Climate and aspect

Pears grow well in areas with warm, hot summers and low humidity. A north or north-easterly aspect is best for providing maximum sunlight during winter months – slopes facing south are colder and more frost prone.

Winter Chill

Chilling requirements

Pears, like most deciduous fruit trees require a certain amount of winter chill in order to break dormancy in spring and begin growth and flowering. Pears require between 500 to 1,500 hours of chilling.

Why is winter chill important?

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

How is winter chill measured?

The four most common models are:

7.22°C and under model The traditional model used to record chill units. In this model, each hour below 7.2°C is equal to one chill unit.
0 to 7.22°C model This model records one chill unit as an hour between 0 and 7.22°C. An hour spent below or above 7.22°C is recorded as zero chill units.
Utah Model Chill is recorded in ‘Richardson’ chill units. This model recognises the influence that different temperatures have on chill.For example: temperatures between 2.5 to 9.1°C are considered to contribute the most towards the completion of dormancy and are therefore given the maximum 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 lower (0.5) or negative values. The following table outlines the calculation of chill units using the Utah Model.
Dynamic Model The Dynamic Model records chill in units called “chill portions” (CP). It is the most biologically accurate model as it measures chilling on an hourly basis. At maximum chilling conditions the Dynamic Model accumulates 0.83 CP per day.Visit the UC Davis website for more information.
Temperature °C Chill Units Chilling Level Chill Portions
<1.4 0.0 High >70
1.5 – 2.4 0.5 Medium 30 – 70
2.5 – 9.1 1.0 Low <30
9.2 – 12.4 0.5 Table 2: Guide to chill portions in pome fruit
12.5 – 15.9 0.0
16 – 18 -0.5
>18 -1.0
Table 1: Calculating chill units using the Utah Model
A. Erez, Temperate Fruit Crops in Warm Climates. 2000. Kluwer Academic Publishers

Winter chill requirements

European pears require between 500 and 1,500 hours of chilling depending on variety.

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

  • High chill – Williams, Beurre Bosc, Winter Nelis, Comice, Lemon Bergamot
  • Medium chill – Packham’s, Josephine
  • Low chill – Corella

How can I determine the winter chill in my area?

An estimate of winter chill for your area can be determined by using the Department of Environment and Primary Industries chill unit calculator.

Frost susceptibility

New pear tree growth is highly susceptible to spring frosts. The financial risk associated with crop loss in an intensive orchard is high so minimising crop loss through frost damage is important. Initially growers need to avoid planting in frost prone areas. Planting sites need to also have good air drainage to prevent accumulation of cold air. For example, avoid planting at the bottom of slopes or in hollows.

Figure 1: Frost fan

Frost fan.

When establishing new orchards, installation and budgeting for frost protection infrastructure such as warning systems, frost fans or sprinkler systems needs serious consideration.

Management of the orchard floor is also important to minimise frost damage. All inter-rows should be slashed as close to the tree line as possible and weeds in tree lines sprayed. The aim is to have most of the orchard soil exposed to warming sunshine during the day so accumulated ground heat can be released in the early mornings to reduce frost risk. More information about frost and its management can be found under further information.

Wind

Wind can severely damage fruit and reduce fruit quality through rubbing; can increase evaporation and lead to water stress; adversely affect pollination; and damage newly planted trees. Support structures for trees should be installed prior to or soon after planting to minimise root damage in young trees.

Water availability

Intensive orchards require secure access to good quality water for irrigation, spraying and possible frost protection and evaporative cooling needs. Good delivery infrastructure is essential. It is often recommended that growers ensure they have at least six megalitres of water available per hectare of orchard.

Aspect

A north or north-easterly aspect is best for providing maximum sunlight during winter months. Slopes facing south are colder and more frost prone.

Figure 2: Orchard planting scenarios.

Figure 1: Orchard planting scenarios.

In the scenario in Figure 2 the orchard at site A (right) is a warm location and will receive more sun as it is north facing. It will be less prone to spring frosts as the cold air will drain to the lower lying area (site B). Site B at the bottom of the valley will be the most susceptible to spring frosts because of the cold air draining to it from higher up the slopes. Site C (left) is colder and will warm up more slowly than at site A because it is south facing.

Further information

The following websites may be a useful information source for growers. Any specific recommendations may be outdated or irrelevant for Australian conditions and growers should seek local advice.

Australian resources

Measuring winter chill

Winter chill requirement for pears

International resources

References (Note full access may incur a fee)

Boucher, W 2008, Chilling Requirement, Tree Fruit Magazine, October 2008.

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.

Darbyshire, R, Webb, L, Goodwin, I & Barlow, S 2011, Winter chilling trends for deciduous fruit trees in Australia, Agricultural and Forest Meteorology, 151: 1074-1085.

Kretzschmar, AA, Brighenti, LM, Rufato, L, Pelizza, TR, Silveira, FN, Miquelutti, DJ & Faoro, ID 2011, Chilling requirement for dormancy bud break in European Pear, ISHS Acta Horticulturae 909: 85-88.

Click here to return to the Intensive Pear Production home page.

The Intensive Pear section of the APAL website was compiled as part of the Profitable Pears project, funded by APAL, HAL and DEPI Victoria. First compiled in 2009, it was extended and updated in 2014. The information contained in this website is for general reference only. Growers should seek local technical advice before deciding on changes to production and postharvest practices.