Sowing successfully at home: heat, light and potting mix

Your tomato seedlings are 20 cm of stem for two leaves and lean towards the glass. They lacked light, not warmth. A successful sowing at home takes three settings, in this order. First heat to germinate. Count 20 to 30 °C for tomato, pepper, aubergine and basil, 15 to 20 °C for onion and leek. Once it is up, the seedling wants as much light as possible and a little less heat. The last setting is a fine, light potting mix. In February, the window remains the weak link. Often, it is better to change the sowing date than the equipment.

At what temperature each seed comes up

Each species germinates within its own temperature range. Cycle en Terre, a Belgian organic seed company, publishes these values based on the ISTA standards. ISTA is the international association that sets the rules for seed tests. The RHS, the British horticultural society, gives more practical landmarks for the home.

The ISTA values come from a laboratory test. Above all, keep the order. Pepper and aubergine want the most heat and come up the slowest. Onion and leek are happy with a cool room.

The RHS advises a small heated propagator. Failing that, take a warm windowsill and cover the pots with a clear plastic bag to keep the moisture in.

A February window gives little light

Everything is decided here. At Liège, according to the RMI, the sun brings on average 1.4 kWh per m² per day in February, against 5.2 in June. It shines 2.8 hours a day in February, 6.9 in July. February light is therefore barely more than a quarter of June's. And this figure is measured outside, flat, without glass.

Seedlings ask for a lot. Researchers count the light useful to plants in moles per m² per day. Purdue, the Indiana university, has put figures on the needs of vegetable transplants.

A windowsill looks more like a shelter than the open field. The glass filters the light. The plant only sees the sky from one side. We found no published measurement for a Belgian window. If the glass and the frame remove as much as the shelter measured by Purdue, a few moles a day remain. That is far from the 15 of a tomato. Purdue describes what follows. Emergence is uneven, the stem stretches and stays fragile, the roots are weak. The plants take longer to be ready.

You have two solutions. Sow later, when the days lengthen. The dates in the table below take this into account. Or add a lamp. The guide Windowsill or LED lamp compares the two.

After emergence: cooler, brighter

As soon as the first shoots appear, change the setting. The RHS says to remove the lid or the bag right away. Put the seedlings in as much light as possible so they do not stretch. The tomato generally comes up in two weeks.

Then lower the heat a little. The RHS keeps chillies between 16 and 18 °C after emergence. Tomatoes stay at 16 °C at least, in full light.

Prick out when the seedling has two true leaves. These are the ones that follow the first two round leaves. The RHS advises lifting it with a dibber and holding it by a leaf, never by the stem. If a plant has stretched, prick it out deeper, with the first leaves just above the potting mix.

Potting mix: fine, light, peat-free

The RHS recommends a peat-free seed compost, or a peat-free multipurpose compost passed through a sieve.

To choose a potting mix, the guide Peat-free seed compost compares the published trials. If your seedlings suddenly flop over, pinched at the base, read Damping off. The RHS cites too much water among the causes, in basil especially.

In your region: when to sow at home

Count the sowing date backwards from the day the plant will go outside. For Belgium, the Vegiwise catalogue places the sowing of tomatoes six to ten weeks before they go out, from 15 May.

One rule holds everywhere. The later your last frost falls, the later you sow. Sown too early on a windowsill, the plant waits without enough light. Purdue notes that it then takes longer to be ready.

Crop by crop

Tomato. Comes up at 20 °C in one to two weeks. Prick out at two true leaves. In Belgium, the Triomphe de Liège is sown from 15 February to 5 April according to the catalogue. On a windowsill, aim for March instead. At Liège, the sun brings 2.5 kWh per m² per day then, against 1.4 in February (RMI). The full detail is in Sowing your tomatoes.

Pepper and chilli. These are the most demanding. They want 21 °C at least to come up, then one to two weeks of waiting. For them, an early sowing under a lamp is justified. The Rosso sweet pepper is sown from 1 February to 31 March in Belgium.

Aubergine. Same heat as pepper, 7 to 14 days to come up. The Rosso di Napoli aubergine is sown from 1 February to 31 March in Belgium.

Basil. Comes up around 20 °C. The RHS sows it from March to July. It warns that the seedlings damp off when watered too much. Water little, from below. The Grand vert basil is sown from 1 March to 10 May.

Lettuce. Comes up fast, in 4 to 7 days, around 20 °C. It needs less light than tomato, 10 to 15 mol a day according to Purdue. The Reine de Mai de Liège is sown from 1 February to 15 March.

Leek and onion. Come up between 15 and 20 °C, in one to two weeks. No need to heat, a cool, bright room is enough. The Poireau de Liège is sown from 1 March to 15 May, the Oignon de Stuttgart from 1 February to 31 March.

Before sowing, check that your old packets still come up with the 10-seed test. When the plants are ready, do not put them out all at once. Hardening off your seedlings gives the day-by-day programme.

Your sowing dates change with your climate and your varieties. Tell the app which seeds you have, and it gives you the window for each. Start here.

FAQ

Why do my seedlings stretch?

They lack light, especially when it is warm. Purdue describes stretched, fragile stems when daylight is too weak. Remove the lid as soon as they are up, lower to 16 to 18 °C and bring the plants closer to the glass.

Should you sow tomatoes in February?

Not on a windowsill. In Belgium, the Vegiwise catalogue places the sowing of tomatoes from 7 March to 1 April, for planting out from 15 May. A February sowing needs a lamp.

How long before pricking out?

Wait for two true leaves, according to the RHS. Then prick out into an individual pot and keep the tomato at 16 °C at least, in full light.

Do you need a heated propagator?

It helps for pepper, chilli and aubergine, which want 21 °C and more. For onion, leek or lettuce, a heated room is enough.

Which potting mix for seed sowing?

A peat-free seed compost, or a sieved peat-free multipurpose compost, according to the RHS. The guide Seed compost details the trials.

Photo: maks_d, Unsplash

Sources

1. Cycle en Terre, Calendrier des semis 2022 (température idéale et durée de germination par espèce, d'après les normes ISTA ; dates pour la Belgique)
2. RHS (Royal Horticultural Society), How to sow seeds indoors (environ 18 °C, terreau de semis sans tourbe, couverture, arrosage)
3. RHS, How to grow tomatoes (levée en deux semaines, lumière dès la levée, repiquage à deux vraies feuilles, au moins 16 °C ensuite)
4. RHS, How to grow chilli peppers (21 °C pour germer, 16 à 18 °C après la levée)
5. RHS, How to grow basil (20 °C pour germer, fonte des semis si on arrose trop)
6. Purdue University, Vegetable Crops Hotline n° 749, L. T. Wuetcher et W. G. Owen, Managing Daily Light Integral to Improve Vegetable Transplant Quality, 11 mars 2025
7. Purdue Extension, A. P. Torres et R. G. Lopez, Measuring Daily Light Integral in a Greenhouse, HO-238 (lumière naturelle de décembre à mars, perte de 40 à 70 % sous abri)
8. IRM, Statistiques climatiques des communes belges, Liège (1991-2020) : rayonnement solaire global et durée d'insolation par mois