How to Grow Lettuce from Seed: Light, Soil Warmth and Summer Sowing

Barely cover the seed

Lettuce seed germinates better in light. Bury it and the light does not reach it, so the seedlings come up late and patchy, or they never come at all. Sift a little fine soil over it, just enough that the seed is faintly hidden.

Do not leave it bare, though. Uncovered seed dries between waterings and rolls out of place, so the spacing ends up a mess. An eighth of an inch (a few millimeters) is the target, and the effort goes into keeping the surface damp rather than into the covering.

1. Level the drill An uneven drill bottom lets seed fall into hollows where the covering is thicker. Run a fingertip or a board edge along it first and the seedlings come up level.
2. Sift the covering Lumpy soil puts heavy particles on the seed that a lettuce shoot cannot lift. Sift compost or vermiculite over it thinly.
3. Firm it lightly Press the surface with your palm so the seed is in contact with the soil. Without firming, the first watering floats the seed up and it rolls out and dries.

Not letting the surface dry before germination matters more than anything else. Cover with newspaper or row cover and take it off the day the first shoots show.

Above 77F the seed goes dormant

Lettuce germinates best at 59 to 68F (15 to 20C). Above 77F (25C) the seed enters dormancy and above 86F (30C) it barely germinates at all. There is a lower limit too: below about 39F (4C) nothing moves. Seed that fails in high summer is not old, it is too warm.

Soil temperature lags behind air temperature. Sowing in the shade in the evening, keeping the tray somewhere cool indoors, or shading the bed will buy you a few degrees. In the hottest weeks that is not enough, and you chill the seed itself.

Chilling breaks summer dormancy

For a late summer sowing, soak the seed, then chill it in the fridge to break dormancy before you sow. That one step is the difference between about half the seed germinating and about nine tenths of it, from the same packet.

1. Soak for half a day Stand the seed in a glass of water until it has taken up water fully. Chilling dry seed does not break the dormancy: it has to be hydrated.
2. Drain and wrap Drain in a sieve, wrap in damp kitchen paper and put it in a bag. Left submerged it runs out of oxygen and starts to smell of fermentation.
3. Chill for two to seven days Use the main body of the fridge rather than the salad drawer, which is warmer. As soon as a white root tip shows, stop and sow.
4. Sow in the cool of the evening Chitted seed is vulnerable to drying, so sow in the evening and water well. Shade the bed afterwards to slow the soil warming.

If the root grows too long it snaps as you sow and you lose the seedling. Stop while the root is a millimeter or two.

Sow in modules or direct, depending on the type

Heading lettuce needs to be transplanted at an accurate spacing, so raise it in modules. Loose leaf types can be thinned and eaten as you go, so direct sowing suits them. Whether you want a head changes which method makes sense.

Working out what went wrong in the seedlings

When raising goes wrong, narrow it to one thing. Patchy germination, stems that stretch, pale leaves, and seedlings collapsing at the base cover almost all of it, and each has its own cause and its own fix.

Get them into full light the day they emerge. Left in the dark the stem stretches and flops, and a stretched lettuce seedling never recovers, so it will not head up however well you treat it afterwards.

Water in the morning and let them go into the night on the dry side. Compost that stays wet overnight gives you soft growth and shallow roots. Stand the tray on a bench with air moving under it rather than on the ground.

1. Thin at the first true leaf Pull anything uneven or misshapen so you have one plant per cell. Crowded, they compete for light and the roots tangle and tear at planting.
2. Feed weak and often When the color pales because the compost has run out, give a liquid feed diluted below label strength every few days. One strong feed makes soft, stretched plants.
3. Harden them off Three days before planting, stand them outside in the wind and sun and water a little less. Planted straight from shelter they check and stop.

Plant at four or five true leaves, and plant shallow

Modules take about three weeks and pots five to six, and four or five true leaves is the point to plant. Beyond that the plant is pot bound: the outer leaves never get big and the head ends up small and tight.

1. Do not break the rootball Lettuce roots spread shallowly and are slow to recover if torn. Slide the plant out and set it in the hole as it is, without shaking soil off.
2. Do not bury the crown A growing point under soil rots and the head forms unevenly. Set the shoulder of the rootball level with the surface or very slightly above.
3. Water at the base Either fill the hole with water and let it drain before planting, or pour gently at the base straight afterwards. Watering over the top does not wet the middle of the rootball.

You cannot undo an overgrown plant. Anything past six true leaves should go in straight away, on the understanding that the head will be small.

Spacing and container size

Aim for a slightly acid to neutral soil, about pH 6.0 to 6.5. Strongly acid soil slows growth, and too much lime scorches the leaf edges. Lime two weeks before planting and work in compost and a base dressing a week before.

The roots are shallow, so eight inches (20 cm) of depth is enough. What runs short is water, so check the surface daily.