Growing Degree Days – 0c or 6c?

If you've been studying the Greencast GDD calculator you'll see this is the time of year when there is huge differences between 0c and 6c base temperatures.

6c base temperature
0c base temp

Of course the immediate reaction is that something is wrong.

But it's important to note:- by using different base temperatures we are measuring very different things and those differences are really highlighted at this time of year.

Now is a good time to revisit it.

What does the maths look like?

The base temperature is the point where a GDD model stops counting. The below model shows what the equation looks like if the average temperature for the day was 9c with a base temperature of 6c.

Average temperature of 9c with a base temp of 6c = 3 GDD

The below diagram shows how the equation would work on a day with 9c average temperature using 0c as a base temperature.

Average temperature of 9c with a base temp of 0c = 9 GDD

So why choose one or the other?

6c is good for measuring grass growth (which starts around 6c)

0c is good for measuring product degradation (which slows down but doesn't stop until 0c)

Which should you use to help time intervals between Primo Apps?

You can use either

If you use 6c aim to reapply every 150GDD

The advantage of 6c is the number of GDD will reflect turf growth activity

The disadvantage is, if the average temperature is floating around 5,6,7c's then turf will be growing (slowly), Primo will be degrading but your GDD counter will not be ticking - the implication being the gap between Primo apps should be a very long time.

This is fine if you wait until you get genuinely consistent temperatures until you start your primo programme, but in the UK that kind of weather doesn't arrive until much later than we all hope for (usually end of May) and most people have started long before that.

If you use 0c aim to reapply every 200GDD

The advantage to using 0c is the application intervals will be far more accurate during these colder periods of the year when average temperatures float around 6c.

What it won't do is indicate grass growth

If you ran both methods 6c on 9 greens and 0c on 9 greens you would find:

During the summer months very little difference.

But during the cooler months 6c would indicate Primo intervals much wider than the 0c model will.

In the UK we spend alot of the year in these cooler months where the average hovers around 6c which is why I don't think its great for measuring product intervals but its great for measuring grass growth.

On a final note GDD is a useful guide, it can help you get more out of your Primo programme. But don't let it rule your life - like all models - its very simple and always close to being right but never absolutely spot on, it cant be. It will help you improve you application intervals. Don't worry about being a few days out.

Blog covering the same subject in more depth below.

More maths here ... because the calculator is not actually wrong.

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