Most people using a PGR on their lawn get told the same thing: reapply every three to four weeks. Whether that's Primo Maxx, Limitless, or something else, the advice is nearly always the same. So they set a reminder, count the days, and spray on schedule.
The issue is that PGR breakdown is not driven by time. It's driven by heat. And in Australia, the difference between three weeks in June and three weeks in January is enormous. Following a calendar in summer means under-applying and losing suppression. Following the same calendar in winter means over-applying when the product is barely breaking down at all.
Growing Degree Days (GDD) is the scientific solution to this. Once the logic lands, the calendar approach stops making sense.
What exactly are Growing Degree Days?
A Growing Degree Day is a unit of heat accumulation. It measures how much thermal energy has built up over a 24-hour period above a base temperature threshold, which is the point below which biological activity effectively stops.
For most PGR products used on warm-season turf, the base temperature is 10°C. Anything below 10°C contributes zero GDDs. Anything above it adds to the running total.
So on a day where the max is 32°C and the min is 22°C, you accumulate 17 GDDs. On a day where the max is 16°C and the min is 9°C, you get 2.5 GDDs. The biological activity, including PGR breakdown, reflects that difference.
Why the calendar method fails
South East Queensland is a useful example because the seasonal swing is big enough to make the problem obvious. Hot summers, mild winters, not a lot of grey zone. Using a 200 GDD reapplication threshold (typical for most trinexapac-ethyl products), here's how the numbers actually fall:
| Month | Avg max temp | Avg min temp | Daily GDD avg | Days to 200 GDD |
|---|---|---|---|---|
| January | 29.4°C | 21.2°C | 15.3 | ~13 days |
| April | 25.8°C | 17.1°C | 11.5 | ~17 days |
| July | 20.5°C | 9.8°C | 5.2 | ~38 days |
| October | 27.1°C | 17.4°C | 12.3 | ~16 days |
If you're applying every 21 days year-round, you're over-applying by 62% in July and under-applying by 38% in January. That's the real reason a lawn looks stunted in winter and surges in summer even when you're supposedly on schedule.
The same 200 GDD threshold that takes 13 days to reach in January takes 38 days to reach in July in SEQ. A rigid 21-day calendar schedule is wrong for most of the year, in both directions.
How to use GDD in your lawn program
Step 1: Know your product's GDD threshold
Most trinexapac-ethyl PGRs break down somewhere between 150 and 200 GDDs. The exact number varies by product concentration and application rate, so check the label or the manufacturer's technical data. For Primo Maxx at standard home lawn rates, 200 GDDs is the number most people use. But confirm it for whatever you're running.
Step 2: Record your application date and start accumulating
From the moment you apply, you start the GDD clock. You need daily temperature data for your actual location, not the nearest capital city. Microclimates matter and a station 20km away can give you meaningfully different numbers. Weather APIs can pull this automatically if you're using an app.
Step 3: Reapply when you hit threshold
When accumulated GDDs hit your threshold, the product is breaking down and suppression is fading. Reapply then. Not at 21 days. Not when the lawn looks like it's getting away from you. When the number says so.
CoreTurf pulls live weather data for your location, calculates GDD accumulation from your last application date, and shows you a real-time progress ring. You never have to do the maths manually. When you're approaching threshold, the app tells you.
Common questions about GDD and PGRs
Does GDD apply to other products, not just PGRs?
GDD is most commonly applied to PGR timing because breakdown is so directly temperature-dependent. But the concept applies to any biologically-driven process: soil microbial activity, fertiliser uptake, germination. Some people also use it to time pre-emergent herbicide applications for summer grass control.
What base temperature should I use?
For trinexapac-ethyl products (the active ingredient in most home lawn PGRs, including Primo Maxx and Limitless), 10°C is the standard base temperature. In subtropical climates this barely factors into summer, but it matters more than you'd think in winter when overnight temps drop. Southern states feel this more than most.
What if my weather data isn't accurate?
Closer is better. A station 30km away in a different microclimate can give you meaningfully different totals. If you want real precision, check whether there's a personal weather station near you. Some are publicly accessible through weather networks.
Is GDD the same as Growing Degree Units (GDU)?
Same concept, different name. GDU shows up more in American agricultural research. GDD is the term you'll see in Australian turf management. If you're reading overseas literature, treat them as interchangeable.
The bottom line
Heat breaks down the product. More heat means faster breakdown. Reapply when the heat accumulation hits threshold, not when three weeks have passed on a wall planner.
Most home lawn owners don't bother with GDD timing because it used to require real effort — daily temperature lookups, manual calculations, a running log you have to maintain yourself. That friction is real, and most people end up back on the calendar after a few weeks.
CoreTurf removes that friction. Log your application, and the app watches the weather at your location, accumulates the GDDs, and shows a live progress ring counting toward your threshold. When you're close, it tells you. No spreadsheet, no manual maths, no guessing at temperatures from a station two suburbs away.