Germination temperature
Best about 16-21 °C (60-70 °F); germination is inhibited from about 25-35 °C (77-95 °F) depending on variety.
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Lettuce germinates in a few days when the seed is kept cool, moist and close to the light, but warm soil can stop it completely. This guide covers sowing, the heat problem that matters most in India, and how to keep plants from bolting.
Best about 16-21 °C (60-70 °F); germination is inhibited from about 25-35 °C (77-95 °F) depending on variety.
On the surface or under no more than 3-6 mm (1/8-1/4 in) of fine soil.
Usually 7-14 days in cool conditions; faster in mild soil, much slower in cold soil.
Light helps germination; do not bury the seed.
None needed; in hot weather start the seed somewhere cool.
Both work. Head types (crisphead, iceberg) are easiest from transplants.
Lettuce seed is small and sown dry. Mix it with a little fine sand if you broadcast it, so that it spreads evenly.
Not needed. The seed coat is thin and takes up water within hours.
Not needed. Lettuce has no cold requirement for germination; the risk is the opposite, seed that is kept too warm.
Not needed. In hot weather, what helps is keeping the seed cool during its first hours of water uptake: in trials with a heat-sensitive variety, seed held moist at 20 °C (68 °F) for 16 hours before exposure to 35 °C (95 °F) still reached 97% germination, while seed placed directly at 35 °C did not germinate.
Aim for 16-21 °C (60-70 °F). Extension sources give an acceptable range up to about 27 °C (80 °F), but depending on variety germination is inhibited somewhere between 25 and 35 °C (77-95 °F), and untreated seed will not germinate in soil above 35 °C (95 °F). Long exposure to heat can push the seed into secondary dormancy (thermodormancy), so it fails to sprout even after the soil cools.
Press the seed onto the surface or cover it with no more than 3-6 mm (1/8-1/4 in) of fine soil or vermiculite. Deep sowing is a common cause of poor stands.
A fine, moist seed compost or a well-prepared seedbed. Lettuce grows best at soil pH 6.0-6.5.
Keep the surface constantly moist until seedlings emerge; a light cover of shade cloth or paper helps in sun, but remove it at emergence.
Lettuce germinates better with light. In research on a heat-sensitive variety, seed in darkness germinated well only below 22.5 °C (72.5 °F), while seed in light still germinated up to 27.5 °C (81.5 °F). This is why shallow sowing matters most in warm weather.
About 7-14 days at 16-21 °C (60-70 °F). In warm field soil seed can start within a day; in cold winter soil it can take a week or more.
Move seedlings when they have several true leaves and a root ball that holds together. Harden them off for a few days first. Space loose-leaf types 15-25 cm (6-10 in) apart and head types about 30 cm (12 in) apart.
Lettuce grows best at about 13-18 °C (55-65 °F). Water lightly and often: roots are shallow, and the first weeks set the size of the plant.
Loose-leaf types (Lollo Bionda, Lollo Rossa, Salad Bowl) can be picked leaf by leaf within weeks and cope best with warm spells. Romaine (cos) forms an upright head. Crisphead or iceberg types (Great Lakes, Iceberg) need the longest cool period and are the first to bolt when it turns hot.
Cause: soil above about 25-28 °C (77-82 °F) causes thermoinhibition or thermodormancy. Fix: sow in the evening, start trays in the coolest shaded place or an air-conditioned room until emergence, sow shallowly in light, and resow when nights are cooler.
Cause: heat, long days and dry soil. Butterhead types turn bitter readily above 24 °C (75 °F). Fix: grow in the cool season, give afternoon shade, water evenly, and pick leaf types early.
Cause: calcium not reaching fast-growing inner leaves, mostly due to drought or uneven watering. Fix: keep soil moisture steady and soil pH around 6.0-6.5.
Cause: slugs and snails, or bottom rot and grey mould in crowded, wet beds. Fix: use iron phosphate bait, thin plants, and water the soil rather than the leaves.
Lettuce is a cool-season crop. In most Indian plains it does best when soil is below about 25 °C (77 °F), which usually means the cooler months; in hill areas it can be grown from spring to autumn.
Sow a short row every 2-3 weeks so you harvest young leaves continuously instead of a whole bed bolting at once.
Steady nitrogen and calcium supply keep leaves tender; one side-dressing with calcium nitrate is a common recommendation.
Pick outer leaves of leaf types when large enough to use. Head lettuce can be ready from about 55 days, depending on variety; cut it while the head is firm and before the centre starts to elongate.
Choose crisphead, romaine or loose-leaf lettuce seeds for your next cool-season sowing.
Most likely the soil was too warm. Depending on variety, lettuce germination is inhibited from about 25-35 °C (77-95 °F), and long exposure to heat can make the seed dormant. Start seed in a cool, shaded spot or wait until nights are cooler.
Only very lightly. Light helps lettuce germinate, especially in warm conditions, so press it onto the surface or cover with no more than 3-6 mm (1/8-1/4 in) of fine soil.
Loose-leaf types such as Lollo Rossa, Lollo Bionda and Salad Bowl, harvested young. Crisphead and iceberg types need the longest cool period and bolt first when it gets hot.