Starting tomatoes early is the best way to get a reliable harvest in Nordic-ish weather. The trick is warm roots, steady light, and a gentle transition outdoors.
When to sow
Count backward 6–8 weeks from your usual last frost. In much of Sweden, that’s late March to mid-April for planting out late May–June. If you’re growing only on a balcony or in a greenhouse, you can start a bit earlier.
Soil, heat, and light
Use a fine seed-starting mix; pre-moisten so it clumps lightly in your hand. Sow seeds 0.5–1 cm deep. Tomatoes germinate fastest at 22–25 °C. A heat mat speeds things up; remove it once most seeds sprout. Give 14–16 hours of bright light daily—a south window with a reflector or LED grow lights 20–30 cm above the seedlings.
Watering and feeding
Mist to keep the top evenly moist, never soggy. Once the first “true leaves” appear, feed every 7–10 days with a half-strength, balanced fertilizer. Aim for steady, slow growth—not tall and weak.
Potting on
Transplant to individual pots when roots fill the cell (2–3 weeks after sprout). Bury stems a little deeper; tomatoes grow roots along buried stems, making sturdier plants.
Hardening off (the step many skip)
7–10 days before planting out, start giving the plants outdoor time:
Day 1–2: bright shade, no wind, 1–2 hours.
Day 3–4: dappled sun, 3–4 hours.
Day 5–6: morning sun, light breeze, 5–6 hours.
Day 7+: full sun if possible, bring in only for cold nights.
If a night below 8 °C is forecast, keep plants inside or cover them.
Planting out
Soil should be 12 °C+; colder soil stalls growth. Space 45–60 cm for indeterminates with a tall stake or string, 35–45 cm for determinates/bush types with cages. Mulch with straw or shredded leaves to stabilize temperature and reduce splashing diseases.
Quick checklist
Droopy afternoons = normal if they recover by evening; if not, water earlier.
Purple leaves = cold roots or phosphorus hunger → warm the soil, light feed.
Leggy seedlings = too little light → lower lamps or add a second fixture.
