Every number below is either a published model or a threshold from an agronomic source, and each one is cited. If you think one of them is wrong, the source is right there to argue with.
Growth potential
Growth is scored with the PACE Turf Growth Potential model: a Gaussian on mean daily air temperature about the optimum for cool-season grasses, then modified by day length.
It is the established turf-industry standard, and it captures the thing calendar advice misses — that a mild December and a mild April are not the same, because the light is not the same. Below a growth potential of roughly 0.2 the sward is ticking over and does not need cutting.
Soil temperature is reported at 6cm — germination depth — and compared against a species-aware threshold. The default is a ryegrass and fescue blend in the UK and Ireland, and a bluegrass and tall fescue blend in the US and Canada.
A single germination number is wrong in both markets. UK lawns lean perennial ryegrass and fescue, which germinate around 8–10°C; US cool-season lawns lean Kentucky bluegrass and tall fescue, which need 10–13°C. Four degrees is several weeks of spring.
Surface water on the canopy is simulated hour by hour. Rain and dew add water; evaporation removes it at a rate set by net radiation, wind and vapour pressure deficit. Dew burn-off is an output of that balance, not a fixed time of day.
Wet grass tears instead of cutting, clogs the deck and spreads disease, so this carries the heaviest weight. "It has not rained today" is not good enough: a clear, still night lays dew that holds until mid-morning, while a breezy night lays none at all.
An antecedent precipitation index — a decaying weighted sum of rainfall minus evapotranspiration over the preceding ten days — combined with modelled soil moisture in the top 9cm.
Soft ground ruts under a mower and badly under a ride-on, and the damage lasts months. A single moisture reading understates the memory of a wet fortnight, so the index leads and the moisture reading corroborates it.
Mowing before 8am on a weekday, before 10am on a Sunday or bank holiday, or after 8pm on any day is discouraged. It is a preference applied to the score, never a block, and it can be switched off.
There is no UK law setting mowing hours. Councils act under the statutory nuisance provisions of the Environmental Protection Act 1990, which weigh a persistent pattern rather than a single Sunday morning. Many councils publish advisory "considerate hours"; those are what this encodes. It is etiquette, not law.
Any hour at or below 1°C air temperature is blocked outright. So is any hour where the grass canopy itself is estimated to be at or below freezing, even when the air is warmer.
Mowing frosted grass ruptures the leaf cells, and the damage shows for weeks. Grass radiates heat to a clear sky and sits several degrees below air temperature on still nights — which is why a 3°C clear morning can carry a frost that an air-temperature check would miss.
Forecasts come from Open-Meteo: hourly air temperature, dewpoint, humidity, precipitation, wind, shortwave radiation, evapotranspiration, and modelled soil temperature and moisture at four depths. Cached for an hour and refreshed on a schedule.
Soil temperature is modelled rather than measured — there is no dense network of soil probes in anyone’s garden. The site says so wherever the figure appears. An independent estimate, a damped and lagged rolling mean of air temperature, is computed alongside it as a sanity check.
We do not state a verdict without the reason. Every day and every window exposes its factor breakdown and names the limiting factor.
We do not fabricate a fallback. If the forecast service is down, or the soil never reaches your seeding threshold inside the forecast, the site says exactly that instead of estimating.
We flag low confidence. Anything beyond about seven days is marked as such, because a ten-day forecast is not a ten-day promise.
Profiles never override the physics. They change how much risk you are willing to take. Frost, rain, snow, frozen ground and waterlogging block mowing on every setting.