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 so they will 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.
Chill units are measurements of temperatures within a certain temperature range measured during the 3 main months of winter (June, July and August).
A simple way to calculate chill units
The available chill units is the number of hours during winter the temperature, during the day and night, is below 7°C. The total amount of chilling can be affected by hot days which offset the number of hours of night or minimum chilling.
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 lower down on the right hand side 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.
Taking Ballarat, for example:
Mean Max + Mean Min = 13.2 2 = 6.6° for July
Therefore ave. monthly temperature for July is 6.6°
Determining the Chill Units for your area
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.
There are generally three chilling categories used when considering suitable varieties for your locality – and these examples are very general:
Low chill areas < 450 chill units
You can only grow low chill varieties in low chill areas.
Medium chill areas from 450 to 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.
Guidelines to Chill Units and how they affect the blossom
• Select varieties that have a chilling requirement at least 20% less than local averages.
• Selecting a low chill variety in a cold area will result in trees flowering too early and being damaged by late frosts.
• Selecting a high chill variety in warm areas will result in little or no fruit production.
• Early flowering varieties are best in warm climates, late flowering varieties are best in cooler areas.
• Early ripening varieties are best in areas with intense summers, late ripening varieties are best in cooler summers.
• Climate extremes may eliminate certain varieties that would otherwise meet the chilling requirements.
• Terrain can affect the chilling hours received. Open slopes may receive more chilling hours than sheltered areas next to warm buildings.
• Various sellers of fruit trees publish significantly varying chilling hour requirements for the same variety. It is difficult to know the exact requirements. Experiment and ask around for promising local cultivar success stories.
The effect of the Chill Units to the fruit tree buds
The chilling requirement of a fruit is the minimum period of cold weather after which a fruit-bearing tree will blossom.
Deciduous fruit and nut trees develop their vegetative and fruiting buds in the summer. As winter approaches, the already developed buds go dormant in response to both shorter day lengths and low temperatures. This dormancy or sleeping stage protects buds from the effects of cold weather. Once buds have started dormancy, they will be tolerant to temperatures much below freezing and will not grow in response to mid-winter warm spells.
Buds remain dormant until they have accumulated sufficient chilling units (CU) of cold weather. A chill unit is allocated when temperatures occur within certain parameters (chill accumulation models). When enough chilling accumulates, the buds are ready to grow in response to high temperatures. As long as enough CU have been accumulated the flower and leaf buds develop normally. If the buds do not receive sufficient CU during winter to completely release dormancy, trees may have uneven flowering, poor fruit set and shoot dieback.
Fruit and nut species and cultivars have different requirements for CU and the selection needs to be matched to the climate of the area to be planted. When selecting cultivars consideration of the future effects of climate change and likely reductions in CU for the production area.