What are chill units?

Firstly, both chill units and chill hours refer to the same thing – the total amount of time a fruit tree needs to be exposed to effective winter temperatures to help them break dormancy and flower and set fruit normally.

The exposure time to these particular temperatures is what is referred to as chill units or hours.

Another way that people refer to chill units with regard to fruit trees is to ask whether a fruit tree needs high, medium or low chill conditions in winter.

A good example of this is stone fruit such as peaches and nectarines. There are low chill stone fruit varieties which can be grown in the subtropics such as Queensland and there are high chill stone fruit varieties which can be grown in cold climate areas such as Tasmania and Victoria.

How are chill units measured?

It must be noted that there is no single scientific way to measure chill units and horticulture scientists use different practices to measure chill units.

We are not here to explain the different sciences of chill units, but merely to give you a gardener’s guide to help you understand which fruit trees are right for you.

Chill units are measurements of temperatures within a certain temperature range measured during the 3 main months of winter (June, July and August).

Before explaining more about the methods of measuring chilling it must be noted that there are many factors that can affect the amount of chill units measured.

All deciduous fruit trees have limiting factors that affect the way they grow and fruit. For example, sunlight, aspect, soil type, shade, wind and even the fruit variety itself, can all influence how your fruit tree will grow. Some of these factors can influence the amount of chill units received by your fruit tree.

The simplest chilling model to look at it in this way is: How many hours during winter is your fruit tree exposed to temperatures below 7°C? The number of chilling minimum or nightly temperatures can be affected by hot days. This will offset the number of hours of night or minimum chilling.

How can I determine the chill hours where I live?

There is a simple way to determine your suburb/area’s chill units.

First you need to determine the average temperature of the coldest month (eg July) for your suburb/area. The Elders weather website can help you to establish this: http://www.eldersweather.com.au

Select your suburb or town in the Local Weather search button at the top right of the webpage and once your local weather is displayed, look for the Climatology button on the bottom right hand of the screen displaying your local data.

Once the ‘Long Term’ averages are displayed, find July and add the maximum and minimum temperatures together and divide this total by 2 to give you the average monthly temperature for July. It’s that easy!

Once you have determined the average temperature for the coldest month you can use the chart or table below to determine the approximate amount of chill units your suburb/area receives. You can then use this as a basis in determining the best varieties that will grow and fruit in your region…we have included some examples to help you.

Low, medium and high chill fruit varieties and a bit of common sense….

There are generally three types of chill groups used to find stone fruit varieties you can grow in your suburb/area, but note that these examples are very general.

• Low chill areas = up to 450 chill units. You can only grow low chill varieties in low chill areas.
• Medium chill areas = 450 – 650 chill units. You can generally grow all low and medium chill fruit varieties providing low chill plants are protected from late spring frosts.
• High chill areas = >650 chill units. You can generally grow all low, medium and high chill fruit varieties providing low and medium chill plants are protected from late spring frosts.

Generally speaking, you don’t need to be too concerned about winter chilling and chill units if you follow a guide as below:

• Tropical and sub-tropical areas will generally be low chill areas – for example coastal areas from the east coast near Port Macquarie north to Queensland are generally sub-tropical becoming more tropical as you move further north.
• Temperate regions are generally medium chill areas – such as on the east coast from Port Macquarie south to Victoria. Many fruit varieties fall into this category which means most Australians living in these areas can enjoy growing peaches and nectarines in their back yard.
• Cool to cold regions are generally medium to high chill areas – this includes an area that runs from Toowoomba down to Armidale, Tamworth, Bathurst, Canberra, Shepparton, Melbourne and right down to Tasmania.

See the map above and examples from the Elders website to get detailed information on the east coast regions of Australia.

This completes your crash course in CHILL UNITS! Happy gardening!

For further information on where PlantNet fruit trees and ornamentals grow see the growing regions map on every product page.