Every spring, windowsills across the country fill with trays of seedlings that start promisingly and end up pale, stretched and flopping by April. The seeds were fine; the timing and the light were wrong. Sowing indoors is genuinely easy once you understand a handful of principles — when to start, why light matters more than warmth after germination, and how to move young plants outside without shocking them.
Time it from your last frost, not the packet photo
Seed packets give generous sowing windows — "February to April" — because they are written for the whole country. What actually matters is your local last frost date, because that is when tender plants can finally go outside. In much of southern England that is early to mid May; in the north, Scotland and frost pockets it can be late May or even early June. The Met Office's climate data for your area is a better guide than the packet, and so is your own memory of recent springs.
Count backwards from that date. Tomatoes need about eight weeks indoors, so a mid-May planting-out date means sowing in mid March. Courgettes and squash need only three to four weeks, so mid to late April is plenty. Sow too early and you end up with root-bound, stressed plants queuing on the windowsill for weeks while frosts continue outside — they rarely outperform later, better-timed sowings.
- February–early March: chillies and aubergines (slow starters that need the long run-up), onions from seed.
- March: tomatoes, sweet peppers, celeriac; half-hardy annual flowers such as Cosmos and Antirrhinum towards the end of the month.
- April: courgettes, squash, cucumbers, sweetcorn, French and runner beans, sunflowers, Zinnia.
- Any time the space exists: hardy things like lettuce, brassicas and peas, which do not care about frost and can go out young.
Heat germinates seeds; light grows seedlings
The single most common indoor sowing failure is confusing these two needs. Most seeds germinate happily somewhere between 18 and 21°C — an airing cupboard, a heated propagator, or the top of a warm kitchen cupboard all work, and darkness is fine at this stage for most vegetables.
The moment shoots appear, the priorities flip. Seedlings now need maximum light and, ideally, slightly cooler conditions. A seedling kept warm but short of light does the only thing it can: it stretches towards the window, producing the pale, elongated, weak-stemmed plants known as leggy seedlings. Leggy plants never fully recover their sturdiness.
Practical fixes:
- Move trays off the heat and onto the brightest windowsill you have — south or west facing — the day germination shows.
- Turn pots a quarter-turn daily so seedlings do not lean permanently towards the glass.
- In February and March, natural window light in the UK is marginal. Either wait and sow later (usually the best answer) or use an inexpensive LED grow light on a timer for 12–14 hours a day.
- Keep night temperatures cooler than day where you can; a constantly warm room encourages stretching.
Choosing and using compost
Use fresh, peat-free seed compost or a fine-textured peat-free multipurpose. Seed compost is deliberately low in nutrients — seeds carry their own reserves and high fertiliser levels can damage emerging roots. Sieve out the coarsest lumps of multipurpose if that is what you have; a seed needs consistent contact with fine, moist material.
Do not reuse last year's opened bags for seed sowing if they have been sitting wet outdoors, and never use garden soil indoors — it compacts in trays and carries weed seeds and fungal spores. Fill containers, firm very lightly with the base of another pot, and water before sowing so the surface is damp but not sodden.
How deep to sow: the two-times rule
The near-universal rule of thumb is to cover a seed to about twice its own diameter. Big seeds like beans and squash go 2–3cm down; medium seeds like tomatoes get a light 5mm covering; and dust-fine seeds such as begonias, foxgloves and lettuce are surface-sown and merely pressed in, because many small seeds need light to germinate. When in doubt, sow shallower — a seed buried too deep exhausts itself before reaching daylight, while a seed sown too shallow usually manages.
Cover trays with a clear lid or a sheet of glass to hold moisture until germination, then remove it promptly. Water from below by standing trays in shallow water, or use a fine rose; a jet from a can will redistribute your entire sowing into one corner.
Pricking out without casualties
Seedlings sown in trays need moving to individual cells or small pots once they have their first true leaves — the leaves that look like the adult plant, appearing after the initial rounded seed leaves.
- Water an hour beforehand so the compost holds together.
- Loosen under a clump with a dibber, pencil or old teaspoon.
- Hold seedlings only by a seed leaf, never the stem. A bruised leaf is expendable; a crushed stem is fatal.
- Lower each seedling into a pre-made hole in its new pot, deep enough that the seed leaves sit just above the surface — tomatoes and brassicas positively benefit from being buried up to their seed leaves, rooting along the buried stem.
- Firm gently, water in, and keep out of direct sun for a day or two while they settle.
Large seeds (beans, squash, sweetcorn, sunflowers) skip this stage entirely: sow them one per pot or module from the start.
Hardening off: the step everyone skips
An indoor-raised plant has soft growth that has never met wind, direct sun or a cold night. Put it straight outside and the leaves scorch, bleach or shred. Hardening off is simply a graduated introduction, and it takes one to two weeks:
- Days 1–3: outside in a sheltered, lightly shaded spot for a few hours, back in at night.
- Days 4–7: outside all day, including some direct sun, in at night.
- Week 2: outside day and night unless frost is forecast, then plant out.
A cold frame makes this easier — lid off by day, on at night — but a sheltered spot by the house wall and a nightly carry indoors works fine. Watch the forecast throughout: one late frost in May can erase two months of work, and covering plants with fleece on a cold night is far cheaper than resowing.
Preventing damping off
Damping off is the fungal collapse of seedlings at soil level — one day a healthy forest, the next a patch of toppled stems with pinched brown bases. There is no cure once it starts, but prevention is straightforward because every contributing factor is under your control:
- Use clean pots and trays (wash last year's in warm soapy water) and fresh compost.
- Sow thinly. Overcrowded seedlings create the still, humid canopy the fungi love. If you can't see compost between seedlings, they are too thick.
- Water with mains tap water rather than water-butt water for seedlings, and keep compost moist, never saturated.
- Remove propagator lids as soon as germination is complete, and ventilate on mild days.
- Prick out promptly rather than leaving seedlings jostling in the tray for weeks.
Kit that earns its place — and kit that doesn't
Worth buying or improvising:
- A heated propagator or windowsill heat mat — the one gadget that genuinely widens what you can grow, especially chillies. A £20 unheated windowsill propagator plus a warm spot achieves most of the same result.
- Module trays and 9cm pots — reusable rigid ones last a decade. Yoghurt pots and toilet-roll tubes with drainage holes are free and work.
- A basic LED grow light if you insist on February sowings.
- Plant labels and a pencil — every gardener who has skipped this has ended up with trays of anonymous green.
- Horticultural fleece for frost protection during hardening off.
Generally not worth it: propagators with built-in apps, self-watering seed kits, seed-sowing gadgets that dispense one seed at a time (fingers and folded paper do this), and "intelligent" pods locked to branded refills. The plants cannot tell the difference; your bank account can.
Sources and further reading
- Royal Horticultural Society — sowing guides for individual vegetables and flowers
- Garden Organic — organic seed raising, peat-free growing and the Heritage Seed Library
- Met Office — frost forecasts and local climate averages for timing sowings and planting out