Finding your last frost date

It is the one piece of information that makes a sowing calendar usable, and almost nobody knows it for their own garden.

Once you have it, everything else follows. Tomatoes get sown six weeks before it, planted out two weeks after it, beans get sown once it has passed. Without it you are following dates written for an average garden that sits nowhere.

What a last frost date actually is

It is not a date, it is a probability, and that is the first misunderstanding to clear up.

The value in most tables is the date after which a 10 % risk of frost remains. In other words: one year in ten, it still freezes afterwards. Some sources give the 50 % threshold instead, meaning one year in two, which is a very different thing to know when you are putting plants outside.

So keep two numbers rather than one: the average date, and the date after which the risk becomes negligible. There are often three weeks between them.

Three ways to find it

The nearest weather station. National meteorological services publish normals by station, and American university extension services publish particularly readable tables. Look for the station that resembles yours in altitude and distance from the sea, not merely the closest as the crow flies.

Neighbours who garden. The most underrated source, and often the most accurate. Someone who has worked the same slope for twenty years knows the date to within a week, including the fact that the bottom of the valley freezes eight days longer than the hillside.

Your own records. A minimum thermometer left in the vegetable patch, read every spring, beats any table after three years. It is slow, and it is the only method that accounts for your actual garden.

Why your garden is not your town

The gap between two points a kilometre apart routinely reaches ten days, sometimes more. Four things explain it, and you can judge them without instruments.

Altitude. Reckon on roughly one day later per thirty metres of elevation.

Valley bottoms. Cold air is heavier and flows downhill. A hollow freezes reliably before the slope above it, even ten metres above. That is the classic frost pocket.

Water nearby. A large body of water, sea or lake, delays the last spring frost and pushes back the first autumn one. It is why coastal Brittany grows things that are impossible inland at the same latitude.

The town. A built-up centre easily gains one to two weeks on the surrounding countryside, purely from heat stored in the fabric.

Using it without getting caught out

Once you have the date, three habits are enough.

Sow indoors by counting backwards. Six weeks before for tomatoes, peppers and aubergines, four for squash. Sowing too early gives leggy plants that suffer at transplanting.

Do not put plants out on the date, put them out after it. The useful rule is not frost, it is night temperature. A tomato outside when nights drop to 6 °C does not freeze, but it stops growing and takes three weeks to restart. Wait for nights that stay above 10 °C.

Keep fleece within reach for three weeks afterwards. That is what turns a one-year-in-ten residual risk into a mild annoyance.

And the first autumn frost

It always gets forgotten, and it is the one that decides when the season ends.

It tells you whether a sowing still has time. A crop needing seventy-five days, sown sixty days before the first frost, will give you nothing. That calculation is the one most often missing from late-summer sowings, and it is why serious calendars show a window closing date, not just an opening one.

Keep a rule rather than a date

Everything above is done by hand, once, and lasts a decade.

The principle to keep: never write down a sowing date, write down a rule. Forty-two days before the last frost stays true every year and in every garden, whereas a date only holds for the place where it was written. With your last spring frost and your first autumn frost, each rule becomes a date in ten seconds.

But start by finding it. Even roughly, it changes how you read a seed packet straight away.