Although the weather has been cold and wet, this week Susie Murphy White provides an interesting update on the current status of chill hours recorded compared to previous years, citing the optimum conditions required for pome fruits and managing our current conditions. Information is also available on our website and socials.

As we all know, this autumn started with unusually warm days and nights, causing the accumulation of winter chill hours to be slow this year. Bickley and Donnybrook experienced their first chill period in late May, while Manjimup had theirs in the first week of May. Winter chill accumulation has been slow, and we are currently well below the lowest 10% of years, making 2024 a very warm winter. For most locations, we have not yet reached 70% of the average winter chill hours required to break dormancy. However, we still have officially four weeks of winter to go, so hopefully, we will experience some colder and wetter days in August. some colder and wetter days this month. Check out the data tabled and described below.

Table 1. Winter chill portion accumulation data for the South West Region of WA. 2024 Winter chill portions were calculated on 29 July 2024. The average chill portions are 1994-2023; 70% are the average.

If you check your records, 2014 was also a year of very low chill and 2013 also had a late start and low chill. Further investigation of previous low chill years can be done using the My Climate View resource My Climate View developed by the Climate Services for Agriculture program funded by the Australian Government’s Future Drought Fund (FDF). As shown in the past and projected winter chill accumulation graph in figure 1, the chill accumulation is set to decline in the future.

Figure 1. Past and projected future chill accumulations for Manjimup, My Climate View.

What can you expect after a low chill year for pome fruit? When we monitored flowering in the climate change project (2014-1016), the green tip started earlier, and the time between the first flowers and full bloom was extended. This was consistent with the research findings that when chilling requirements are not met during dormancy, there is a risk of light and variable flowering with a protracted flowering period, poor fruit set, distorted fruit, delayed foliation, and extended blooms.

In the next couple of weeks, it will be time to consider using dormancy breakers to compact flowering and ensure more uniform fruit maturity. Consider whether the application will set back or induce early bud break, and then wait for the right weather conditions to apply. As always, ensure you follow the label directions.

The dynamic chill model (Erez et al. 1990) is the current best practice model, especially in warmer climates. It calculates chill in Chill Portions based on hourly temperatures. The Dynamic model has many features that capture known temperature-winter chill relationships lacking in other models, including the Chill Hours model. In the graphs below, the shaded area represents the highest 10% of years and the lowest 10% of years. The solid blue line is this year’s winter chill accumulation. Follow the link below to calculate today's winter chill accumulation for your location using the Bureau of Meteorology weather stations. https://www.daf.qld.gov.au/news-media/campaigns/agtech/about/types

The low down on winter chill to date