Hortiforum

When purchasing trees such as Apples, Peaches or Almonds, gardeners frequently encounter a specified ‘chilling requirement’ for the plant, expressed numerically as ‘400 hours’ or ‘600 hours’ or simply as ‘low-chill’ or ‘high-chill’. Regardless of presentation, this ‘chilling requirement’ reflects the natural ‘vernalization’ process necessary for many plants. During vernalization, a plant’s flowering process is induced by exposure to an amount of cold winter temperatures, after which it can initiate blossoming in warmer spring temperatures.

The ‘chilling requirement’ for plants is a measurement of the cool temperatures required in this process and is commonly presented in ‘chill hours’. The measurement of ‘Chill Hours’ can vary significantly, but broadly speaking, a chill hour is considered an hour during which temperatures drop to or below 7 degrees Celsius. In Australia, climates are generally classified from low-chill (<450 hours) to medium-chill (450-650 hours) and high-chill (>650 hours).

The measurement of chill units can vary significantly depending on the model employed. Nevertheless, the simplest model is also that most commonly used by nurseries and will be outlined below.
This model involves accumulating the hours within winter (June-August) when temperatures fall below 7 degrees Celsius. This can be easily estimated this for your local climate by determining the average mean daily maximum and minimum temperatures for July, dividing the sum by 2, and identifying your approximate chill hours using the table below. For individuals in larger towns or cities (for larger cities this will vary considerably depending on where within the city you are located), this approximate amount has already been approximated and is provided in the table below:

Average July Temperature	Approximate Chill Units/Hours
0	1250
7.9	1100
10.2	800
12.7	600
13.6	500
14	450
15.3	330
15.5	300
19.7	0

City	Avg. Temp Coldest Month	Approx. Chill Units	High, Medium or Low Chill
Sydney	12.2	640	Medium
Newcastle	12.6	600	Medium
Wollongong	13.3	510	Medium
Port Macquarie	12.5	600	Medium
Coffs Harbour	13.1	520	Medium
Melbourne	9.7	830	High
Bendigo	7.5	1120	High
Ballarat	6.6	1180	High
Mildura	9.8	810	High
Wodonga	8	1100	High
Perth	13.1	520	Medium
Bunbury	12.3	620	Medium
Albany	11.9	650	Medium – High
Kalgoorlie	10.9	700	High
Geraldton	14.5	400	Low
Adelaide	11.4	730	High
Murray Bridge	10.8	700	High
Mount Gambier	9.1	900	High
Port Augusta	11.1	710	High
Brisbane	14.2	300	Low
Gold Coast	16.6	<300	Low
Toowoomba	11.5	730	High
Rockhampton	16.3	<300	Low
Bundaberg	16.1	<300	Low
Stanthorpe	8.2	1050	High
Hobart	8.1	1100	High
Launceston	7.4	1100+	High
Devonport	8.7	1050	High
Canberra	5.6	1200+	High

While the above method and amounts are likely to provide a relatively accurate estimation of chill hours, it is also important to note other complexities which are noted regarding chill hour calculation. Please see the below two major points noted from studies and research:

1. Research has indicated that winter temperatures above 16 degrees celcius can offset accumulated chill hours.
2. Temperatures ranging between 7 and 13 degrees Celsius can result in varying levels of chill hours. Additionally, research suggests that temperatures below freezing do not contribute to chill hours, and the most effective chilling temperatures fall within the range of 0-7 degrees Celsius.

Although these factors may partially offset each other, some may be of particular interest. Despite potential flaws, the method of calculation based on temperatures below 7 degrees Celsius is ideal as it allows for effective comparison to the chilling requirements listed by nurseries.