Growing tomatoes, from the March sowing to the last fruit in October

In Belgium, a tomato is sown in March, planted in mid-May and gives fruit from July to October. Between the seed and the first fruit, there are six steps. Sowing is done in the warm, followed by pricking out into a pot. After ten days of hardening off, the plant goes into the ground when the nights are mild. You then train it all summer and harvest until the first cold. Elsewhere, the dates change. The table below gives them for five climates.

The tomato waits for warmth

The tomato only really grows in the warm. At Liège, the mean temperature goes from 10.4 °C in April to 14.2 °C in May, then 19.3 °C in July (RMI, 1991-2020 normals). The mean of a day reaches 10 °C from 11 May to 11 October. These five months are the window for the tomato in the ground.

So the whole season is calculated backwards. You start from the moment your soil and your nights will be warm enough. Then you count back the time the plant needs to be ready. A cold month delays everything, a warm month brings everything forward. Growing degree days measure exactly that, the heat received rather than the days on the calendar.

Step 1: sow in the warm, in March

Sow in a fine, moist potting mix, half a centimetre below the surface. The seed needs warmth to come up. The RHS, the British horticultural society, counts about 18 °C. ProSpecieRara, the Swiss foundation that conserves heirloom varieties, gives 20 to 25 °C. The seedlings generally come up in under two weeks.

Once they are up, they mostly need light. Without it, they stretch and stay fragile. In Belgium, a February sowing spends weeks on a dark windowsill. Better to wait for March. Sow in early March if you have a lamp, in mid-March on a well-exposed window. Either way, the plant is ready in mid-May. The sowing step by step is in the guide Sowing your tomatoes.

Step 2: prick out, burying the stem

When the plant has two true leaves, move it into a pot of its own. ProSpecieRara does it three or four weeks after sowing.

Step 3: harden off for ten days

A plant raised behind a window has never seen wind or direct sun. ProSpecieRara puts it out by day as soon as the air goes above 5 °C, in partial shade at first. At night, it comes in. After two or three days, it can take direct sun. Count seven to ten days before planting. The step by step is in Hardening off your seedlings.

Step 4: plant when the nights are mild

At Liège, the RMI counts no frost days in May on average. The nights still stay cool, with an average minimum of 9.4 °C. The RHS waits for the temperature to stay above 16 °C. ProSpecieRara does not plant before the Ice Saints, on 11, 12 and 13 May. On the Belgian plain, aim for mid-May. You can plant earlier if the forecast minimums stay above 5 °C over seven days. The full rule is in the article on the Ice Saints.

Keep tomatoes away from potatoes. Potato blight passes onto the tomato (sheet from the University of Göttingen and the Dreschflegel association).

Step 5: train the plant from June to September

Side shoots

On a tomato with a single stem, a shoot grows in the axil of each leaf. This is the side shoot. The RHS pinches it out at every watering. On a bush tomato, you leave them. The guide Tomato side shoots says what to do variety by variety.

To top or not to top

Outside, the RHS pinches the tip two leaves above the fourth truss of flowers. The Göttingen sheet does the opposite for vigorous varieties. It does not top them and gives them a tall bean pole. Both can be defended. The RHS wants trusses ripe before autumn, while Göttingen lets vigorous varieties produce as long as they can.

Water and feed

Water at the base, never on the leaves. A leaf that stays wet a long time opens the door to blight. In the ground, the Göttingen sheet does not make watering compulsory. When you do water, do it at regular intervals. In a pot, the RHS feeds the plant every two weeks with a high-potash fertiliser, as soon as the first fruits swell. A well-enriched ground can do without fertiliser.

Step 6: harvest, then bring in the last fruit

On the Belgian plain, the first tomatoes arrive in July. A ripe tomato is a little softer and comes away easily (ProSpecieRara). Pick it with its small stalk.

The season ends with the cold. At Liège, the mean falls to 11.7 °C in October. The 10 °C period stops on 11 October. Outside, green fruit no longer ripens. The RHS suggests two solutions. Pull up the whole plant with its fruit and lay it on straw, under cover. Or pick the fruit and let it ripen somewhere warm and dark, with a banana in the same drawer. The guide Ripening green tomatoes compares the methods.

The following year, do not put tomatoes back in the same bed (RHS).

Which varieties to start with outside

In a humid climate, blight is the number one enemy of the outdoor tomato. Two German universities have long looked for varieties that hold up. At Kassel, a programme has observed outdoor tomatoes since 1996. It has run national trials since 2003, with blight resistance at the centre of the selection. At Göttingen, with the Dreschflegel association, a first selection came out of 3,500 varieties tested from the North Sea to the Alps. For a difficult spot, their advice is to start with cherry tomatoes or wild tomatoes.

In the catalogue, three varieties have close dates in Belgium:

The Triomphe de Liège has its portrait, with its history and its growing. For other colours, go and see the Rose de Berne or the comparison of yellow tomatoes. To save the seeds of your open-pollinated varieties, follow the fermentation method.

Two or three varieties are enough for a family. If you tell the app which seeds you have, it sets the dates of each step to your garden. Start here. The shop's tomato seeds are in the Tomate Lovers collection.

FAQ

When to sow tomatoes in Belgium?

In March. Early March under a lamp, mid-March on a windowsill. The catalogue opens the window from 15 February. But a February sowing lacks light and gives stretched plants.

When to plant tomatoes outside?

On the Belgian plain, from mid-May, after the Ice Saints. Earlier only if the forecast minimums stay above 5 °C over seven days. In the south of France, the catalogue plants from 25 March.

Should you remove the side shoots?

Yes on a single-stem tomato, at every watering according to the RHS. No on a bush tomato.

How many tomato plants to plant?

For a first season, two or three varieties and a few plants of each are enough. Count 45 to 60 cm between two plants.

What to do with green tomatoes in October?

Bring them in before the cold. Let them ripen somewhere warm and dark, or pull up the plant and lay it on straw under cover.

Photo: Viviana Piper, Unsplash

Sources

1. RHS (Royal Horticultural Society), Tomatoes: grow your own (semis, repiquage, plantation, gourmands, quatre bouquets dehors, fin de saison)
2. ProSpecieRara et Tomates urbaines, Guide étape par étape Tomates (semis à 20 à 25 °C dès la mi-mars, plantons prêts environ six semaines plus tard, pot de 20 litres, récolte de juillet à l'automne)
3. Dreschflegel e.V. et Université de Göttingen, Tomaten im Freiland : Sortenempfehlung und Anbauanleitung (sélection parmi 3 500 variétés, essais de la mer du Nord aux Alpes depuis 2003, conseils de culture)
4. Université de Kassel, The organic outdoor tomato project (observations depuis 1996, essais nationaux depuis 2003, résistance au mildiou au centre de la sélection)
5. RHS, Tomatoes: fruit splitting and cracking (variations d'humidité du sol et de température, arrosage régulier)
6. IRM, Statistiques climatiques des communes belges, Liège (1991-2020 : températures mensuelles, jours de gel, dernier et premier gel, période de végétation)