Lettuce

Lactuca sativa

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At a Glance

Lettuce (Lactuca sativa) is a temperate annual leafy vegetable in the family Asteraceae, cultivated for its edible foliage and widely grown in UK gardens, allotments and commercial holdings. It is one of the most commonly grown salad crops in Britain, valued for its fast-maturing habit and tolerance of cool conditions. Modern cultivated forms are derived from wild lettuce (Lactuca serriola) and were first domesticated in the Near East several thousand years ago.

Overview

Lettuce is a quick-growing annual grown primarily for its leaves, although some forms are also harvested for thickened stems (stem lettuce / celtuce). It is suitable for the whole of the UK, including cooler northern regions, and can be cropped almost year-round under cloches, fleece or in unheated polytunnels. As a half-hardy to hardy annual it is sown successionally from late winter through to early autumn, with the main outdoor cropping season running from late spring to mid-autumn.

The RHS notes lettuce as a generally reliable crop for the UK climate, though it is prone to bolting in hot, dry midsummer weather and to a number of fungal diseases in damp conditions. Lettuce is not a toxic plant, but wild lettuce and certain ornamental relatives contain lactucin and related sesquiterpene lactones that can have mild sedative properties; cultivated varieties used in salads are safe to eat, though mature bolted plants become bitter.

Appearance

Lettuce plants form a basal rosette of leaves arising from a short compressed stem. Leaves vary considerably by type: in butterhead and cos forms they are broad, smooth and obovate, forming a loose to fairly tight head; in crisphead (iceberg) types the leaves are crisp, blistered and tightly folded; in loose-leaf and oak-leaf types they are frilled, lobed or deeply cut and do not form a true head. Leaf colour ranges from pale yellow-green to deep green, with red-tinged, bronzed and purple-leaved cultivars common.

Stems are short and inconspicuous until the plant bolts, when a tall flower stalk (typically 60–120 cm) emerges and bears branched inflorescences of small yellow, dandelion-like composite flowers. The seed head is reminiscent of a small dandelion clock, with each "seed" technically being a single-seeded achene attached to a pappus of fine hairs that aids wind dispersal.

Growing Conditions

Lettuce grows best in an open, sunny position, although it will tolerate light dappled shade, particularly in southern England during midsummer. Soils should be fertile, moisture-retentive yet free-draining; very acid or waterlogged ground should be avoided. A pH of around 6.0 to 7.0 is ideal, which suits most UK garden soils. Lighter sandy soils are workable if enriched with well-rotted organic matter and mulched to conserve moisture.

Hardiness is generally described as "hardy" rather than given a specific RHS H-rating: lettuce tolerates light frost (typically down to about −5 °C for short periods when hardened off) and is grown as a cool-season crop. It will bolt (run to flower) in prolonged heat above about 24 °C combined with dry soil, which is the main reason successionally sown crops fail in midsummer. In the UK the main outdoor sowing window is from March (under cloches or fleece) through to August, with main-crop harvest from late May to October. Winter crops of hardy varieties can be grown under cloches or in unheated polytunnels for harvest from October through to the following April.

Planting and Care

Lettuce is raised from seed, either sown direct or in modules for transplanting. Sow thinly 1 cm deep in drills 30 cm apart, or in modules of peat-free multipurpose compost at the same depth. Module-raised plants transplant with minimal root disturbance at the three- to four-true-leaf stage, typically 3–4 weeks after sowing. Space butterheads and cos types 25–30 cm apart, crispheads 30–35 cm, and loose-leaf types 20–25 cm; closer spacing gives smaller, "baby" leaves.

Watering is the most important ongoing task. Lettuce is shallow rooted and responds badly to drying out, which causes bitterness and bolting. Water generously during dry spells, ideally in the morning so that foliage dries before nightfall, and apply a 5 cm organic mulch (garden compost, leaf mould or straw) to conserve moisture. Drip irrigation or seep hoses beneath the mulch are more efficient than overhead watering and reduce disease.

