If you live in a part of the world that gets low chill hours, there is a great list here that gives you a range of low chill fruit that you can try. It is heartening to see that there are even several varieties of blueberries which are worth searching out if you need low chill requirements as they can be grown in areas of low chill hours:

  • Emerald (150)
  • Misty (150)
  • Pink Lemonade (200)
  • Sharpblue (150)
  • Sunshine Blue (200)

Here are the chill hours required by some of your favourite summer stone fruits

Variety
Chill Hours Required
"Snow Queen" Nectarine
650
"Snow Zee" Nectarine
900
"Fairlane" Nectarine
700
"Early O'Henry" Peach
900
"Fuji" Apple
200- 300
"Santa Rosa" Plum
150
"Florda Gold" Peach
300

If you are growing plants with a chill hour requirement that matches your area, yet are still not getting good fruit set, then you may need to consider additional factors that affect fruit set include the age of the tree, nutrition that has been supplied to it during its growing period, availability of compatible pollen and what the weather was like during blooming – if it has been really windy you may get less fruit set and rain can have a detrimental effect too. Talk to your neighbours who grow fruit trees and compare notes – you may discover that in any given year you all have the same issues and you may discover someone who has a solution or who has tried something new to get good fruit set. And they may well share their ideas with you.

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