Frost Date Finder

A frost date is not a prediction for any single year. It is a statistical summary of decades of weather records. When we say the "last spring frost date" for your location is 15 April, we mean that across 30 years of records, frost had typically stopped by that date. The same logic applies in reverse for autumn: the "first autumn frost" is the date when temperatures historically begin dipping below 0°C again. The dates in this tool come from daily minimum air temperatures in a gridded climate dataset (see the note on the data below), summarised as percentiles over 1981-2010.

What the 10%, 50%, and 90% thresholds mean

The probability thresholds reflect how much risk you are willing to accept. The 10% date (early/aggressive) is the date by which only 1 year in 10 has had its last frost, so in about 9 years out of 10 frost still comes after it. Plant out then only if you are willing to gamble or can protect plants with fleece at short notice. The 50% date (typical) is the median: half of all years see frost after this date, half do not. Most gardeners treat this as their baseline planting date. The 90% date (cautious) means only a 1-in-10 chance of frost remaining. This is the conservative choice, and the one I recommend for expensive transplants like grafted tomatoes or tender perennials you have been nurturing indoors for weeks. In practice, many experienced growers use the 50% date for hardy crops and the 90% date (plus a week of buffer) for anything frost-sensitive.

Why frost dates matter for your garden

Not all plants respond to frost the same way. Tender crops such as tomatoes, peppers, courgettes, basil, and runner beans suffer cellular damage when air temperature hits 0°C and will die outright in a hard frost. These need to stay indoors or under cover until your frost date has safely passed. Hardy crops are a different story: broad beans, garlic, kale, spinach, and most alliums actually benefit from cold exposure and can be sown or planted well before the last frost. Knowing your frost dates lets you build a sowing schedule that starts hardy crops early (maximising your growing season) while protecting tender ones from a late cold snap.

Microclimates can shift your frost date

The frost date for your grid square is an average over a broad area, so your actual garden may be warmer or cooler depending on local conditions. A south-facing wall absorbs heat during the day and radiates it overnight, creating a pocket that can be 2-3°C warmer than the surrounding area and can shift your last frost date a week or two earlier. Urban gardens benefit from the heat island effect (concrete, buildings, and traffic all radiate warmth), while gardens on hilltops or in frost hollows (low-lying areas where cold air pools) may experience frost a week or more after the official "safe" date has passed. If you are in any doubt, a cheap min/max thermometer placed at plant height for a few spring nights will tell you more about your specific microclimate than any dataset can.

A note on the data

This tool uses the frost days indicator from the Copernicus Climate Change Service (C3S) agroclimatic indicators dataset. It was computed from WFDEI data (the WATCH Forcing Data method applied to ERA-Interim reanalysis) for 1981-2010, on a 0.5° grid. Individual years still vary a lot, and a single late frost in an otherwise warm spring can catch you out whatever the averages say. Use these dates as your planning baseline, but keep an eye on your local 10-day forecast during the critical weeks around your frost date. Weather apps are free; replacing a flat of dead seedlings is not.