GDD Calculator for Lawns
Track growing degree days at your location — see seasonal accumulation since January 1, calculate GDD from any date, and check pre-emergent and greenup milestones for your grass type.
Fetching seasonal weather data…
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Base 50°F · Jan 1–today
GDD from a specific date
Pick a date — shows PGR GDD at base 32°F and turf GDD at base 50°F
Daily GDD — Last 21 Days (Base 50°F)
PGR Reapplication Reference
These windows start from your application date. Use the app to track your personal countdown from a specific date.
| Product | Active Ingredient | Base Temp | Reapplication Interval |
|---|---|---|---|
| Primo Maxx / T-Nex | Trinexapac-ethyl | 32°F cool-season · 50°F warm-season | ~200 GDD on cool-season grasses (Cornell); ~216–230 GDD on ultradwarf bermuda (Reasor et al. 2018, Crop Science) |
| Anuew | Prohexadione calcium | 32°F cool-season · 50°F warm-season | Shorter residual than trinexapac-ethyl; ~120–130 GDD on ultradwarf bermuda (Reasor et al. 2018); shorter interval documented on annual bluegrass (Husiny et al. 2023) |
| Trimmit / Pillar | Paclobutrazol | 32°F | ~270–300 GDD on bentgrass (Kreuser 2018, Crop Science); soil absorption makes interval more variable than foliar PGRs |
The base temperature used for PGR GDD should match the grass type: 32°F (0°C) for cool-season (C3) grasses, 50°F (10°C) for warm-season (C4) grasses, reflecting each type's minimum metabolic temperature. Most published GDD research for PGRs was conducted on bentgrass putting greens — apply these as tracking benchmarks on warm-season turf and calibrate with your own mowing response data. Always confirm with the product label.
PGR research sources
- Cornell Turfgrass Program: Plant Growth Regulators — GDD timing for trinexapac-ethyl on cool-season turf
- Reasor et al. 2018, Crop Science — GDD models for PGR applications on ultradwarf bermudagrass (base 50°F)
- Kreuser 2018, Crop Science — GDD models for paclobutrazol on creeping bentgrass
- Husiny et al. 2023, Crop Forage & Turfgrass Management — Prohexadione calcium residual and rebound on annual bluegrass and bentgrass
Want a notification when it's time to reapply? The web calculator shows your current GDD, but it won't alert you. Download Lawn Dominator to set a PGR countdown from your exact application date and get a notification when you hit your threshold.
Track your PGR reapplication countdown from your exact application date in the app.
Weather data provided by Open-Meteo (open source, no API key required).
Why Different GDD Calculators Show Different Results
If you compare this tool to GreenCast or another GDD calculator and see a difference of 100–250 GDD, that is expected — not an error. The main sources of variation between tools:
- Max temperature cap: This tool applies an 86°F maximum — the standard modified GDD method used by university turfgrass programs. Calculators that do not apply this cap accumulate more GDD on hot days.
- Weather data source: This tool uses Open-Meteo, a gridded weather model. Tools using local weather stations will read differently based on proximity to official sensors.
- Start date: This tool accumulates from January 1. Some tools start March 1 or from a user-defined date, which changes the total significantly.
- Calculation method: Some tools use hourly temperature data rather than daily high/low averages, which produces slightly different results.
Use this tool to track trends at your location over multiple seasons. For cross-tool comparison, confirm the base temperature, max cap, start date, and weather source match before drawing conclusions from the difference.
Growing Degree Days for Lawn Care — What You Need to Know
Growing degree days (GDD) are a heat accumulation metric used by turf managers to predict biological events that calendar dates cannot reliably capture. Because plant development responds to temperature — not the date — GDD gives you a more accurate signal for pre-emergent timing, greenup prediction, and PGR reapplication windows.
This calculator uses a base of 50°F with a maximum temperature cap of 86°F — the modified GDD method used by most university turfgrass extension programs. Accumulation starts January 1 each year.
Pre-Emergent Timing with GDD
Crabgrass and other summer annual weeds germinate when soil temperature reaches 50–55°F for several consecutive days — University of Minnesota Extension and Penn State Extension both cite this range. GDD accumulation is correlated with warming soil, so tracking seasonal GDD gives you a secondary heat-trend signal alongside the soil temperature reading. Apply pre-emergent before soil temperature is consistently above 55°F or you will miss the window. The most reliable trigger is the soil temperature reading itself, not a fixed GDD number.
Bermuda and Zoysia Greenup
GDD is a useful trending tool for comparing spring heat accumulation across years, but university extension sources do not establish a single confirmed GDD threshold for bermuda or zoysia greenup. Texas A&M AgriLife Extension identifies 65°F soil temperature as the active growth threshold for bermuda. Track GDD alongside soil temperature and visual greenup at your own location over multiple seasons to build a personal baseline — this combination is more reliable than any single number.
PGR Reapplication Windows
Primo Maxx (trinexapac-ethyl) and similar PGR products wear off based on heat accumulation from the application date, not elapsed calendar days. The standard reapplication interval is 150–200 GDD from your last application using a base of 32°F. Use the date picker above to calculate GDD from any past date — then compare to the PGR reference table to judge whether it is time to reapply. For soil temperature data at your location, see the Soil Temperature Tracker.
Frequently Asked Questions
What are growing degree days for lawn care?
Growing degree days (GDD) measure accumulated heat above a base temperature — typically 50°F for warm-season turf. Each day contributes GDD based on the average temperature above that base. Lawn professionals use GDD to time pre-emergent herbicide applications, track greenup, and schedule PGR reapplication windows.
When does bermuda grass green up based on GDD?
University extension literature does not establish a confirmed GDD threshold for bermuda greenup. Texas A&M AgriLife Extension identifies 65°F soil temperature at depth — with consistent nighttime temperatures above 60°F — as the signal for active bermuda growth and recovery. GDD is most useful as a year-over-year trend comparison at your own location, used alongside soil temperature and visual observation.
When do I reapply Primo Maxx based on GDD?
Primo Maxx (trinexapac-ethyl) is typically reapplied every 150–200 GDD from the previous application, using a base of 32°F. Higher application rates extend the interval. Use the date picker on this page to calculate exactly how many GDD have passed since your last application, then compare to the threshold in the reference table above.
What base temperature is used for GDD in lawn care?
Seasonal GDD for warm-season turf uses base 50°F (10°C) — the minimum metabolic temperature for C4 plants like bermuda and zoysia. For PGR reapplication, the base temperature depends on grass type: Cornell Turfgrass Program uses base 32°F for trinexapac-ethyl on cool-season grasses; Reasor et al. 2018 (Crop Science) used base 50°F for PGR timing on ultradwarf bermuda. This tool uses base 50°F with a max cap of 86°F for seasonal accumulation, consistent with the modified GDD method used by university turfgrass extension programs.