Harden off tomatoes: two weeks before planting out

The plants on the windowsill look finished. It is early May, one mild week sits in the forecast, and the bed is dug. Harden off tomatoes before that move. A tender plant from still, warm air can collapse after one breezy afternoon. The RHS and Gardeners' World agree that the step exists. They do not share, in the pages used here, a single outdoor temperature, and neither prints an hour-by-hour chart.

A qualifying purchase from a product link can earn us a commission, at no extra cost to you. The packet temperatures below come from amazon.co.uk listings in the tomato-seed comparison, read as seller instructions, not as a forecast. No Met Office frost map was opened, and nobody here ran a planting trial.

In short

Harden off tomatoes gradually for a couple of weeks, then plant out once frost risk has passed. The RHS waits until it is reliably above 16°C. Some packets say 12°C. Pick one threshold, and wait if frost is forecast.

Why a windowsill plant fails outside

After pricking out, the RHS wants tomato plants kept at least at 16°C while they are still indoors. A bright windowsill is how most people meet that line. The leaves grow soft. The stem has never had to hold itself in wind. That plant can look stocky in the pot and still be unfit for the garden on the day you first carry it out.

Tomatoes are tender. The RHS says so in the planting guidance, and it holds the final move until early summer, once the outdoor temperature stays reliably above 16°C. "Reliably" is doing real work. A sunny Saturday with a clear night afterwards is not the same as a run of days and nights above that line. Gardeners' World makes the frost point in plain language: outside only after the risk has passed.

The failure usually shows after the plant is already in the soil, which is the expensive moment to discover it. Leaves that were flat indoors curl, pale, or blacken at the edge. A plant still in its pot is easier to carry back inside than a stem you have buried. That is why the fortnight happens in the container you already own. The sowing guide covers the indoor stage before this, including the warmth after pricking out. Hardening starts when that plant is large enough to move, not when the seed has only just germinated.

A couple of weeks, not one afternoon

Gardeners' World asks you to harden off gradually over a couple of weeks. The RHS names the practice before planting out and, in the extract used here, does not add a day count. Neither source prints a timetable of hours. This article will not invent "two hours on Monday, four on Tuesday". What "gradually" and "a couple of weeks" do support is a longer outdoor spell as the fortnight goes on, and a return indoors when the day turns cold or the wind is harsh. One afternoon on the patio, then into the bed before supper, is a shorter exposure than those words describe.

Put the pots outside for part of the day, out of the strongest wind, then bring them back under cover. Stretch that outdoor spell across the fortnight. Skip a day when the forecast falls back towards frost. Leave them out overnight only near the end of those two weeks, once frost risk has passed and you are inside the temperature rule you chose.

The mistake is to treat a warm lunchtime as the whole test. Light is stronger outdoors than through glass, and wind pulls water from leaves that have never met it. A plant can wilt in the afternoon and look recovered by morning on the windowsill, then fail the next time because you read the recovery as proof.

16°C, May, and the 12°C packets

Two thresholds are in front of you, and they are not the same. The RHS puts the final position in early summer once the temperature is reliably above 16°C. Gardeners' World times the outdoor move to after frost risk, usually sometime in May, and does not state 16°C on the page used here. A kit listing in the tomato-seed comparison, and the Mar Azul and Montserrat packets, tell you to move plants outdoors only when temperatures stay above 12°C. That 12°C line is the seller's instruction. It is not a restatement of the RHS.

Choose which rule you are following and keep it in view. If you follow the RHS, you wait until conditions stay above 16°C, nights as well as days. If you follow the packet, you are accepting 12°C because the listing said so, and you have stepped off the RHS figure. Averaging the two numbers is a third rule that nobody published. The tomato seeds comparison is where those packet lines sit next to each other, including Just Seed Sungold F1, the top pick in that set at £3.65 with 4.5 from 111 reviews. Sungold's listing does not carry the 12°C sentence. Do not borrow another packet's temperature for a variety that never stated it.

"Usually sometime in May" is not a date for your postcode. This page did not open a Met Office last-frost table, so it will not award one. In a colder garden, sometime in May can mean the later part of the month. A warm spell in April does not cancel the condition that frost risk has passed. The threshold you chose, 16°C or 12°C, is the number. May is the typical month.

Planting deeper once they are ready

When the fortnight is done and the temperature test is met, the RHS planting line is specific. Set the plant deeply enough that the first leaves sit just above the soil. The buried stem makes more roots. Water it in so the compost and the surrounding soil meet.

Do the deep planting after hardening, not instead of it. Burying a soft windowsill plant on the afternoon it first goes outside stacks two stresses. If the plant is drawn, the deep set hides some bare stem, and the leaf rule still holds: the first leaves finish just above the surface. Leaves left under wet soil are the risk that wording avoids.

Where the plant will live is a separate choice from how deep you set it. The greenhouse or outdoor guide is the page for that decision. Hardening off, as these sources use it, is for plants that will live outside. A crop that stays in a heated greenhouse, already near the indoor 16°C, is not being asked to survive a garden night. Opening the house still puts outdoor air on the leaves. The RHS wants those vents for pollinators, and Gardeners' World treats the same opening as a blight route in wet, warm weather. An unheated house in a cold May can drop with the garden. The sources do not give it a separate timetable.

If a late frost is forecast

Wait. Gardeners' World keeps tomatoes inside until frost risk has passed. The RHS keeps the final planting for early summer weather that stays warm enough. A forecast that puts frost back on the week means the move stops, including for plants that are ten days into their couple of weeks. Carry them back under cover. A half-finished hardening is allowed to pause. It is not a reason to plant and hope.

The pages used here do not describe a fleece weight or a grade of cover. This article will not invent one. If you need a number, use the threshold you already chose. Below it, or with frost in the forecast, the plants stay in. Above it, with the fortnight done and no frost forecast, you plant, and you plant deeply.

Check the plants the morning after any night they did spend outside near the end. Blackened tips mean that night was too cold and the rest of the batch should go back in. Pale, limp leaves after wind often pick up once the pot is out of the draught, which is a reason to finish the fortnight before you commit them to the soil.