How do I find out my last and first frost dates?

Almost every timing instruction in gardening is expressed relative to a frost date. Sow six weeks before it, harden off in the fortnight before it, plant out after it, lift tender tubers before the other one. None of that works if you do not know what your dates are, and most people never look them up — they use a month they half-remember, which in Britain can be six weeks wrong in either direction.

What you'll need

• Your postcode or nearest town
• Somewhere to write two dates down where you will find them again

Step by step

1. Understand what you are looking for before you search.You want two averages: the mean date of the last air frost in spring, and the mean date of the first air frost in autumn. Both are long-run statistics from a weather station, usually averaged over thirty years. Neither is a prediction of this year.
2. Find your nearest Met Office climate station.The Met Office publishes long-term averages for its network of climate stations. Pick the closest one, and note its altitude and whether it is coastal or inland, because you will need both to correct the figure.
3. Read the frost figures, not the temperature figures.Some station summaries give days of air frost per month rather than a date. That still works: the last month with a meaningful count of frost days in spring brackets your last frost, and the first in autumn brackets your first.
4. Cross-check against a second source.The RHS, a local gardening club, an allotment association or a county-level growers' group will often publish a working date for your area. If two independent sources agree within a week or so, you have a usable figure.
5. Write both dates down as a fortnight, not a day.A frost date is the middle of a probability distribution. Recording it as a two-week window rather than a single day stops you treating it as a promise and reminds you the risk does not fall off a cliff.
6. Correct the figure for your own site.This is the step almost everyone skips and it is worth more than the original number. Altitude, aspect, whether you sit in a hollow, urban heat, and distance from the sea all shift your garden's dates from the station's. There is a separate guide for the arithmetic.
7. Start recording your own.Note the date of the last frost you actually got each spring and the first each autumn. After three or four years your own record beats any published figure, because it is a measurement of your garden rather than of a field a few miles away.

Tips

• The published date is an average, which means roughly half of years frost later than it. If a whole tray of plants depends on the decision, use the date plus a fortnight, or plant on the date and keep fleece to hand.
• Ground frost happens more often, and earlier and later in the season, than air frost. Published dates are usually air frost. If you grow at ground level — strawberries, seedlings, low bedding — assume your effective season is shorter at both ends.
• Ask three people who have gardened nearby for ten years or more. Their answers will cluster, and the cluster is a better local figure than a station twelve miles away at a different altitude.
• Allotment sites are frequently on low, open, formerly unwanted ground. If yours is, its frost dates can be a fortnight worse than the housing estate that overlooks it.

Common mistakes

• Treating the last frost date as a date after which frost cannot happen — It is a mean. Late frosts after the average date are entirely normal and they are the ones that catch out a whole year's tender planting, because everything has been planted out by then.
• Using a date from a national gardening article or a seed packet printed for the whole country — Britain's frost dates span a couple of months from the mildest coastal south-west to upland Scotland. A single national date is wrong nearly everywhere.
• Looking up only the spring date — The autumn date is what gives you a growing season length, and it is the deadline for lifting tender tubers, bringing pots in and picking the last of the tender crops. Half the value is in the second number.

Questions people ask