How do I grow plants from seed on a windowsill with no greenhouse?

Almost everything that goes wrong with windowsill seedlings comes from one mismatch. Indoors, it is warm enough for June and as bright as a dull December afternoon. Plants respond to that by stretching for the light they are not getting. Every technique here is about narrowing that gap, and the most powerful one is simply sowing three weeks later.

What you'll need

• Seed compost
• Trays or modules with drainage
• The brightest windowsill available
• White card or foil to reflect light
• A grow light (optional)
• A tray for watering from below (optional)

Step by step

1. Sow later than you want to.This is the single most effective change and it costs nothing. Light levels in a UK February will not support a seedling however warm it is. A sowing made in late March or April on a windowsill routinely overtakes one made in February.
2. Choose the brightest window, then improve it.South or west-facing, glass clean, nets and blinds out of the way, nothing shading it outdoors. Every layer between the sky and the seedling costs you.
3. Put white card or foil behind the tray.The side facing the room gets almost nothing. A folded piece of white card, or foil on card, bounces light back and halves the leaning. It costs nothing and works.
4. Turn the tray every day.Seedlings lean towards the light within hours. A daily quarter turn keeps them upright and the stems even.
5. Keep them cool once they are up.Move them off a radiator shelf, and if you can put them in the coolest bright room you have — a spare bedroom or a porch — do. Cool and bright makes stocky plants; warm and dim makes weak ones.
6. Move them off the sill at night in cold weather.The space between a closed curtain and the glass can approach freezing on a clear night. Bring the tray into the room in the evening and put it back in the morning.
7. Water from below and sparingly.Stand the tray in shallow water for a few minutes and drain it. Indoor air is still, so wet compost stays wet, and damping off follows.
8. Sow thinly and prick out promptly.Crowded seedlings shade each other and hold humidity between them. Thin sowing and moving them at the first true leaves gives the best plants a windowsill can produce.
9. Add a grow light if you want to sow early.A simple LED bar 100 to 300mm above the seedlings for fourteen to sixteen hours a day genuinely changes what is possible indoors, and it is what makes a February sowing viable.
10. Choose crops that suit the conditions.Lettuce, brassicas, leeks, herbs, peas, beans, courgettes and most hardy annual flowers do well from a windowsill start. Chillies and aubergines need warmth for germination and then more light than a sill offers, so they are the hardest.
11. Plan for the handover outdoors.Windowsill plants have met no wind, no cold night and no direct sun. They need a proper week or two of hardening off in a cold frame or a sheltered spot before they are planted out, or they will scorch and check badly.

Tips

• The three free changes that matter most: sow later, add white card behind the tray, and keep them cooler once they germinate.
• A bright unheated porch or a cool spare room beats a warm kitchen windowsill almost every time.
• Label everything at the moment of sowing. On a windowsill in March everything looks the same by week three.

Common mistakes

• Sowing in January and February with no grow light — There is not enough light in a British midwinter for a seedling to grow properly, and the warm indoor temperature makes it stretch. It is the commonest beginner mistake and it wastes a lot of seed.
• Growing seedlings above a radiator — Warm dry air and dim light. It is the exact combination that produces long weak plants, and the compost dries unevenly as well.
• Leaving trays between the curtain and the glass overnight in early spring — That gap gets far colder than the room and can freeze. Bring them in at night.
• Planting straight out from the windowsill — The leaves have never met ultraviolet light or wind. They scorch and shred within days. Harden off properly.

If it doesn't work

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