How to Grow Cos Lettuce (Romaine)

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Cos Lettuce (Romaine)

Lactuca sativa var. longifolia

vegetable

Romaine lettuce is a crisp, upright-growing leafy green characterized by elongated leaves arranged in a tight head with a firm, pale inner core. It's prized for its sweet, mild flavor and crunchy texture, making it ideal for salads and cooking. This cool-season crop is more heat-tolerant than butterhead lettuce and stores better than other lettuce varieties.

Looking for a specific variety?

Parris Island Cos Jericho Valmaine Red Romaine Little Gem Freckled Romaine

When should you plant Cos Lettuce (Romaine) in your zone?

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Example Cos Lettuce (Romaine) timeline · Pittsburgh, PA (Zone 6b)

Plant Cos Lettuce (Romaine)
Around Apr 17

Near Pittsburgh's average last spring frost.
Tend & protect
Late spring–summer

Watering, feeding, and pest-watch reminders.
Harvest Cos Lettuce (Romaine)
Around Jun 16

≈ 60 days after planting

Sample dates for Pittsburgh, PA. Sign up to get exact Cos Lettuce (Romaine) dates for your own zip code.

Growing Conditions

☀️

Sun: Full sun to partial shade; 6+ hours of sunlight daily (afternoon shade beneficial in hot climates)

💧

Water: Consistently moist soil; provide 1-1.5 inches of water per week through drip irrigation or soaking. Water at soil level to keep leaves dry and prevent disease. More frequent watering needed during hot weather.

📏

Spacing: 8 inches

📅

Days to maturity: 60-80 days from seed; 30-45 days from transplant

🌱

Planting depth: 0.25 inches (just barely cover seeds); transplants at same depth as original soil line

Soil

Type: Well-draining, fertile loamy soil rich in organic matter

pH: 6.0-6.8

Amendments:

Compost Well-rotted manure Peat moss or coco coir Balanced fertilizer (10-10-10)

Growing Zones

Find your zone →

Zones 3-9; grows best in cool seasons (spring and fall)

2a 2b 3a 3b 4a 4b 5a 5b 6a 6b 7a 7b 8a 8b 9a 9b 10a

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Growth Stages

Seed & Germination

7-10 days

Seeds germinate in 7-10 days at soil temperatures of 60-70°F. Seedlings emerge with two cotyledons (seed leaves).

Keep soil consistently moist but not waterlogged. Maintain cool soil temperature for best germination. Thin seedlings to proper spacing once true leaves emerge.

Seedling & Establishment

2-3 weeks

True leaves develop and plants establish a small root system. Seedlings are tender and vulnerable to pests and drying out.

Protect from harsh sun and wind. Provide bright, indirect light. Maintain consistent moisture. Apply gentle fertilizer at half strength. Thin crowded seedlings.

Vegetative & Leaf Development

3-4 weeks

Plant grows upright with elongated leaves spiraling outward. Inner core begins to pale as the head starts forming. Plant reaches 6-8 inches tall.

Keep soil consistently moist. Apply balanced fertilizer every 2-3 weeks or use slow-release fertilizer. Remove any damaged or yellowing outer leaves. Watch for pest infestations.

Head Formation & Maturity

2-3 weeks

Leaves wrap tightly into a compact, elongated head with a firm, pale yellow-white interior. Plant is 10-12 inches tall. Heart is tender and crisp.

Maintain steady moisture—inconsistent watering causes bitterness and tip burn. Avoid high nitrogen at this stage. Begin harvesting outer leaves or wait to harvest entire head.

Harvest & Post-Harvest

Harvest window 1-2 weeks

Romaine head is fully formed, crisp, and ready for harvest. Can be harvested as individual leaves or as a complete head before bolting.

Harvest in cool morning hours for maximum crispness. Cut heads at soil level with a sharp knife. Store in refrigerator in sealed bags for 2-3 weeks. Watch for signs of flowering/bolting in warm weather.

Companion Planting

Plan your garden →

Plant with:

Radish Carrot Beet Strawberry Chard Parsley Chives Dill Fennel Nasturtium Calendula

Avoid planting near:

Cabbage Celery Fennel (some cultivars) Parsnip

Common Pests

Spray with strong water stream; use insecticidal soap or neem oil; attract predatory insects like ladybugs and lacewings
and
Hand-pick in early morning or evening; use copper barriers; apply diatomaceous earth; introduce parasitic nematodes; beer traps
Plant resistant varieties; control aphid vectors; remove infected plants immediately; use reflective mulch to deter aphids
Ensure good air circulation; avoid overhead watering; apply sulfur or copper fungicide; plant resistant varieties; remove infected leaves
Use cardboard or foil collars around seedlings; apply Bacillus thuringiensis (Bt); hand-pick at night; remove debris where they hide
Use yellow sticky traps; spray with insecticidal soap; apply neem oil; encourage beneficial insects

Uses

🍳

Classic Caesar Salad

Culinary

Romaine's crisp texture and mild, slightly sweet flavor make it the traditional choice for Caesar salads and other fresh preparations. The sturdy leaves also hold up well to cooking, roasting, or grilling. [source]

🍳

Nutritious Green Beverage

Culinary

Fresh romaine lettuce is commonly used in green smoothies and fresh-pressed juices for its high nutrient density and mild flavor that blends well with other ingredients. [source]

💊

Vitamin and Mineral Source

Medicinal

Romaine is rich in vitamins K, C, and A, as well as folate and potassium. It contains antioxidants and has traditionally been used to support bone health and cardiovascular function. [source]

🏠

Biodegradable Garden Waste

Household

Outer leaves and plant scraps from romaine can be composted to create nutrient-rich compost for future gardens, creating a sustainable growing cycle. [source]

🦋

Habitat and Food for Garden Insects

Wildlife

Romaine lettuce plants, especially when allowed to bolt and flower, provide nectar and pollen for pollinators and beneficial insects while also creating shelter for ground-dwelling predatory insects. [source]

This is not medical advice. LizPlants is not a medical resource. Always consult a qualified healthcare professional before using plants medicinally.

Harvest Tips

Harvest in early morning for crispest leaves. For continuous harvest, pick outer leaves starting when plant is 4-5 inches tall, working toward the center—inner leaves will continue growing. For full-head harvest, cut entire plant at soil level with a sharp knife when head feels firm but before flowering begins. Harvest before temperatures consistently exceed 75°F, as heat causes bolting and bitterness. Newly harvested romaine keeps 2-3 weeks in the refrigerator in a sealed plastic bag.

Fun Facts

🌱 Romaine lettuce gets its name from its origins in the Mediterranean region, specifically Rome and the Nile Delta. It was a favorite of Roman emperors.
🌱 The pale, tender heart of romaine (the 'heart of romaine') is the most prized and expensive part, commanding premium prices at farmers' markets because it's sweeter and more tender than outer leaves.
🌱 Romaine is more cold-hardy than other lettuce varieties and can sometimes overwinter in mild climates when protected with row covers, providing early spring harvests.

Gardening Terms

New to gardening? Learn about terms used in this guide:

Bolt Full Sun Germination Partial Shade True Leaves

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