Starting Tomatoes Indoors: When, How and With How Much Light
Tomatoes need a long season before the first fruits are ripe. If you only sow them in May, you harvest late and little. That's why you start them indoors. Here you'll learn when the right time is, how to sow successfully and how small seedlings turn into strong plants.
When can you start tomatoes?
The best window is mid-March to early April. By mid-May, after the last frosts, the plants are then six to eight weeks old and a good 20 to 30 cm tall. That's ideal for planting out.
- Late February only works if you have a grow light. Otherwise the plants grow long and thin.
- Later than mid-April, starting them yourself is hardly worth it. Buy young plants instead.
- Fully indoors, you can sow compact varieties all year round, as long as they get enough light.
Which other vegetables to start when is shown in our Indoor sowing calendar.
Which varieties are suitable?
- Cherry and cocktail tomatoes bear early and produce lots of small fruits. They also forgive mistakes.
- Bush tomatoes and dwarf varieties stay compact. They fit in pots, on the balcony and on the windowsill.
- Beefsteak tomatoes take the longest and need lots of space. They're better suited to a greenhouse or a sheltered bed.
What you need
- Seeds. Seeds from shop-bought tomatoes often germinate, but they don't always produce the same variety.
- Seed compost. It's low in nutrients and loose.
- Small pots or a seed tray with a lid.
- A warm spot. Tomatoes germinate best at 20 to 25 °C.
- Light. After germinating, the seedlings need lots of it.
Sowing step by step
- 1. Sow. Place the seeds about half a centimetre deep in moist seed compost and cover them lightly.
- 2. Keep moist and warm. A lid or cling film holds in the moisture. Air briefly once a day.
- 3. Wait. After five to ten days, the first seedlings appear.
- 4. Into the light. Take off the lid and move the seedlings somewhere as bright as possible.
Light against legginess
The most common mistake when starting plants indoors is too little light. The seedlings stretch towards the window and grow long, thin and pale. Gardeners call them leggy. Such plants flop over easily.
- 12 to 16 hours of light a day keep the plants compact.
- A grow light at a fixed distance and on a timer makes this easy. More on this in our guide Grow light for herbs.
- Cooler after germinating. 18 to 20 °C is enough now. Warmth with little light makes the plants leggy.
Pricking out and potting on
After the round seed leaves, the first jagged tomato leaves appear. Now it's time to prick out: each plant goes into its own pot. Tomatoes have one special feature: if you set them deeper, up to just below the seed leaves, the stem forms new roots. This makes the plant sturdier. When the roots fill the pot, move it into a larger pot with normal potting soil.
Hardening off and planting out
Once the last frosts have passed in mid-May, your tomatoes can go outside. Harden them off to sun and wind over a week: first a few hours in the shade, then longer and sunnier. Outside they need a sunny spot that's sheltered from rain. You can read how tomatoes do well on the balcony and why indoors is sometimes the better choice in Tomatoes on the balcony.
Tomatoes indoors all year round
Compact cherry tomatoes also grow fully indoors, as long as they get enough light. In the GreenBox, the seeds germinate in PlantPlugs instead of seed compost. The GreenBox takes care of light and schedule, and the app reminds you to top up. Varieties like Cherry Tomato, the Red Bubble cocktail tomato or the Summer Sweet wild tomato stay there until harvest. To get started, there's the Cherry Tomato & Chilli Mix. You can read how to start chillies in Starting chillies indoors.
In a nutshell
- Sow tomatoes from mid-March to early April, or from late February with a grow light.
- Sow them half a centimetre deep and keep them moist at 20 to 25 °C.
- After germinating, move them somewhere bright and cooler: 12 to 16 hours of light, 18 to 20 °C.
- Set them deeper when pricking out; this gives a sturdy stem.
- Harden them off and move them outside after the last frosts in mid-May, or grow compact varieties fully indoors.
Happy sowing!
Your BERLIN GREEN team
