Degree Days Unlocked

Degree Days Unlocked

Have you ever wondered why wine writers refer to degree days when they are writing their wine prose? Maybe you haven’t even noticed the reference ‘DD’ or ‘HDD’ or ‘GDD’ before. After reading this article you just might start to see it pop up and now you’ll know why it is there and what people are on about.

DD or Degree Days is the underpinning factor behind the development of what’s known as the Winkler Index from the University of California. A technique used by grape growers all over the world as an effective way to locally classify the climate of their wine growing region and give an indication of the growing season.

What is so interesting about this index is it can be used, in combination with Geographical Indicators (‘GI’), to map out suitable areas in Australia, or more widely globally, which are suitable for growing grapes or any other plant. Or for our purposes, and the wine writer, to help explain climate variation from season to season and hence the stylistic differences in winemaking year on year.

As mentioned, the Winkler Index is a guide based on growing degree days (‘GDD’), a universal measure of heat accumulation; the measure predicting and indicating vine development rates.

The way the system has been worked out is like this:

  1. Each day during the growing season is assigned a value (known as a GDD) equal to the amount the days average temperature exceeds 10°C.
  2. If a day is below 10°C it is assigned ‘zero’.
  3. Then the GDD’s are summed together to determine a region’s classification. A region can be one of five different zones**; which range from cool climate to hot climate.

** Further research expanded using spatial data to break Zone 1 into two zones 1a and 1b and an additional Zone to cater to climates which reach GDD in excess of 2700 GDD (a very hot climate). Both new zones are deemed not suitable for grape growing, not that this would deter any keen grape grower up for a challenge!

In regions such as Australia who use degree centigrade ‘°C’ the following applies, which is an indication of the most common growing season you will find in these places. As we know climate is rarely identical year to year so occasionally you may see a season outside of the normal range listed.

Zone

1a

1b

2

3

4

5

6

GDD

< 850

850 – 1380

1389 – 1667

1667 – 1944

1944 – 2222

2222 – 2700

> 2700

Season

Cool

Mild

Temperate

Warm

Hot

 

Wine regions

Tasmania

Nagambie Lakes, Coonawarra, Adelaide Hills

Clare Valley, Margaret River, Yarra Valley, Heathcote

McLaren Vale, Barossa

Swan & Hunter Valleys

In the context of Australia, this map from the Wine Gourd blog spot shows the spread of GDD across the landscape.

No surprises to see a large part of Australia is not suitable for grape growing. But this doesn’t mean people won’t grow grapes in those locations.

On this map they have also highlighted in red outline where existing grape growing regions are located. If you look closely you will find Nagambie Lakes region is clustered down south in Central Victoria around the blue regions.

Tahbilk is located in Nagambie Lakes, one of those regions influenced by Geographical Indicators.

We are lucky to be surrounded by a large water mass which gives us a mild regional climate which is different to others in the Goulburn Valley Region or close by in Heathcote.  Our regional location without GI would suggest we are a temperate climate but this is not the case.

This unique meso-climate helps us to grow a diverse range of fruit which might not be possible outside of 3kms from the Tabilk site.

So now you know.

Next time you see a reference to degree days you’ll know what’s going on and how to apply it to your wine.

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