Fruit trees unlikely to warm to recent weather

Author: Rebecca
Rebecca Darbyshire
Research Fellow
University of Melbourne
03 5833 5909

Warm weather across SE Australia could limit flowering in apple and pear orchards if winter temperatures don’t get colder over the coming months.

The recent autumn warm spell across south-eastern Australia has attracted much attention in terms of breaking climatological records. Although a delay in winter weather is pleasant for us, what effect might there be on fruit production for the 2014/15 season?

Influence of inadequate chill for cherry trees. Note: joint flowering and leaf out and the light flowering on the trees.
Influence of inadequate chill for cherry trees. Note: joint flowering and leaf out and the light flowering on the trees.

Temperature conditions throughout autumn and winter are important for fruit trees and warm conditions can negatively impact yields. Over autumn and winter, accumulated exposure to cool temperatures is needed to break trees from winter dormancy. Insufficient exposure to these cool conditions can lead to light, sporadic and/or protracted flowering in spring. This can result in poor pollination and fruit set and low final yields (see A Chilling Year: Was 2013 a low winter chill year? and A discussion on dormancy — and why it’s worth thinking about in early December).

Figure 1
Figure 1 Accumulated chill portions for autumn 2012, 2013 and 2014 at Tatura (Vic).

The effectiveness of temperature regimes in breaking winter dormancy in fruit trees is measured via winter chill models. To investigate the influence of the recent warm spell, we compared weather data from Tatura, Victoria, from the autumn of 2014 with 2013 and 2012 using the Dynamic chill model, which converts temperature conditions into chill portions.

Two aspects are clearly different between the autumn of 2014 and those of 2013 and 2012. Firstly, chill accumulation began late in 2014, 19 days later than 2013 and 23 days later than 2012 (Table 1). This late beginning to chill accumulation was due to warm weather in March and early April. Secondly, chill accumulation plateaued throughout the latter half of May (Figure 1), due to warm weather over this period. These two effects have resulted in current levels of accumulated chill being much lower than in previous seasons. Currently, accumulated chill for 2014 is 53% lower than both 2013 and 2012 (Table 1).

Table 1 The beginning of the chill period, total accumulated chill (1st March–31st May) in chill portions and differences between 2014 and 2013 and 2012.
Year First chill portion accumulated Total chill – 31 May (CP) Difference from 2014 (CP)
2012 26 March 35.53 +18.86 (+53%)
2013 30 March 35.50 +18.83 (+53%)
2014 18 April 16.67 -

Whether this deficit in accumulated chill for 2014 will be maintained throughout winter will depend on the temperature conditions over the coming months. The Bureau of Meteorology has predicted that June-August will likely be warmer than normal across south-eastern Australia, particularly for minimum temperatures. This indicates that it will be difficult for chill accumulation to catch up to 2013 or 2012 values.