When Should I Start Seeds Indoors?

Timing is the single biggest factor between stocky, vigorous transplants and spindly seedlings that struggle all season. Start too early and plants outgrow their pots before the soil warms outside. Start too late and you lose weeks of growing season. The good news is that once you know your last expected frost date, the schedule writes itself.

Find Your Last Frost Date First

Every sowing schedule is calculated backwards from the average last spring frost date in your area. In the UK this is typically mid-March to mid-April depending on region; in much of the northern US it ranges from late March (zone 8) to late May (zone 4). Check a local weather service or the RHS frost map for your specific postcode or zip code. Write this date down — it becomes the anchor for everything.

Count Back by Crop

Each vegetable needs a specific number of weeks indoors before it is ready to plant out. Here are the most common ranges:

• Tomatoes and aubergines: 8–10 weeks before last frost
• Peppers and chillies: 10–12 weeks before last frost (they are slow)
• Celery and celeriac: 10–12 weeks — among the earliest sown
• Courgettes, squash, cucumbers: only 3–4 weeks (they grow fast and hate root disturbance)
• Brassicas for transplanting: 4–6 weeks
• Annual flowers (cosmos, marigolds, antirrhinums): 6–8 weeks

If your last frost is 1 May and you want to plant tomatoes out then, count back 9 weeks — you should sow around 26 February.

Why Starting Too Early Backfires

Many gardeners sow tomatoes in January because excitement gets the better of them. By May those plants are potbound, yellowing from the bottom up, and have often been stressed by cold windowsills. A tomato sown in late February under decent light will overtake a January-sown plant within a fortnight of planting out. Resist the urge to go early.

Succession Sowing Changes the Equation

Rather than one big sowing, stagger two or three batches two weeks apart. This spreads germination risk — if a batch fails due to cold or damping off, you have a backup — and it extends your harvest window. For lettuce, basil, and flowers this is especially useful.

Write a Simple Sowing Calendar

Take a sheet of paper or a spreadsheet and list every crop you want to grow. Next to each, write your last frost date minus the weeks needed for that crop. That is your target sowing date. Then add a reminder one week earlier so you have time to get compost and trays ready. Stick this calendar somewhere visible — above the potting bench is ideal.

Check seeds as soon as they arrive. Some need a pre-soak or a period of cold stratification before germination, so factor that in. Parsley, for example, germinates faster after a 24-hour soak in warm water.