TurfTrigger combines validated turf-science models from major university extension programs with daily-updated weather data. Here's exactly what's under the hood: nothing proprietary, all citable.
Modeled hourly soil temperature at 6 cm depth (≈2 inches) is pulled from the Open-Meteo weather API for the ZIP's grid cell and averaged across the last 5 days — a model value, not a sensor in your lawn. Two signals matter: ~50°F is the early-application cue (get the barrier down and activated around here), while soil holding ~55–58°F approximates when crabgrass germination often begins.
For longer history, the chart uses ERA5 reanalysis data (also via Open-Meteo) since the forecast model only provides ~3 days of soil-temperature lookback.
Growing Degree Days (GDD) measure heat accumulation: each day adds the degrees its temperature spends above a base, here 50°F, counted from January 1. We count them two ways, by region, and always cross-check against the 5-day soil average (section 01). Since September and October 2026 most of the country runs the first rule below.
The single-sine rule covers the 17 northern states from 39°N up, plus Maryland, DC, Virginia, North and South Carolina, Georgia, Alabama, Tennessee, Kentucky and West Virginia below about 2,500 feet. In those southern states, below USDA zone 8b no apply text goes out before February 1. Where a ZIP's elevation is unknown we currently treat it as below these limits. The thresholds are TurfTrigger operating choices informed by university GDD research (Calhoun 2004 at Michigan State, the base behind GDDTracker; Ohio State's phenology calendar), not universal biological thresholds, and the product label always governs. Base 50°F means a midwinter thaw adds little, and the soil check must agree too; neither rules out a cold snap after the window opens.
How these rules behaved on seven years of historical weather in 28 cities, and how they compare with published crabgrass emergence records, is in our pre-emergent timing guide.
GDDTracker (MSU)MSU Lawn & Landscape ExtensionOhio State GDD and phenology calendar
For broadleaf weeds, we track a second GDD accumulator with different parameters:
Source: Schleicher et al. (1995), Purdue University. Used to time post-emergent broadleaf herbicide applications and recommend ester vs. amine formulations based on temperature.
To predict when the pre-emergent window will open or close, we use Open-Meteo's 16-day forecast to project daily GDD accumulation forward. For each upcoming day, we apply the same GDD formula to the forecast T_max/T_min and walk forward day by day until each threshold is crossed.
Predictions beyond 16 days are not provided; they fall back to a simple linear estimate based on the recent 7-day accumulation rate.
Each ZIP's hardiness zone is sourced from phzmapi.org, which exposes the USDA Plant Hardiness Zone Map data as a ZIP-lookup API.