Pears, like most deciduous fruit trees require a certain amount of winter chill in order to break dormancy in spring and and begin growth and flowering.

Why is winter chill important?

How is winter chill measured?

What are the winter chill requirements for pears?

Further information

Why is winter chill important? 

If sufficient winter chill is not received it can result in delayed and uneven leaf and flower development and subsequently poor fruit set. This can affect yield, fruit quality and harvest duration.  

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How is winter chill measured? 

The accumulation of winter chill is measured through a number of models. The three most common models are the 7.22^oC and under model, the 0-7.22^oC model and the Utah model.  

The 7.22^o C and under model is the traaditional model used to record chill units. In this model, each hour below 7.2^o C is equal to one chill unit.

The  0-7.22^oC model records one chill unit as an hour between 0 and 7.2^o C. An hour spent below or above 7.2^o C is recorded as zero chill units.

The Utah model records chill in 'Richardson'  Chill Units. This model recognises the influence that different temperatures have on chill. For example temperatures between 2.5-9.1^oC are considered to attribute the most towards the completion of dormancy and are therefore given the value of one chill unit for each hour spent in that range. Lower and higher temperatures can have a negative effect on chill unit accumulation and are assigned values accordingly. The following table outlines the calculation of chill units using the Utah Model.  

                                                                               Table 1: Calculating Chill Units using the Utah Model

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Winter Chill requirement for European pears

Pears generally require between 500 and 1500 hours of chilling. Specific varieties differ in their chill requirements.

Defiinitive data on the chill requirements of pear varieties in Australia are difficult to find. However, the varieties grown commercially in Australia can be roughly divided into those requiring, high, medium or low chill (Campbell 1995).

High Chill - Williams, Buerre Bosc, Winter Nelis, Comice, Lemon Bergamot

Medium Chill - Packham's, Josephine

Low Chill - Corella

For more information on the chill requirement for pears see further information.

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How can I determine the winter chill in my area?

An estimate of winter chill for an area can be determined through using this simple Chill Unit Calculator provided by the Department of Primary Industries, Victoria (external link).

Further information

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Australian Resources

Measuring Winter Chill

For information about the importance of winter chill and its measurement please visit any of the folllowing sites

•  Chill Units of Stone Fruit - Victorian DPI Agnote (external link).
• Winter chilling recorded in Western Australia - Department of Agriculture Western Australia Farmnote (external link). 

Winter chill requirement for pears

For guidance on the chilling requirement of pear varieties in Australia

• Winter Chill - Apples and Pears for Warmer Districts - Conference presentation by Dr J Campbell (external link).
• European Pear Varieties - NSW Department of Primary Industires AgFact (external link).

International Resources

• Chilling accumulation: its importance and estimation - Department of Horticultural Sciences, Texas A&M University (external link).

References

Campbell, J. (1995). ' Winter Chill! - Apples and Pears for Warmer Districts'. in Proceedings of the Sixth conference of the Australasian Council of Tree and Nut Crops , Lismore, NSW, September 11-15 Retrieved January 2009 from http://www.newcrops.uq.edu.au/acotanc/papers/campbel1.htm

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