How Deep Should I Sow Seeds Indoors?

Sowing depth is one of the most underrated factors in germination success. Sow too deep and the seedling exhausts its energy reserves before it reaches light. Sow too shallow and the seed dries out before the root can anchor. Getting the depth right for each crop is straightforward once you know the underlying rule.

The Basic Rule of Thumb

Sow seeds to a depth roughly twice their own diameter. A large seed like a bean or courgette might be 1 cm in diameter, so sow it 2 cm deep. A tomato seed is about 2–3 mm across, so 4–6 mm depth is right. A tiny seed like celery or begonia is barely 0.5 mm — these are sown on the surface and not covered at all, or just pressed gently into the compost so they make good contact.

Seeds That Need Light to Germinate

Some seeds are photosensitive — they will not germinate unless light reaches them. These must be sown on the surface or covered with only the thinnest dusting of vermiculite:

• Celery and celeriac
• Lettuce
• Petunias and antirrhinums
• Begonias
• Lobelia

After placing these seeds on the surface, press them gently with a flat board or the back of a spoon so they make firm contact with the moist compost. Then place the tray under glass or a clear lid to retain humidity without burying the seeds.

Common Depths for Vegetable Seeds

• Tomatoes, peppers, aubergines: 5–6 mm
• Courgettes, cucumbers, squash: 1–2 cm (sow on edge to reduce rot risk)
• Beans and peas (if starting indoors): 2–3 cm
• Brassicas (broccoli, cabbage, kale): 5–8 mm
• Onions and leeks: 1 cm
• Celery, celeriac: surface only

How to Achieve Consistent Depth

Fill your tray or module to within 1 cm of the rim, firm the surface gently, then use a dibber, pencil, or the end of a cane to create holes at the right depth. Drop seeds in and cover with more compost or vermiculite. For very fine seeds, mix them with a little dry silver sand before sowing — the sand helps you see where you have sown and prevents clumping.

After Sowing: Firm and Label

Always firm the compost lightly after covering to eliminate air gaps around the seed — roots cannot bridge air pockets. Then water gently from below (sit the tray in a shallow dish of water for ten minutes) rather than from above, which can dislodge tiny seeds. Label every tray immediately; many seedlings look identical until they are several centimetres tall.