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 over time. We use the simple-average method:
We use base 50°F, the same base as the Calhoun (2004) Michigan State University model behind the widely-adopted GDDTracker tool. We open the window early, at 150 GDD, so the barrier is down and activated with a week or two of lead before germination begins. The standard-preventer phase ends near 360 GDD; from about 360–500, dithiopyr may still control very young crabgrass where the label permits, and by ~500 germination is underway and the spring window is closed. These specific phase boundaries are adapted from university GDD research and calibrated by TurfTrigger; they are timing estimates, not a substitute for the product label. Base 50°F also filters out midwinter warm spells: a February thaw adds almost nothing to a base-50 count, so the window will not open on a false spring.
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.