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Morning All,
This is a longer report than normal but the weather over the past 14 days has been more unusual than normal. I have been making a setting spray for macadamia’s and had an urgent request to ship the product as flowering looks as if it is going to be 6 weeks ahead of average. While we area long way from the macadamia growing areas it is an observation that indicates that the weather pattern is in an anomaly.
We had a later than normal start to the chill season followed up by very cold excellent chill accumulation and over the weekend what could be regarded as a spring break in terms of temperature conditions. While the forecast is suggested to go back to cooler conditions it is the lack of rainfall that will keep those minimum temperatures below critical chill accumulation.
We are seeing chill getting to levels which are theoretically in line with what should set crops, but the question will be has the plant had a long enough resting phase for bud setting. It also raises questions on pollen synchronisation once flowering starts and pollen viability. I will look at the long term (monthly outlook in terms of temperature and see if that gives us an insight as to what we can expect for the coming season and report on this next week. If the weather on the 18-23 July looks like negative chill conditions this will be sufficient to cause bud movement in those varieties over 400 RCU and in that chill range. If these are varieties used for cross pollination, then it is worth trying to delay bud movement in these varieties as they can be out of alignment with the main varieties. The cooling polymers here are worth considering.
If we go back in time to compare 2016 and 2017, we see:
- 2017 ended up as a high chill year;
- 2016 ended up at the lower end of chill accumulation
In 2017 at the same time we had
- Swan Hill 500 RCU;
- Loxton & Renmark 350 RCU
The difference this year is that from the met bureau forecast
- No significant rain until the middle of July;
- Above average temperatures
I thought I would include the calculation of the Richardson model so you can appreciate that very cold temperatures do not contribute to chill accumulation as do temperatures over 16C. In years like this we are expecting to see significant amounts of night time temperatures below 2.5C and daytime temperatures above 16C thus limiting the accumulation of chill
With the cold weather that we have had over the past few weeks it is worth going back to the basics of RCU accumulation. The sweet spot for maximum chill accumulation is 2.5-9.2C and that while there is fringe accumulation outside of these temperatures it is the lack of chill acquired in the very cold periods that needs to be also considered as while it may be cold it is doing nothing to contribute to bud fertility. While one can take the met bureau long term forecasts with a grain of salt it is worth noting that for our region below average rainfall and above average temperatures are supposedly forecast for the rest of winter.
Temperature Celcius Richardson Chill Unit (RCU) or Utah
<1.5= 0
1.5-2.5= +0.5
2.5-9.2=1.0
9.2-12.5=+0.5
12.5-16.5=0
16-18=-0.5
>18=-1.0
Chill Hours Model:
- This model, one of the earliest, counts the number of hours within a specific temperature range (typically 0-7.2°C or 32-45°F).
- Some variations of this model consider temperatures below 1.4°C (34.5°F) as ineffective or even detrimental to chill accumulation.
- Weinberger's model (1950) is a prominent example, using the number of hours at or below 7.2°C.
Dynamic Chill Model:
- This model, widely used, calculates chill portions based on hourly temperatures, with an optimal chilling temperature around 6°C.
- Temperatures above 14°C (57.2°F) can negate previously accumulated chill.
Utah Model:
- This model, also using hourly temperatures, assigns different chill unit values based on temperature ranges, with temperatures between 2.5°C and 12.5°C being most effective.
Other Considerations:
- The total chilling requirement varies by fruit variety and species, with some requiring more chilling hours than others.
- In the southern hemisphere, chilling accumulation is typically measured from April 1st to November 1st.
- Hot days can offset the benefits of chilling hours, particularly in warmer regions.
In summary, while chill hours are a simpler measure, the Dynamic Chill Model is often preferred for its more nuanced approach to temperature and its ability to account for the negative impact of high temperatures on chilling accumulation. RCU and Utah are the same.
Regards,
Shane Phillips | Head Chemist - Research & Development
E: shane.phillips@biocentral-labs.com
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