PlantNet Chill Guide

What the? Chill Factor? Chill Units?

Confused when you here about CHILL FACTOR or CHILL UNITS? Well now you don't need to be as we have written a simple guide as to what to be aware of and what to look for when talking about chill factor.....we know you are not dummies but our non scientific guide gives you the basics to make informed decisions on what sort of fruit tree to buy whether it be high, medium or low chill.....Check it out below:

PlantNet’s CHILL UNITS/HOURS GUIDE for dummies!

The question of what are Chill Units or Hours is a thing that is often requested through the PlantNet office. It seems many people either have no idea of Chill Units or have a confused idea of it! As we like to help, PlantNet has decided to tackle the questions about Chill Units head on....

What are Chill Units?

Chill Units or hours refers to 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 time exposed to these particular temperatures is often 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 stonefruit such as peaches and nectarines – there are low chill stonefruit varieties which can be grown in the subtropics such as QLD and there are high chill stonefruit 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 ONE scientific way to measure chill units and horticulture scientists use different practices to measure chill units. There is no one right or wrong way of measuring chill units, just different ways.

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?
Now before you start going mad trying to work out an equation to measure these hours, there is a much simpler way to determine your suburb’s/area’s chill units.

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

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

Once the Long Term averages are displayed, find July and add the  Maximum and Minimum temperatures together and divide the total by 2 to give you the average Monthly temperature for July. It’s that easy!

SEE ELDERS PICTURE ABOVE

Once you have determined the average temperature for the coldest month you can use the chart or table below to determine approximately the amount of chill units your suburb/area receives and you will then be able to determine what varieties will grow and fruit in your region...we have included a few examples to help you.

Low, Medium and High Chill fruit varieties and a bit of common sense....

There are generally three types of chilling groups used to find stonefruit varieties you can grow in your suburb/area – and these examples are very general:

• Low chill areas = <150 - 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 people do not need to be too concerned about winter chilling and chill units if common sense is used, such as this simple guide:

• Tropical and sub tropical areas will generally be low chill areas – for example coastal areas from the east coast near Port Macquarie north to Qld are generally subtropical and getting more tropical as you go 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 and 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 region’s the east coast of Australia.

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