Starting seedlings indoors
Tomatoes, peppers, aubergines and many others need a 6–10 week head start before they can go outside. A sunny windowsill, a seed tray and a bit of patience turn €3 of seeds into a full summer's harvest — but there's a craft to it. Too warm and they go leggy, too dry and they never germinate, too little light and they collapse at the first real sunlight.
Why start indoors?
Head start — heat-lovers like tomato, pepper and aubergine need 8–10 weeks of growth before the last frost to set fruit in time.
Protection — seedlings are safe from mice, slugs, cold snaps and heavy rain while they're tiny and vulnerable.
Better germination — controlled warmth and moisture give you 80–95 % success rates vs. 40–60 % direct-sown outdoors.
Variety access — seed packets offer hundreds of heirloom and unusual varieties that garden centres never stock as plants.
What to start indoors
Not everything benefits from pre-sowing. Transplant-shock-sensitive crops do much better direct-sown.
Start indoors
Heat-lovers (8–10 weeks before last frost): tomato, pepper, chilli, aubergine.
Cucurbits (3–4 weeks before): cucumber, squash, melon — or direct-sow once the soil is warm.
Brassicas (4–6 weeks before): broccoli, cabbage, kohlrabi, kale, Brussels sprouts.
Slow starters: basil, parsley, leek, celery — they benefit from an early, warm start.
Sow direct
Root crops: carrot, radish, beetroot, parsnip — the taproot forks when transplanted.
Peas & beans: big seeds, fast germination, and they resent having their roots disturbed.
Salads: lettuce, spinach, rocket — cheap, fast, no advantage from indoor start.
Umbellifers: dill, cilantro (coriander), fennel — also dislike transplanting.
Sowing mix
Regular potting compost is too rich, too lumpy and too wet for tiny seedlings.
Buy seed-starting compost, or mix 1 part compost + 1 part coir/peat + 1 part perlite or vermiculite.
Fine-textured, sterile and low-nutrient — the lack of food forces roots to hunt downward, building a strong system.
Moisten the mix before filling trays — dry mix repels water and leaves dry pockets around the seeds.
Containers
Cell trays
Plastic trays with 6×4 or 8×6 cells. Cheap, reusable for years, easy to transplant from. The workhorse of indoor sowing.
Peat / paper pots
Biodegradable — plant the whole pot into the ground. Zero transplant shock, great for fussy roots, but more expensive per pot.
DIY containers
Toilet-paper rolls, yogurt cups with drainage holes, newspaper pots, soil blocks. Free, plenty of space for roots, perfectly fine.
Skip these
Egg cartons — too small, dry out in hours, roots hit the bottom within a week. Fine for cress but nothing you want to grow for weeks.
Light
The single biggest limit on windowsill sowing — seedlings get leggy and collapse without enough light.
South-facing window for late-winter sowings (Feb–Mar) is the minimum, and often still not enough.
A full-spectrum LED grow light (20–40 W) at 15–25 cm above the seedlings, 12–16 hrs/day, solves the problem completely.
Turn the tray 180° every 2 days so seedlings don't all lean toward the window.
Red flags: pale colour, long thin stems, leaves angled steeply upward. Those plants are starving for light — move them closer or add a lamp.
Watering
Water from below: pour water into the tray, let seedlings wick it up for 20–30 min, then drain. Prevents damping-off disease.
Moist, never soaked — roots need oxygen. If the surface glistens, you've watered too much.
Humidity dome or plastic bag only until germination — remove it immediately once cotyledons appear, or mould takes over.
Room-temperature water only — cold tap water shocks tiny roots and slows growth for days.
Temperature
Germination: 20–25 °C for most crops. Top of the fridge, on a radiator, or a heated propagator mat.
After germination: cooler, 15–20 °C, with plenty of light. Prevents the leggy-seedling problem.
Pepper and chilli need 25–28 °C to germinate and can take 2–3 weeks. Don't give up too early.
Check the seed packet — some varieties need specific temps (cold stratification, etc.) and the packet will say.
Pricking out
Moving tiny seedlings from a crowded sowing tray into individual pots once they develop their first true leaves (not the round cotyledons).
Wait for 2 real leaves above the cotyledons — the stem is then strong enough to handle.
Hold only by the cotyledon leaves — never the stem. A bruised stem kills the seedling.
Loosen soil around the roots with a pencil, lift the whole root ball gently, and replant in its own pot.
Plant deep — tomatoes and brassicas can be buried up to the first true leaves. New roots form along the buried stem, making a stronger plant.
Water in gently and keep out of direct sun for 1–2 days so the seedling can recover.
Hardening off
Indoor-grown seedlings have thin cuticles and weak stems. Planted straight out, they get scorched by sun, shredded by wind, and stunted for weeks.
Day 1–2: 1 hour outside in shade, sheltered from wind.
Day 3–5: 3–4 hours, gradually adding morning sun.
Day 6–7: a full day outside, but still bring them in for the night.
Day 8+: leave overnight if temperatures stay above 10 °C — then ready to plant out.
Keep a sowing journal: variety, date sown, germination rate, date pricked out, date planted out. After one season you'll have a personalized schedule that works for your specific windowsill, grow light and climate.