What Seeds Should I Sow Indoors?

Not every seed benefits from indoor sowing — some crops resent the disturbance of transplanting and prefer to be sown exactly where they will grow. Knowing which is which saves you time, prevents wasted compost, and dramatically improves germination rates for the crops that really do need a head start.

Crops That Must Be Started Indoors

These crops have a long growing season or need warm soil temperatures that are impossible to guarantee outdoors early in the year. Starting them inside gives them the head start they cannot get any other way:

• Tomatoes — need 8–10 weeks of warm growing before they are ready to plant out
• Peppers and chillies — among the slowest germinators; 10–12 weeks indoors minimum
• Aubergines (eggplants) — similar to peppers; frost-tender and slow
• Celery and celeriac — tiny seeds, need light to germinate, very long season
• Annual bedding plants — petunias, antirrhinums, busy lizzies — all need indoor warmth

Crops That Benefit From Indoor Sowing

These can technically be sown direct but do better when given an indoor start, especially if your growing season is short:

• Courgettes and squash — sow 3–4 weeks before planting out; they grow fast so do not start too early
• Cucumbers — prefer warm compost; indoor sowing gets them ahead of outdoor soil temperatures
• Brassicas (broccoli, cabbage, kale) — easier to protect young plants indoors from slugs and pigeons
• Leeks — traditionally sown in a seed tray then transplanted
• Marigolds, cosmos, zinnias — flower earlier when given a head start

Crops That Prefer Direct Sowing

These crops strongly dislike root disturbance or simply germinate so quickly outdoors that indoor sowing wastes effort:

• Carrots, parsnips, beetroot — taproots mean transplanting almost always fails
• Peas and beans — germinate rapidly in cool soil; transplanting sets them back
• Spinach, rocket, chard — fast germinators best sown direct in rows
• Sweetcorn — tolerates indoor sowing but only 2–3 weeks before planting out

The Rule of Thumb

If a crop needs more than 4 weeks of protected growing before it can go outside, or if it needs temperatures above 18 °C (64 °F) to germinate reliably, it should be started indoors. If it germinates happily in cool soil and grows quickly, sow it direct. When in doubt, check the seed packet — "sow under cover" is a clear instruction.