When to plant tomatoes: the date is not a date

"Plant tomatoes out after the last frost." It is the most repeated piece of gardening advice there is, and it is half wrong. Half only: it encodes a correct instinct — wait until frost risk has passed — in a rule that stops exactly where the useful part begins.

Late May is sometimes three weeks too late on a mild coast, and sometimes still too early at altitude. The right question is not "which day", it is "which signals".

Signal 1: the last frost

This is the eliminating signal. A tomato does not survive frost, and it is already damaged below 2 to 3 °C.

What matters is not the average last-frost date but the date after which the risk becomes acceptable to you. A gardener with fleece to hand can take the risk ten days earlier. A gardener going away for the weekend should not.

That date varies enormously — from late March on a mild Atlantic coast to early June in the uplands. It is the first parameter Marmanda derives from your location, because the whole rest of the calendar hangs off it.

Signal 2: soil temperature

This is the signal almost nobody checks, and probably the most useful one.

Tomato is a warm-climate plant. Planted into cold soil it does not die: it stops. The roots stop taking up nutrients properly, leaves turn purple, and the plant stalls for two or three weeks. A tomato planted too early is often overtaken in July by one planted a fortnight later.

The commonly used threshold: soil at 15 °C or above, measured in the morning about ten centimetres down, for several days running. A soil thermometer costs very little and settles a lot of arguments.

If your soil is still cold you have two levers: warm the bed with black mulch or a cover for a fortnight beforehand, or simply wait.

Signal 3: the state of the plant

The third signal is in your hands rather than in the sky.

A good transplant has a stocky stem, a root system that holds the rootball without circling it, and five to seven true leaves. A leggy plant — long, pale, leaning towards the window — will make a weak plant even in perfect conditions.

That means counting backwards. Sowing indoors generally needs six to eight weeks before the intended planting date. Planting out in mid-May means sowing in late March. Sowing in February out of impatience produces leggy plants you will have to pot on twice.

Plan for hardening off too: a week of progressive time outdoors before planting. A plant that goes straight from a warm windowsill into a north wind drops its leaves.

The calendar, in order

1. Find your likely last-frost date.
2. Count back six to eight weeks: that is your indoor sowing window.
3. Around the last-frost date, check the soil temperature.
4. Harden the plants off for a week.
5. Plant out when all three signals agree — not when the calendar says so.

What if you plant late?

A late planting is not a loss, it is a trade-off. Every week of delay shortens the tail of the season, so the last trusses will not have time to ripen. Two possible answers: choose an earlier variety, or accept picking green fruit in autumn and ripening it indoors.

If you are three weeks behind, favour a short-season variety over a large late beefsteak. That is a variety decision, not a failure.

Why an article cannot give you your date

Everything above is true everywhere and directly usable nowhere, because all three signals depend on your position, your soil and your shelter.

That is the work Marmanda does: from your location and the description of your garden, it produces dated windows — sowing, planting out, harvest — for the crops you want, with the sources behind them and an explicit abstention when the data is missing for your territory. You get a window, not a proverb.

The honest shortcut

If you keep one rule: wait for the soil to be warm, not for the calendar to feel safe. You will rarely lose anything by planting ten days later, and you will often lose something by planting ten days too early.