How deep to sow seeds: about twice their width, with a few exceptions
Short answer
Sow most seeds at a depth of about two to three times their width. Tiny seeds like lettuce and tomatoes go about 0.5 cm (¼ in) deep, carrots and onions 1 cm (½ in), and big seeds like peas and beans 4 to 5 cm (1½ to 2 in). A few, such as celery and celeriac, need light and go on the surface.
Sow most seeds at a depth of about two to three times their width. That one rule covers almost everything: tiny lettuce and tomato seed goes just under the surface, carrot and onion seed about 1 cm (½ in) down, and big seeds like peas and beans 4 to 5 cm (1½ to 2 in) deep.
The most common mistake is sowing small seeds too deep. A seed only holds enough energy to reach the surface from a certain depth. Bury it deeper and it never comes up.
A depth chart for common crops
| Depth | Crops |
|---|---|
| Surface, lightly pressed in | Celery, celeriac |
| About 0.5 cm (¼ in) | Lettuce, tomatoes, peppers, chillies, aubergines, basil, dill, French marigolds |
| About 1 cm (½ in) | Carrots, parsnips, onions, leeks, spring onions, radishes, rocket, parsley, coriander, calendula |
| About 1.5 cm (½ to ¾ in) | Kale, cabbage, broccoli, sweet peas |
| About 2 cm (¾ in) | Spinach, chard, cucumbers, nasturtiums |
| About 2.5 cm (1 in) | Beetroot, sweetcorn, courgettes, squash, pumpkins, sunflowers |
| About 4 cm (1½ in) | Peas, French beans |
| About 5 cm (2 in) | Broad beans, runner beans |
When in doubt, check the seed packet. It almost always gives a depth, and it knows the variety.
Seeds that need light
Some seeds germinate better with light reaching them, so they’re barely covered or not covered at all. Celery and celeriac are the main ones in the vegetable garden. Press them onto the surface of moist compost and cover the pot with clear plastic to keep the surface damp.
Lettuce also germinates better with some light, which is one reason to cover it with a thin layer of vermiculite rather than compost.
Sowing at the right depth
In a drill outside:
- Rake the soil fine and level.
- Lay a cane or the edge of a rake along the row and press it in to make a straight groove at the right depth.
- Water the drill before sowing if the soil is dry.
- Sow the seed along the drill, then rake the soil back over it and firm gently with the back of the rake.
In pots and modules:
- Fill with seed compost and firm lightly.
- For tiny seeds, sprinkle them on the surface and cover with a fine layer of sieved compost or vermiculite.
- For larger seeds, push them in with a fingertip or a pencil to the right depth.
- Water from below by standing the tray in water, so the seeds don’t wash about.
Adjusting for your soil
- Light, sandy soil dries out fast. Sow a little deeper, especially in summer.
- Heavy clay can crust over. Sow a little shallower and cover the seed with compost rather than clay.
- Hot, dry weather dries the surface. Water the drill first and shade it with a board or fleece until seedlings appear.
For seeds sown in trays to plant out later, see hardening off seedlings, and for raising plants indoors, starting seeds indoors.
Questions people also ask
What happens if you sow seeds too deep?
Small seeds run out of stored energy before they reach the light, so they never appear. That's the most common reason for a gappy row of carrots or lettuce.
What happens if you sow seeds too shallow?
They can dry out, wash away or get eaten by birds. Big seeds near the surface may also push themselves out of the soil as their roots grow.
Should I cover seeds with compost or vermiculite?
Either works. Vermiculite is light, holds moisture and lets light through, which suits small seeds and those that like some light, such as lettuce and basil.
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