Feeding should be modest: a base dressing of well-rotted compost or manure before sowing is usually sufficient, with a light top-dressing of a balanced organic fertiliser (such as a seaweed or blood, fish and bone) at four-week intervals for overwintered or long-running crops. Over-rich nitrogenous feeding produces soft, sappy growth that is more attractive to slugs and to grey mould (Botrytis cinerea).

Pruning is not required in the conventional sense, but "cut and come again" harvesting of loose-leaf types — picking outer leaves rather than the whole rosette — extends the productive life of each plant. For heading types, harvest the whole plant once a firm head has formed but before the stem starts to elongate; cut at the base in the morning while leaves are crisp.

Propagation is by seed only, as lettuce is an annual. Saving seed is straightforward but plants cross-pollinate readily, so more than one variety flowering at the same time will produce hybrids. For reliable, true-to-type plants most UK growers buy fresh seed each year; commercial seed is widely available from garden centres, seed merchants and the major UK catalogues. Seed remains viable for roughly three to five years when stored cool and dry.

Seasonal care centres on the successional programme. Sow small batches every two to three weeks from March to August to maintain a continuous harvest. In southern England a final September sowing of a hardy variety, such as 'Winter Density', can be overwintered under cloches for spring use. Fleece or cloches also protect early and late sowings from cold wind, pigeons and the worst of the slug damage. In prolonged wet weather, remove the worst-affected outer leaves to limit Botrytis and downy mildew.

Common Problems

Slugs and snails are the single most damaging pest, attacking seedlings and the lower leaves of mature plants. Cultural controls (night-time hand-picking, beer traps, copper collars, encouraging hedgehogs and ground beetles) are usually preferred in the UK over chemical pellets, which are restricted and toxic to wildlife. Lettuce root aphid (Pemphigus bursarius) feeds on the roots in midsummer, causing plants to wilt and stop growing; crop rotation and avoiding planting near poplars (the alternate host) help reduce pressure.

Downy mildew (Bremia lactucae) is the most widespread fungal disease and produces pale yellow patches on the upper leaf surface with white downy growth beneath. It thrives in cool, damp weather and is best managed by improving air circulation, avoiding overhead watering, removing infected leaves promptly, and choosing resistant cultivars — most modern UK-bred varieties carry one or more Dm resistance genes. Lettuce mosaic virus and cucumber mosaic virus cause mottling, distortion and stunting; they are spread mainly by greenfly and are controlled by prompt aphid management and removal of infected plants.

Bolting — premature flowering — is physiological rather than a disease but is the most common cause of unusable crops. It is triggered by stress: high temperatures, drought, transplant shock, or long day length. Successional sowing, steady moisture, and avoiding very early or very late sowings into cold, wet soil all reduce the risk.

Popular Varieties

A wide range of cultivars is sold in the UK. The following are widely available from British seed merchants and allotment seed schemes, and have been grown reliably in UK conditions for many years.

• 'All Year Round' — a traditional compact butterhead, popular with exhibitors and allotmenteers; reliable in most UK districts.
• 'Little Gem' — a small, fast-maturing cos / baby romaine; one of the most widely grown garden lettuces in Britain and a staple in supermarket salad packs.
• 'Lollo Rossa' — an Italian loose-leaf type with deeply frilled, red-tinged leaves; grown for cut-and-come-again harvesting.
• 'Winter Density' — a hardy cos/butterhead cross, sown in autumn for overwintering under cloches and harvesting in early spring.
• 'Webb's Wonderful' — a large, crisp-hearted crisphead dating from the late nineteenth century; long-standing favourite in UK gardens.
• 'Freckles' — a romaine-type with green leaves splashed maroon; valued for flavour and visual interest.

Pests and Diseases

For step-by-step help, read Controlling Aphids Naturally, Dealing with Slugs and Snails and Treating Powdery Mildew. Or browse the full plant problem solver to diagnose an issue by symptom.

Lettuce in our guides

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• Charles Dowding: Why Mid-June Is the UK's Busiest Sowing Fortnight