Fit-Score Methodology

What the Fit Score Is

For every U.S. county, we compute an agronomic fit score (0–100) for each vetted grass-seed cultivar, then roll those up into a per-species suitability number. The same number drives both the “Top Grass Fit” card on a county page and the glow on our interactive map — they are computed by one shared function, so they can never disagree.

The score is an informational estimate built from public datasets. It is not professional agronomy advice. Where a threshold is our own editorial judgment rather than a measured agronomic constant, we say so explicitly below — we do not dress an invented number in a borrowed institution’s name.

The Four Weighted Factors

The fit score is a weighted sum of four factors. The weights below are pulled directly from the scoring engine, so this table always matches the live formula. The weights themselves are a LawnByCounty editorial weighting, not a measured agronomic constant.

These four factors are the only inputs to the additive score. We do not score growing-season length (unavailable in our source data) or extreme-heat days (not yet wired) — listing an input we don’t actually read would misrepresent the score.

How Each Factor Scores

The Survivability Gate (Our Immune System)

Before any factor is scored, each cultivar must pass a hard survivability gate keyed on its own rated zone band — never a coarse “this region is too cold for warm-season grass” proxy, which would wrongly reject grasses that are genuinely rated for those zones. A cultivar fails the gate only when the county’s USDA zone falls outside the cultivar’s effective range.

Tolerance at the edges is directional. A transition-zone-tolerant cool-season grass is allowed to stretch one extra zone colder and one zone warmer. A warm-season grass is never stretched colder on the strength of a generic tolerance flag — that is the exact mistake that would let Bermudagrass “survive” a zone where it winter-kills. So a warm-season grass rated to zone 7 correctly rejects in zone 5.

When a cultivar is ruled out, the reason string reports the exact effective zone range the gate compared against — nothing more. We never assert a winter-kill or survival mechanism the code did not evaluate. Ruled-out cultivars are quarantined so they can never appear as a recommendation.

Species Rollup and the Coverage Cap

The headline number on a county page is a per-species suitability, not a single cultivar’s score. We take the best gate-passing cultivar for that species, then apply a cap: the species can never glow higher than the species’ own broad idealZones would allow. This stops a single, perfectly-marketed cultivar from pinning a thinly-covered species to a perfect score. The capped number is what both the card headline and the map glow display; an individual cultivar’s own score is shown only as a clearly-labeled secondary line.

When a species is backed by only one vetted cultivar, the card flags “limited product coverage.” When a county is missing soil or precipitation data, the dependent factor is dropped and the remaining weights are renormalized; if two or more key inputs are missing, we degrade to a category-level recommendation rather than show a falsely-precise number.

Per-Species Agronomic Ranges

These are the numeric agronomy values the scorer reads for each grass species. For the four species that currently have reviewed products and therefore drive live scores — Tall Fescue, Kentucky Bluegrass, Bermudagrass, and Zoysiagrass — the zone bands and pH windows are attributed to specific university Cooperative Extension publications (linked in the Sources column). Other species shown here have no reviewed products yet; their ranges are LawnByCounty editorial estimates pending individual sourcing. Annual water figures are editorial estimates for every species: no Extension publishes a single annual-inches-per-year number — the citable figure is a weekly rate (≈1–2 in/week during active growth), which we convert to an annual range using regional growing-season length.

^* Water (in/yr) is a LawnByCounty editorial estimate for every species — derived from university-extension weekly irrigation rates (≈1–2 in/week during active growth) multiplied by regional growing-season length. No Cooperative Extension publishes a single annual-total-per-species figure. Source links cover the cited zone bands and pH windows of the four scored species only.

Data Sources

• USDA Plant Hardiness Zone Map (2023) — county hardiness zones and species zone bands.
• NOAA Climate Normals (1991–2020) — growing-degree-days, frost dates, and precipitation.
• USDA SSURGO — soil pH and available water capacity.
• University Cooperative Extension turfgrass publications — species zone bands and pH windows for the four scored species: Clemson HGIC, Penn State Extension, UGA Extension, Iowa State Extension, Kansas State Extension, NC State Extension, Missouri Extension, Purdue Turfgrass Science, University of Tennessee UTIA, and the USGA Green Section (drought tolerance). Per-figure links appear in the Sources column above.
• The Lawn Report editorial cultivar reviews — the product-level review excerpts, ratings, and recommendations.