How to grow lettuce

A packet of lettuce sown in one go gives you a fortnight of glut followed by a bed of bitter, bolting plants you feel guilty about. It is the single most common vegetable-growing mistake and it has one fix: sow a short row — three or four feet is plenty for most households — every two weeks from early spring until late summer.

The plants themselves cost you nothing to stagger. What staggering buys is a continuous supply from the same small piece of ground, and it removes the pressure to eat a crop faster than you want to.

Our sowing calendar puts the first outdoor sowing about four weeks before your last frost, which for most people is earlier than feels reasonable. Lettuce germinates in cold soil perfectly well; what it will not do is germinate in hot soil, which is why the late-summer sowings are the ones that fail and why they are best started in a shaded corner or indoors.

Lettuce bolts — sends up a flower stem and stops making leaves — in response to heat and lengthening days. As it does, the plant fills with a milky latex, and that latex is what makes a bolted lettuce bitter. The bitterness is not a sign the plant is old; it is a sign the plant has switched jobs.

There is no reversing it. Once the centre begins to elongate and the plant starts to look like it is standing up rather than sitting down, you have a few days at most. Cut it and use it, or pull it.

Everything you can do is preventative. Choose bolt-resistant varieties for summer sowings — seed catalogues label them and the labelling is broadly honest. Give afternoon shade once the weather warms, which can be as simple as sowing on the east side of taller crops. Keep water steady, because a plant that has been checked by drought bolts sooner. And accept the seasonal shape of the crop: in most climates midsummer lettuce is a fight, and the spring and autumn crops are the ones worth planning around.

Solomon puts the maritime Northwest version of this plainly. Spring-sown lettuce will size up beautifully with no irrigation at all if it has room — a foot apart with two feet of elbow room either side — but lettuce cut after mid-June gets bitter without regular heavy watering, and he keeps a dedicated irrigated bed for exactly that reason.

Lettuce is almost entirely leaf and its roots are shallow, so it has very little buffer against a dry spell. A plant that dries out and recovers is checked, and a checked lettuce is tougher, more bitter and closer to bolting than one that grew evenly.

That does not mean drowning it. Steady and moderate beats heavy and occasional, and mulch does more for this crop than for almost anything else in the garden because it holds moisture in exactly the top few inches where the roots are.

The other reason to water carefully: wet leaves that stay wet overnight invite rot, particularly in a dense heading variety where water sits in the heart. Water in the morning, at the base, and give the plants enough room for air to move between them.

Unusually, lettuce is happy either way, and the choice is about what you want rather than what the plant needs.

Direct sowing is less work and gives slightly better plants, because nothing is ever checked. Sow thinly a quarter-inch deep and thin to your final spacing, eating the thinnings, which is the whole point of the exercise.

Transplanting gives you control. You can start plants in modules while the bed is still occupied, set them out at exactly the spacing you want, and skip the thinning entirely. It also dodges the slug problem at the seedling stage, which in a wet spring is not a small thing. Set plants out no deeper than they sat in the module — a lettuce buried to its growing point rots.

Commercially the decision is made for you, and the photograph below shows why: a mechanical planter sets thousands of identical transplants at identical spacing in a fraction of the time. The home-garden version is a trowel and an afternoon, and the principle is the same.

Loose-leaf types can be harvested by taking outer leaves and leaving the centre to keep growing, which turns one plant into weeks of picking. Take no more than a third at a time and always leave the growing point.

The better method for most people, and the one the 1912 manual recommends for spinach for the same reason, is to cut whole plants and let the neighbours take the room. Cut about an inch above the soil with a knife and many varieties will regrow a second, smaller flush.

Heading types — butterhead, crisphead, and the romaine in the photograph at the top of this page — are cut once, whole. Take them as soon as the heart feels firm rather than waiting for a size, because the days between firm and bolting are few in warm weather.

Cut in the morning if you can. Lettuce cut in the cool of the day is crisp and keeps for days; the same plant cut at four in the afternoon is limp before it reaches the kitchen and never fully recovers.

Slugs and snails are the main enemy, and they take seedlings rather than mature plants — which is the argument for transplanting rather than direct sowing in a wet spring. A lettuce that has four true leaves is largely out of danger; one that has just germinated can vanish overnight.

Aphids colonise the undersides of leaves and, worse, get down into the heart of a heading variety where they cannot be washed out. Catch them early, and encourage the hoverflies and ladybirds that eat them rather than reaching for anything.

In wet weather the characteristic failure is rot starting in the heart or at the base — grey mould and bottom rot both thrive where leaves stay wet and air does not move. The fixes are spacing, morning watering and not planting into ground that sits wet.

One physiological problem worth naming because it looks like a disease and is not: tipburn, where leaf margins go brown and papery. It is a calcium delivery problem caused by fast growth and irregular water, not a deficiency in the soil, and even watering is the answer rather than feeding.