When most people talk about what Primo Maxx II does they say “It stops growth”. This is a bit of an over simplification.
Strictly what it’s going is reducing the amount of GA (gibberellic acid) in the grass plant. – why does that matter?
It lead us on to why application intervals is so important.
Context
GA is the plant hormone responsible for cell elongation.
When a plant is stressed by shade (not just thinking shade of a nearby tree here, this would also be the shade of the grass plant next to it) it produces more GA so it can grow taller and hopefully get out of the shade, and soak up some rays.
The issue is each and every grass plant in our sward is doing this, so awkwardly no-one gets out of the shade, they all just get taller, and then mown.
Importantly a grass plant which is under Primo regulation, and one which is not both experience the same amount of vertical growth, but only in terms of number of cells.
Lets say in this example both get 100 cells taller as they grow over the next few days. Those cells in the untreated, with the higher levels of GA are each longer, and so the amount of vertical growth (in terms of cm) is more.
The Primo treated leaf still grows 100 cells taller, but each sell is less long so when the mower comes, less of those valuable cells, that the plant has ‘invested’ precious resources in to create, are lost in the cut.
As a side note this is why primo treated turf appears greener once in regulation, because the cell, and so chlorophyll density is higher.
The cumulative effect of this over multiple applications, and successive cuts over a season is that the plant doesn’t need to invest so much in replacing that material lost at the top of the plant.
And so can invest more in root development, side shoots and tillers.
This is why primo treated turf is more drought tolerant.
The levels of carbohydrates in turf which has been in a Primo program are higher than untreated, these sugars are the fuel the plant can use to support rooting, enhance density and initiate internal defenses from pests and disease.
Turf Advisor can assist
What we want is to keep that plant in regulation all season long and GDD (growing degree days) is a great way to do that.
I won’t go into that here, but this blog covers the key points, and why our advice would be to target 200 GDD on a 0°C base temperature model.
The turf advisor “Growing degree days 0°C” module is a super easy way to keep track, and work out what you’ve had since your last PGR application.
Click on the ‘bald man icon’ in the bottom left corner of the home screen and then click “Edit dashboard” to add it to your list as below.
I have mine dragged near the top of the page for the summer period.
Once you’ve added that module you can select the date of your last PGR spray and it will instantly display the GDD total from then to now, as below:
But getting back to the point of the blog.
Rebound
What if we don’t keep the plant in PGR regulation?
If we have applied a PGR, we’re in a program. Then we stopped. Middle of the warm period and we will apply no more applications.
As the plant is in a ‘better place’ than in the untreated equivalent.
Due to that extra investment it rooting, density, less loss of plant material and raised carbohydrate reserves, when the effect of the PGR wares off completely it has the capacity to deliver more growth than the untreated equivalent.
This is termed the ‘rebound effect’.
Put simply the more healthy a plant is the more growth it can deliver, they are essentially just growth machines.
You would expect to see the same if I set up two plots, one received sufficient inputs of water, fertility and light, and the other got 30% less of each resource.
You’d see more growth and so clipping yield from the sward with the full supply.
As you can see in the graph at the top its by no means an instant thing.
It’s very much a curve, as more time goes on less regulation is given to the sward. So that is not GDD 201 = an explosion of growth.
it could be GDD 250 or 300+ depending on the grass species, height of cut etc. until all the GA suppression has fully ended.
At that point we would expect to see growth rate, and so clipping yield higher in the formerly PGR treated turf than in an equivalent never PGR treated area of turf.
It’s a cautionary tale
As turf managers the aim would be to keep turf growth as even as possible and so avoid any big flush of growth.
Situations where this could occur:
- You stopped your PGR program mid season for some reason
- Your application intervals are too wide.
Remember as temperatures rise time between intervals needs to fall, 14 day application intervals may well not be suitable for greens and other irrigated/fed areas of the course at the warmest times of the year.
A late application in June at 16 or 17 days post your last PGR spray wouldn’t see you near rebound territory, but in a really hot spell if we’ve got 14 day application intervals and we reach 200 GDD in 7 days, we may well be entering rebound territory…
PGR’s are widely used to deliver high quality turf surfaces, the best use advice is to base application intervals on what the climate is delivering.
Keep the plant in a state of regulation to maintain the turf benefits, temperatures cooling down at the start of winter will naturally extend application intervals.
Please do let us know any feedback on the Turf Advisor app and specifically the GDD module, it’s great to hear so many are using it to inform intervals already!
