Fertilising guide

How to fertilise Romaine Lettuce (Lactuca sativa var. longifolia)— schedule & NPK

Also called Romaine Lettuce, Cos Lettuce, Roman Lettuce.

More about romaine lettuce

About Romaine Lettuce

Lactuca sativa var. longifolia · also called Romaine Lettuce, Cos Lettuce · edible

Romaine (cos) lettuce forms tall, upright heads of long, crisp, dark-green leaves with a prominent white midrib and robust, slightly bitter flavour. More heat-tolerant and bolt-resistant than butterhead types; the standard lettuce for Caesar salads. Matures in 60–75 days. Popular varieties include 'Little Gem', 'Parris Island Cos', and 'Fordhook'. Suitable for containers and cut-and-come-again harvesting.

Growth habit: Tall, upright rosette forming an elongated, cylindrical head; leaves long, stiff, and cupped with prominent white midribs

Watch for — Lettuce mosaic virus (LMV): Causes mottled, distorted, or stunted leaves spread by aphids. Use resistant varieties; control aphid populations with insecticidal soap; remove and destroy infected plants. Buy certified virus-free seed where available.

What fertiliser romaine lettuce actually wants — and why

Romaine Lettuce is grown entirely for its leaves, so nitrogen is the priority — steady, nitrogen-leaning feeding keeps it growing fast, tender and unbolted.

For the language behind the three numbers on the bottle — what nitrogen, phosphorus and potassium each do — see the NPK ratio explained entry. The short version for romaine lettuce: match the feed to the job the plant is doing right now, not to a generic “plant food” on the shelf.

How often to feed romaine lettuce, and which months

Feeding only earns its keep while the plant is in active growth and can use the nutrients — pour feed into a dormant or low-light plant and it simply builds up as root-burning salt. For romaine lettuce:

The dormant-season rule matters more than the exact interval: skip feeding entirely when romaine lettuce is resting. For the wider context on indoor feeding rhythms across the seasons, the houseplant fertiliser schedule walks through the year month by month.

What strength to mix for romaine lettuce

Use the vegetable-feed label rate for romaine lettuce. Steady availability matters more than a strong dose — a check in growth makes leaves tough and can trigger bolting.

Feeding always goes onto already-damp soil, never dry roots — water romaine lettuce first if the soil is dry, then apply the diluted feed. The companion question is when to water at all, covered in the romaine lettuce watering schedule.

Signs you are over-feeding romaine lettuce

Over-feeding is far more common — and more damaging — than under-feeding for most plants. The classic tells for romaine lettuce:

Very soft, floppy, dark-green growth that attracts aphids.
Excess leafy growth at the expense of hearts/heads in cabbage and the like.
Salt crust and scorched leaf edges in containers; nitrate-heavy leaves.

Signs you are under-feeding romaine lettuce

Pale, yellow-green leaves, oldest first, and slow growth.
Small, tough, bitter leaves and premature bolting.
Weak, stunted heads in cabbage, broccoli and cauliflower.

If the symptoms point at watering, light or roots rather than nutrition, the full romaine lettuce care brief covers soil, humidity and the common problems for this species.

Flushing and leaching the salts

For container-grown romaine lettuce, water until it drains freely each time and flush pots monthly with plain water to stop nitrogen salts accumulating; in the ground, good compost levels naturally buffer this.

Organic vs synthetic feeds for romaine lettuce

Organic options

Well-rotted manure or compost dug in, plus nitrogen-rich liquid feeds like diluted chicken-manure pellets or nettle feed. UK: pelleted chicken manure or Westland; US: Espoma Garden-tone or blood meal. Steady and soil-building.

Synthetic / liquid feeds

A balanced feed at planting then a high-nitrogen liquid or granular side-dress — UK: Growmore then a nitrogen feed or Phostrogen; US: a 10-10-10 then a high-N (e.g. 21-0-0) side-dress or Miracle-Gro.

Brand names are examples, not endorsements, and UK and US ranges differ — check the label’s own NPK and dilution rate, since formulations change.

Fertilising romaine lettuce — frequently asked questions

What fertiliser does romaine lettuce need?

A nitrogen-leaning feed (higher first number) or compost-rich soil — nitrogen drives the fast, tender leafy growth this crop is grown for. Phosphorus and potassium matter far less here than for fruiting crops. Romaine Lettuce is grown entirely for its leaves, so nitrogen is the priority — steady, nitrogen-leaning feeding keeps it growing fast, tender and unbolted.

How often should I feed romaine lettuce?

Apply a balanced liquid fertiliser every 2–3 weeks during active growth. Nitrogen supports the tall, leafy structure but excessive feeding in warm conditions promotes bolting. A pre-planting compost incorporation is generally sufficient for garden beds; container crops need regular feeding throughout. Apply a balanced liquid fertiliser every 2–3 weeks during active growth. Nitrogen supports the tall, leafy structure but excessive feeding in warm conditions promotes bolting. A pre-planting compost incorporation is generally sufficient for garden beds; container crops need regular feeding throughout. In practice: a balanced or compost-rich start, then a nitrogen side-dress or liquid feed every 3-4 weeks through the cropping period in the main season (spring through early autumn).

What strength of feed for romaine lettuce?