Methodology

1. Locating your zone

When you enter a ZIP code, we map it to its primary county and look up that location in the 2023 USDA Plant Hardiness Zone Map (PHZM). The PHZM is built by the PRISM Climate Group at Oregon State University for the USDA Agricultural Research Service from 30 years of station weather data. Each zone represents a 10°F band of average annual extreme minimum winter temperature; each half-zone (the "a" / "b") is a 5°F band. That winter low is the single strongest predictor of whether a perennial or cool-season grass will survive where you live.

2. Soil profile

We pull county-level soil characteristics from the USDA SSURGO database via the Soil Data Access (SDA) service: dominant soil series, surface texture, typical pH range, drainage class, and organic matter. Soil pH and drainage decide which grasses and amendments actually work in your yard — a zone-correct grass planted in the wrong pH still struggles.

3. Climate & frost windows

First and last frost dates and growing-degree-day estimates come from NOAA National Centers for Environmental Information 1991–2020 Climate Normals. We combine these with the US Drought Monitor's weekly severity classification to flag locations where overseeding or watering guidance should shift.

4. Turning data into a plan

We classify each zone as cool-season, transition, or warm-season turf territory, then match recommended grass species and a seasonal action calendar (seeding, fertilizing, aeration, overseeding) to your zone and frost windows. Recommendations are rules-based and deterministic — the same ZIP always produces the same plan — so results are reproducible and auditable rather than guessed.

We do not invent numbers. Every figure shown traces back to one of the public sources on our data sources page. Where a county lacks a value in the source data, we say so rather than estimate.

Limitations

Zone maps describe averages, not guarantees — a hard winter can still damage a marginal planting, and microclimates (slopes, urban heat, shade) can shift your effective zone. Soil data is county-dominant and may not match a specific lot. Treat our output as a strong starting point, then verify with a local extension office for high-stakes decisions.