Romaine lettuce (cos)

Sun

Full sun in cool weather; afternoon shade in warm weather.

Water

Keep the root zone evenly moist, watering when the soil surface begins to dry. Avoid alternating drought with heavy watering.

Soil

Fertile, moisture-retentive but well-drained soil, ideally pH 6.0-7.0.

Height

12-16 in (0.30-0.40 m) before flowering

Spread

10-12 in (0.25-0.30 m)

Foliage

Upright, elongated green leaves with substantial white midribs, forming a loose outer head around a paler heart.

Growth

Fast; full heads usually mature about 65-80 days from sowing, depending on conditions.

Care needed

Medium

• Low allergen
• Edible

Northern hemisphere months. In Australia, shift them by six.

Difficulty

Moderate

Toughness

Needs looking after

Pick your zone, country, light and whether you have pets.

1. Planting day

   0.5 ft (0.15 m)

2. After 1 year

   1.1 ft (0.35 m)

   full size

Full size in one season.

From a typical nursery size, at an average rate for this kind of plant. A rule of thumb, not a forecast.

Cats and dogs

Safe

Growth rate

Fast

• Space plants 10-12 in (0.25-0.30 m) apart so heads have room to develop.
• Mix mature compost into the bed before planting; in poor soil, give a light application of balanced vegetable fertiliser.
• Water at soil level and keep moisture steady while heads develop.
• Apply a thin mulch once seedlings are established, keeping it clear of the crowns.
• Use afternoon shade or shade cloth during warm spells, and sow small batches every two to three weeks during suitable cool weather.

Pruning. No routine pruning is needed. Remove damaged or decaying outer leaves, and pull out plants once they bolt unless you want to save seed.

Pick a few outer leaves once they are large enough to eat, leaving the central growing point intact. For whole heads, harvest when the upright head feels filled out but before the central stem stretches; cut at soil level, preferably in the cool morning. In good conditions, full heads are usually ready about 65-80 days after sowing.

Late winter

In cold climates, sow under cover about four to six weeks before you expect to plant outdoors.

Early spring

Harden off seedlings and plant into workable soil; protect young plants from hard frost.

Spring

Direct-sow short rows every two to three weeks, thin promptly and keep seedlings moist.

Late spring

Harvest developed heads before hot weather triggers bolting.

Summer

In hot districts, pause sowing or start seedlings in a cool, shaded place for an autumn crop.

Autumn

Plant another crop as temperatures cool; use fleece or a cold frame where early frosts threaten.

Winter

In mild, frost-light districts, continue growing outdoors; elsewhere clear spent plants and plan spring sowings.

• Plant in repeated rows in a potager, using the upright heads to give beds a clear pattern.
• Use as a temporary edible edging beside parsley, chives or other compact herbs.
• Grow in raised beds with radishes and carrots, allowing enough space for mature lettuce heads.
• Plant in deep patio containers for accessible leaf picking close to the kitchen.

Garden styles it suits: Potager (kitchen garden), Edible landscape, Homestead, Cottage garden, Small-space and Family-friendly.

Plant it with: Radishes, Carrots, Chives, Parsley, Calendula and Sweet alyssum.

Wildlife: Leaves attract slugs, snails and sometimes rabbits. Plants left to flower provide small yellow flowers visited by insects, though harvesting usually prevents flowering.

• Slugs and snails chew seedlings and leaves: inspect after dark, remove hiding places and use an approved iron-phosphate bait if needed.
• Aphids gather inside leaves and hearts: inspect young growth, rinse off small colonies and encourage hoverflies and ladybirds.
• Downy mildew causes pale patches above and whitish growth below leaves: improve spacing, water at soil level and remove affected plants.
• Heat and drought cause bolting, bitterness or scorched leaf edges: maintain even moisture, provide afternoon shade and grow mainly in cooler seasons.

Cultivated lettuce leaves are not known to be toxic to people, cats, dogs or horses. Wash leaves before eating; spoiled or mouldy lettuce should not be eaten or fed to animals.

• Little Gem lettuce (Lactuca sativa 'Little Gem')

  Another cos-type lettuce, but it forms much smaller, shorter, compact heads; 'Parris Island Cos' develops taller leaves and larger heads.

• Chinese cabbage (Brassica rapa Pekinensis Group)

  Also forms an elongated head, but usually has more crinkled leaves, broader white ribs and a denser, barrel-shaped head.

Lactuca comes from the Latin word for milk, referring to the milky sap released when lettuce stems are cut.

Propagation. Grow from seed, sowing about 1/8 in (3 mm) deep in moist soil or seed compost. Germination can be poor in hot soil, so start warm-season batches somewhere cool. This open-pollinated cultivar can be saved for seed, but crossing with other flowering lettuces is possible.

Accepted botanical name

Lactuca sativa

In the US and Canada

Introduced in the lower 48 states, Pacific Basin, Puerto Rico and US Virgin Islands.

Native range

Iraq.

Measured

Average height 42 cm.

Sources: GBIF (CC BY 4.0), USDA PLANTS and Trefle (CC BY 4.0). The care and design notes are LandscapeGPT's own.