GDD 101 – And reasons to keep the program ticking over even though Summer’s done

Over the last few years GDD (Growing Degree Days) has been a hot topic, we get lots of questions relating to its use to optimise Primo Maxx II applications and the GGD tool on the Syngenta turf website is the most visited page!

Primo Maxx II Keep Using It

GDD is a powerful tool for monitoring a number of things which are affected by temperature; development of insect pests, grass growth, fungicide longevity and how long Primo Maxx II will keep grass under regulation.

We’ll focus on the latter here, and touch on some of the benefits of keeping the sward under regulation even though were about to leave Summer for Autumn.

If you haven’t already seen it click here for the Growing Degree day tool. If you’re signed into the Syngenta Turf website it can bookmark your location so its quick and easy to calculate GDD for your golf course or location.


Full disclosure; you don’t need to use GDD models to use Primo Maxx, our advice if you don’t want/need to go down the GDD road is:

Growing well = apply every 14 days (2 weeks)

Growing slow = apply every 30 days (4 weeks)

In-between = In-between 

Following that path won’t lead you to any disasters.

But the benefit for those who tread the GDD road is your applications will be more in tune with what’s going on climatically:

  • If its unseasonably warm and wet, following GDD will keep the sward in regulation, and avoid the dreaded REBOUND EFFECT (not really that dreaded). It just means more growth than in unregulated turf for a period – watch the video below for more info on that (jump to minute 5.50 Glenn explains with graphs). If you stuck to an application every 14 days we would expect you to come out of regulation in this scenario.
  • Cooler than expected – in a situation where we didn’t get the warm weather you expected, and you stayed in the 14 day application intervals, you could see over regulation, where the previous application is still giving suppression. Its not dangerous to the plant, but its more regulation that you bargained for, you could be using more PGR than you need to in that situation and could save by holding off.

GDD recommendations:

Our current advice for Primo application intervals is 200 GDD with 0c base temperature.

Our advise has changed over the years as more data has come to light and from user feedback, and we will probably change it again as we learn more.

Whilst no model is perfect we think that following the above gives the best chance for smoother growth across the year, avoiding coming out of regulation if someone turns up the heat, and avoiding over regulation.

The following graph (snatched from one of Glenn’s previous blogs – Primo Maxx II – when to start?) is great for thinking about your PGR program over the year.

We can see that ‘middle Summer’ period (June – August) has pretty consistent application intervals (bare in mind this is an average of 5 years so doesn’t show the yearly quirks, which is where GDD really helps (blue and red zones)).

But the rest of year can be pretty variable, so following a GDD model will keep you at the right level of regulation for you.

Why they keep on a PGR program right up to snow cover in the Nordics:

The next question which tends to come up is: When should I stop my Primo program?

And the answer is of course, it depends on lots of factors, and personal preference. But I wanted to highlight some research done in the Nordics which talks of the benefit of keeping the plant in regulation later into the season. In the trials shown below they kept up with the primo program until snow cover.

Ignore the fungicide treatment (this is not a blog to relaunching Headway in the UK) the trial is from 2011/2012.

This is in no way suggesting that Primo has fungicidal properties, if you spray some fuz in a petri dish with primo not much would happen, but because we are keeping that plant in a healthy state with good carbohydrate reserves right into the winter disease pressure the plant can do a better job defending its self.

You can also see the same trend for snow mold below, not an issue we struggle with so much here!

Its not even Autumn yet and I’m now feeling Christmassy already!

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