Growing Degree Days vs Calendar Days for Forecasting

If you want to increase the accuracy of yield predictions it’s crucial to understand the importance of growing degree days (GDD) vs calendar days. Growing degree days are a huge part of how we forecast with GreenView. Take a look at the image below – this is a customer from Tasmania and we looked at how the crop ripened 2 different years in a row. Blue line = April 2023. Orange line = April 2024.

Growing Degree Days vs Calendar Days for Forecasting

Y axis shows us how many berries are at each phenological stage in the footage. When we look at how the crop ripened in 2023 in calendar days versus 2024, we can see that there’s quite a deviation between the two years. This is why forecasting is so challenging.

If we compare April 2023 and April 2024, we can see it took twice as long for all the red berries to ripen and be picked – this was due to a cold snap at the beginning of April 2024. Weather changes every year and you can see with discrepancies like this it’s really hard for growers to forecast accurately with just year-on-year data – because what the crop did last year isn’t necessarily what it’s going to do this year.

Now take a look at this image below – when we start to look at this data against GDD instead of calendar days we can properly understand how many heat units it takes to get to each phenological stage.

Growing Degree Days vs Calendar Days for Forecasting

You can see there is still a little bit of a deviation for the reds, but it really comes together very closely as the season progresses. We know the heat units required are consistent year-on-year. This means we can use the same ripening curve every year, and not worry about what the weather is doing day-to-day because we’re taking the ambiguity of calendar days out of the equation.

The overall result is that we can forecast extremely accurately using these curves instead of calendar days.

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