Indoor sowing calendar: what to start when
Many vegetables and flowers need a head start to give you a big harvest or lots of flowers in summer. They get this head start indoors, on the windowsill or under a grow light. Here you'll find, month by month, what to sow when, what seedlings need and which mistakes to avoid.
Sowing calendar: January to May
January
- Very hot chillies like Habanero or Carolina Reaper. They take up to four weeks to germinate and a long time until the first pod.
- Physalis and pineapple ground cherries. They grow slowly too.
February
- Mild chillies and sweet peppers. More on this in our guide Starting chillies indoors.
- Aubergines. They like it warm, around 25 °C.
- End of February: tomatoes. If you have lots of light, you can start now. Otherwise, wait until March.
March
- Tomatoes. Mid to late March is the best time for most varieties.
- Herbs like basil, thyme, oregano and marjoram.
- Salad greens like cut-and-come-again lettuce or rocket. They germinate at just 15 °C.
- Summer flowers like zinnias, petunias and sunflowers.
April
- Cucumbers and courgettes. They grow fast. Sown too early, they get too big before they can go outside.
- Basil. A second sowing gives you fresh supply for the summer.
- Nasturtiums and pot marigolds.
May
- Once the last frosts have passed in mid-May, your young plants move outside. Harden them off to sun and wind over a week.
- Fast herbs and salad greens can now be sown directly outdoors.
Indoors all year round
You can sow herbs, salad greens and compact varieties of chilli or tomato indoors all year round, as long as they get enough light. In winter, that means using a grow light.
What do seedlings need?
- Light. This is the most important point. Seedlings need 12 to 16 hours of light a day. In January and February, a window is rarely enough.
- Warmth. Most vegetables germinate at 20 to 25 °C. Chillies and aubergines like it even warmer. Salad greens and many herbs also germinate in cooler conditions.
- Seed compost. It's low in nutrients and loose. Normal potting soil has too much fertiliser and is often too coarse.
- Moisture. The soil should be moist, but not wet. A lid holds in the moisture during germination. Air once a day.
Grow light for seedlings
A grow light makes raising seedlings in winter predictable. Hang it at a fixed distance above the seedlings and control it with a timer. That way the plants get the same amount of light every day and stay compact. You can read what matters in a lamp in our guide Grow light for herbs.
Common mistakes
- Sowing too early. The plants get too big before they can go outside. Stick to the calendar.
- Too little light. The seedlings grow long and thin. Gardeners call them leggy.
- Too warm after germinating. Warmth without enough light makes seedlings leggy. Once they have germinated, move them somewhere a little cooler, at 18 to 22 °C.
- Too much water. Wet soil encourages mould and root rot.
- Pricking out too late. As soon as the first true leaves appear, each plant needs its own pot.
- Going outside without hardening off. Direct sun burns the leaves of plants that only know window light.
Starting seeds in the GreenBox
In the GreenBox, seeds germinate in PlantPlugs instead of seed compost. The GreenBox takes care of light and schedule, and the app reminds you to top up the water. So you can start in January without relying on window light. Compact varieties like Cherry Tomato or Chili Mild stay in the box until harvest. Larger plants get their start there and move outside in May. For your own seeds, there's the Experimental Set with PlantPlugs without seeds. To get started with tomatoes and chillies, there's the Cherry Tomato & Chilli Mix.
In a nutshell
- January: very hot chillies. February: mild chillies, sweet peppers, aubergines.
- March: tomatoes, herbs, salad greens and summer flowers.
- April: cucumbers, courgettes, a second round of basil.
- May: after the last frosts, move your young plants outside.
- 12 to 16 hours of light, 20 to 25 °C for germinating, a little cooler afterwards.
Happy sowing!
Your BERLIN GREEN team
