Starting seeds indoors

Starting seeds indoors means germinating and growing young plants in the house before they go outside. You gain weeks, you can grow varieties nobody sells as ready-grown plants, and you control the conditions.

When to begin

Count back from your last frost date. Most seeds are started indoors about 6 to 8 weeks before it.

It varies by crop:

Every packet states what applies to that variety. Start a little late rather than far too early: seedlings that wait indoors for weeks outgrow their pots and turn long and weak.

What you need

• Pots or a seed tray, at least 5 to 8 centimetres deep and with drainage holes
• Fresh seed compost — light and fine, not garden soil and not old potting mix
• A mister or a can with a fine rose
• Plenty of light: the brightest windowsill you have, or a grow light
• Labels and a waterproof pen

Useful but not essential: a clear lid or propagator, a heat mat, and a small fan for airflow.

If you are reusing old pots, wash them first. That alone removes much of the risk of damping off.

Step by step

1. Prepare compost and pots. Moisten the seed compost first in a bowl — damp like a wrung-out sponge, not dripping. Fill pots to about three quarters and firm lightly. Do not pack it down; it should stay open and airy.

2. Sow. Sow about twice as deep as the seed is wide. Large seeds such as beans can go 1 to 2 centimetres down, small seeds need only 3 to 6 millimetres of covering, and light-loving seeds stay uncovered. Sow one or two seeds per cell; if both come up, snip out the weaker one.

3. Moisten and cover. Mist gently so the seed is not washed out. Put a clear lid or a bag over the tray to hold the moisture in.

4. Keep it warm. Most seeds germinate at 21 °C or a little warmer. A heat mat holds the compost around 21 to 27 °C for heat-lovers. At this stage warmth matters more than light.

5. Light as soon as they show. Take the cover off the moment the first shoots appear, and move them somewhere as bright as possible. With a grow light, run 14 to 16 hours a day. Too little light gives long, weak seedlings within days. Normal room temperature of 18 °C or more is fine now, and the heat mat can come off.

6. Water from below. Stand the pots in a little water so the compost draws it up, then pour off the excess. That keeps the surface drier and pulls roots downward.

7. Thin and feed. Once the first true leaves appear — the second set, which look like the adult plant's leaves — the seed's own store is used up. Feed weekly with a heavily diluted liquid fertiliser, at a quarter to half the normal strength.

8. Pot on. If roots poke out of the drainage holes or the plant is clearly cramped, move it into a larger pot with fresh compost. Tomatoes and peppers often need this if you started early.

Then: hardening off

Do not put seedlings straight outside. See Hardening off seedlings for the 7 to 10 day routine that gets them used to wind, sun and cool nights.

Common mistakes

• Sowing too early, so plants outgrow their pots indoors.
• Too little light, which gives long, pale seedlings.
• Too much water, which invites damping off.
• Using garden soil or old potting mix instead of fresh seed compost.
• Forgetting to label.
• Skipping hardening off.

The full version of this guide is on seedschoice.com.

Ready to sow? What you can sow now

• Vegetables you can sow this month
• Flowers you can sow this month
• Herbs you can sow this month
• Vegetables to sow directly outdoors
• Fast-growing vegetables — quick results, ideal for beginners

New to SeedsChoice? Sign up for our newsletter with the "10% off" button in the shop and get 10% off your first order. Orders leave our warehouse in Meppel within 1–2 business days.

Updated on: 05/10/2026