Primo Maxx II – Can I trust the model when its this hot?!

If you are in the UK you will know its been very hot recently, and Ireland has seen its share of the heat wave also; Dublin's Phoenix Park recorded 33.1C (91.4F). The hottest day in 135 years across the island of Ireland.

I saw on the news that the UK had its hottest night on record on the 18th July. It didn't drop below 25C all night in some parts according to the Met office. The all time high temperature for the UK was also broken when it reached a balmy 40C!

Record high overnight temperatures make for an unpleasant night sleep - need to get in the loft looking for that fan

Seeing all these extreme temperatures in the news, like any normal person, made me think just one thing:

How's that going to affect GDD modelling for PGRs like Primo Maxx II?!

The short answer is it depends....


GDD (Growing Degree Days) is at the end of the day are just a model. But it's the best model we have for many of the things we are trying to keep track of:

  • Grass growth (using 6C base temperature) - Cool season grasses, those grown in the UK & Ireland
  • Grass growth (using 10C base temperature) - Warm season grasses, those grown in parts of the world that see consistently high temperatures
  • Speed of insect pest development in the soil (base temperature depends on the pest)
For irrigated greens with good wetting agent programs 200 GDD could be reached in as little as 9 days due to a heat wave, but interpret with caution as this would not be so relevant for dry fairways...

As the model is able to do lots of different things its not really tight and should only be used as a guide.

For example it doesn't have an upper 'off switch' temperature, as that would not be relevant for both warm and cool season grass growth. There is a temperature at which cool season grass will not grow more rapidly (its reached its maximum potential - an increase in temperature will not result in increased growth). But lets say that's 30C (just an example as it will depend on grass cultivar etc), warm season turf cultivars will still be functioning fine at that temperature, and so the model would ring true for them. In our cool season turf example at temperatures above 30C you may not need to reapply a PGR as quickly as your GDD model would suggest.

But this is just a quirk to bare in mind, it would rarely affect the application timing of a PGR in the UK by more than a few days, so it's important not to be too precious about the exact timing!


An upper 'off switch' temperature is not something we considered necessary even just a few years ago when we started using the GDD calculator in the UK, as it would have so rarely been relevant. But we may look to add this if we continue to see temperatures touching 40C in the future!


Another quirk would be if you don't have irrigation of fairways then the GDD model would be screaming at you to 'apply now!'. If the plant has no water its not going to be growing, in fact unlikely to even show any evidence of activity! You delaying that application in these situations until growth and recovery starts. The model doesn't take grass stopping growth, due to being too dry or not enough available nutrients into account. Just watch out for any big down pours or thunder storms, as that could well see the growth kick on, and that will be different for all depending on how dry and hot things got and how well the turf coped.

So considering all the recent heat do we still back the GDD model for PGR application intervals?

Whilst like any model its not perfect and we will always see quirks, following a GDD model for Primo application intervals will still give great results. Improved over just applying on a fixed interval over the summer period, where you are more likely to be in periods of over or under growth regulation depending on weather conditions. But remember growth regulation does not switch off dead on 200 GDD, its a gradual reduction in growth suppression with each day so you have wiggle room.

Stay cool everyone, and hopefully we'll see those overnight temperatures slip back down a little.

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