Understanding GDD: smarter timing for Primo Maxx II

Growing Degree Days (GDD) is one of those terms we’ve all heard, often mentioned when discussing growth or product timing. But beyond the acronym, how well do we really understand what it measures, how it builds, and how it helps us make better decisions, particularly when applying Primo Maxx II?

When turf growth kicks into gear and expectations are high, our decisions on plant growth regulation can make or break surface performance. Get the timing right and we stay ahead of growth, keep clipping volumes under control, and avoid the unneeded additional growth that can sometimes be seen when a growth regulator is allowed to completely run out.

That’s where GDD come in, a more intelligent way to manage timing based on what the weather is doing, rather than what the calendar says.

How GDD works

GDD tracks the amount of accumulated heat the turf has received, heat that directly influences growth and rate of product degradation. Each day, we calculate GDD by averaging the maximum and minimum temperatures, then subtracting a base temperature. For cool-season turf, we use 0 °C as that base: If the result is positive, it adds to the running total. If it’s below 0, it doesn’t count. Day by day, this tally builds, like a mileage counter ticking up as warmth accumulates.

Plant growth starts at around 6 °C so why use 0 °C? We’re not measuring grass growth when using GDD for Primo intervals, were using it to measure the rate of product degradation. The rate of degradation is slow at low temperatures, but it does happen.

How GDD builds and leads to reapplication

If you wanted to work out GDD for yourself, this is how you’d do it:

Let’s say a day records a low of 6 °C and a high of 14 °C:
(14 + 6)/2 – 0 = 10 GDD
On a warmer day, say 16 °C low and 24 °C high:
(24 + 16)/2 – 0 = 20 GDD

The GDD model powered by Syngenta, on their website and in the Turf Advisor app, uses a more accurate method to assess the daily average using hour temperature.

In cool spring conditions, it might take three or four weeks to reach 200 GDD. In summer, it could be just 9 days. That’s the beauty of GDD, it reflects the speed of product degradation and usage, not the calendar.

Why 200 GDD is your Primo Maxx II trigger

Primo Maxx II is broken down in the plant faster as temperatures rise. Around 200 GDD after an application, its effect starts to wane and by 250 GDD growth can rebound.

Applications at 200 GDD give us the best chance of overlapping product applications to ensure consistent levels of growth suppression. So instead of spraying every two or four weeks, you’re guided by the weather and turf response.

This keeps regulation steady:

• No over-suppression during cool spells
• No loss of control in hot weather
• Fewer clippings and more consitent growth

Using GDD to adapt to weather, not the calendar

The start of a Primo Maxx II programme should coincide with reliable turf growth, not a warm weekend. A good marker is when growth potential (GP) hits 35%, usually at a daily average of 12 °C.

What is growth potential?

Growth potential (GP) is a simple model that scores how daily average temperatures influence turf growth. GP ranges from 0% (no growth expected) to 100% (perfect temperature for growth). For cool-season turf, optimum growth is predicted when the daily average is 20 °C. When daily averages are 12 °C, for example, growth potential is roughly 35%, when 35% is seen regularly you are in a growth period where Primo Maxx II will reduce clipping yield, improve plant health and appearance.

Once underway:

• Greens: Reapply around every 200 GDD
• Fairways/semi-rough: Apply at higher rates (1–1.5 L/ha), at these rates GDD becomes less critical and a more calendar-based approach work.

At the lower rates suitable for low cutting heights adjusting intervals to what the temperature is doing makes a big difference.

Tracking made easy with the Turf Advisor app

You don’t have to crunch numbers or track weather data yourself. Syngenta’s Turf Advisor app takes care of it. Plug in your location and spray date, and it:

• Tracks GDD automatically
• Displays daily GP, so you know when the turf is truly active
• Flags reapplication windows based on your local conditions
• Adjusts timing to your site for more accurate decision-making

With both GDD and GP at your fingertips, you can fine-tune your regulation strategy with confidence.

Final words

GDD can seem complex but using Turfadvisor and the 200GDD figure it’s both simple and powerful. Once you start following it, you’ll wonder how you ever timed Primo Maxx II without it.