The one date that decides everything
Every sowing date counts back from your last frost date. Tomatoes sown 6 weeks before it, peppers 8 weeks, lettuce 4 weeks. If you do not know your last frost date, your local extension service publishes the average for your area. Use the seed starting calculator to turn that date into sowing dates.
January–February: slow growers only
Only sow the slowest crops now: peppers, chillies, aubergines and onions. They need 10–12 weeks indoors and grow slowly at first. Most other seeds sown now will be leggy and root-bound long before they can go out.
March: the main sowing month
March is when most sowing happens: tomatoes, brassicas, lettuce and herbs. Give seedlings strong light — a sunny windowsill is often not enough, and a simple grow light a few inches above the leaves prevents legginess far better than a warm room does.
April: harden off and transplant
April is for hardening off: put seedlings outside for a few hours a day, increasing the time over a week, before transplanting after the last frost. Direct-sow peas, beans and root crops outside now — they dislike transplanting and grow better sown where they will live.
May–June: succession sowing
Keep sowing salad leaves, beans and courgettes every two to three weeks for a continuous harvest. A gap in sowing is a gap in eating, and it is the most common reason a garden goes quiet in August.
July–August: sow for autumn
Sow autumn and winter crops now: kale, winter lettuce, spinach and overwintering onions. Seeds sown in high summer germinate fast, but keep them watered — a dry seed tray in July can fail in a day.
Seed starting calendar: crop by crop
The rule behind every date in this guide is simple: find your last frost date, then count back by the weeks each crop needs indoors. Peppers need 8–10 weeks because they grow slowly; tomatoes need 6–8 weeks; lettuce and spinach need only 3–4 weeks and can also be direct-sown; cucumbers, squash and melons need just 2–4 weeks because they grow fast and dislike root disturbance; onions and leeks need 10–14 weeks, and celery 10–12 weeks, which is why they are January crops. Use the seed starting calculator to turn your own last frost date into exact sowing dates, and remember that a packet's "weeks before transplanting" means weeks indoors before the transplant date — usually the last frost date for tender crops — not weeks from today.
| Crop | Weeks indoors before last frost | Notes |
|---|---|---|
| Onions, leeks | 10–14 | Slowest; start in January |
| Celery | 10–12 | Needs steady moisture |
| Peppers, chillies | 8–10 | Needs warmth to germinate, 80–90°F |
| Tomatoes | 6–8 | The classic March sowing |
| Brassicas | 6–8 | Cabbage, broccoli, cauliflower |
| Basil | 4–6 | Very cold-sensitive |
| Lettuce, spinach | 3–4 | Or direct sow outdoors |
| Cucumbers, squash, melons | 2–4 | Dislike transplanting; sow late |
Light: the make-or-break factor
Light decides whether seedlings are sturdy or leggy, and a sunny windowsill is often not enough — winter sun is weak, days are short, and glass cuts light further. Seedlings stretch toward the only light they can find, growing tall, pale and weak. A simple grow light fixed 2–3 inches above the leaves, on for 14–16 hours a day, prevents this far better than a warm room does; raise it as the plants grow so the distance stays the same. Rotate trays every day if you only have a window, and keep the room cool rather than warm — warmth without light is exactly what makes seedlings leggy. If your seedlings are already leggy, move the light closer, add a gentle fan for a few hours a day to strengthen stems, and bury tomato stems deeper when you transplant them, since tomatoes grow roots along a buried stem.
Hardening off, step by step
Seedlings raised indoors will shock and stall if they go straight outside, so harden them off over a week. Start with a few hours in a sheltered, shady spot, bring them in at night, and increase the time and sun each day until they spend a full day and night outside. Skip this step and even healthy seedlings can wilt, sunburn or stop growing for weeks. Transplant on a cloudy day or in the evening, water the hole before you plant, and water again after — never transplant in midday sun. Keep frost protection ready even after the last frost date, because averages are not guarantees and one cold night can undo a month of work.
Direct sow vs start indoors: which crops hate transplanting
Not every crop belongs in a tray. Root crops — carrots, parsnips, radishes — must be direct-sown, because transplanting damages the root you are trying to grow. Peas and beans germinate fast outdoors and resent disturbance, so direct sow them too. Cucumbers, squash and melons tolerate transplanting only if you sow them late (2–4 weeks indoors) and handle the root ball gently, ideally in a pot that goes into the ground whole. The crops that repay indoor starting are the slow, tender ones: tomatoes, peppers, aubergines, brassicas and onions, which need weeks of protected growth to produce in a short season.
Common seed starting problems and fixes
Seeds that never germinate are usually too cold, too dry, or too old — check the packet date, keep the tray moist (not wet), and use a heat mat for peppers, which need 80–90°F to wake up. Seedlings that fall over at the base have damping off, a fungal disease that thrives in wet, airless conditions: use fresh seed-starting mix, do not overwater, give airflow, and thin crowded seedlings so air can move. Pale yellow leaves usually mean too little light or too much water, not a lack of fertilizer — fix light and water before feeding anything. And seedlings that stop growing after transplanting are usually cold-shocked or root-bound: check night temperatures, and transplant before roots circle the pot.
Frequently asked questions
Why are my seedlings leggy?
Almost always not enough light, or too much warmth. Move the light closer — 2–3 inches above the leaves — and keep the room cool rather than warm.
Can I start seeds earlier under grow lights?
Lights help with light, not with temperature or space. Seedlings started too early still become root-bound before transplanting, even under lights.
How many seeds should I sow?
Sow more than you need and thin to the strongest. Work out the number with the seed quantity calculator, which accounts for germination rate.
What is the last frost date and how do I find mine?
It is the average date of the last spring frost in your area, published by your local extension service or a nearby weather station from long-term records. It is an average, not a guarantee — in a cold year frost can come later, so keep protection ready.
Should I use a heat mat for all seeds?
No — only for heat lovers like peppers, chillies and aubergines, which germinate best at 80–90°F. Most other seeds germinate fine at room temperature, and a mat left on too long can dry the tray out.
Related calculators: Seed Starting Calculator · Seed Quantity Calculator · Plant Spacing Calculator · All calculators →
Related guides: When to Plant Tomatoes · Succession Planting · All guides →
Last reviewed: 5 November 2026 · Sources: local extension guidance for regional advice.