Romaine lettuce
Lactuca sativa var. longifolia
Also known as: Cos lettuce, Roman lettuce, Cos
Quick facts
- Category
- leafy greens
- Difficulty
- beginner
- Days to harvest
- 60 to 80 days
- Harvest type
- single harvest then replant
- Spacing
- 25 cm between plants
Environment
- Temperature
- 13–22°C
- pH
- 5.5 to 6.5
- EC (hydroponic)
- 0.8 to 1.4 mS/cm
- Daily light
- 12 to 17 mol/m²/day
Climate and zones
- USDA zones
- 4 to 9 (winter low around -34°C or warmer)
- Frost tolerance
- tolerates light frost
- Season
- cool (spring and fall crops)
Viable growing environments:
- outdoor year-round (in zone)
- outdoor in growing season (annual)
- unheated greenhouse / hoop house
- heated greenhouse
- indoor (heated home)
- indoor hydroponics under grow lights
USDA zone bounds reflect outdoor year-round survival. Anywhere outside the bounded zone range, this crop still grows as an annual in the warm months (outdoor_seasonal), under cover (greenhouse), or indoors under lights.
Growing systems
Romaine lettuce works in:
- deep water culture (rafts)
- NFT channels
- vertical / aeroponic tower
- media bed (ebb and flow)
- wicking bed
- soil bed
Growing media
The substrate the roots sit in. Choice depends on the system (clay pebbles don't fit NFT channels; rockwool isn't used in media beds) and the crop (romaine lettuce works in the media listed below).
| Medium | pH effect | Water retention | Bacterial surface |
|---|---|---|---|
| Rockwool (Mineral wool) | alkaline until pre-soaked | very high | low |
| Expanded clay pebbles (LECA) | neutral / inert | low | high |
| Coco coir (Coconut coir) | slightly acidic | high | moderate |
| Net pot, no medium (Bare-root) | - | - | - |
| Soil-based mix (Potting soil) | varies by source | high | high |
Bacterial surface area matters for aquaponics: clay pebbles, lava rock, and pumice double as biofilter substrate. Low-surface media (rockwool, perlite, pea gravel) work in hydroponics but need a separate biofilter in aquaponics.
Nutrient demand by stage
NPK ratios are relative weights at each growth stage; the nutrient mix calculator scales them to absolute grams or ml. EC targets shift through the plant's life: seedlings need a much lighter solution than fruiting adults.
| Stage | N | P | K | EC target (mS/cm) |
|---|---|---|---|---|
| seedling | 1 | 1 | 1 | 0.6 |
| vegetative | 3 | 1 | 2 | 1.2 |
Aquaponics suitability
Compatible with typical aquaponics nutrient profiles. Fish waste provides enough nitrogen for healthy growth; supplemental potassium, calcium, and iron may still be needed depending on fish stocking density.
Care notes
A standard hydroponic lettuce type, slightly more challenging than loose-leaf but easier than iceberg. EC 0.8-1.5 mS/cm. pH 5.5-6.5. Temperature: 15–22°C (cool-season; above 24°C, bolting begins and the leaves become bitter). Moderate light (DLI 12-17 mol/m2/day). NFT, DWC, raft, or vertical systems. From seed to full head: 7-9 weeks. Baby romaine (harvested young as individual leaves): 4-5 weeks. The upright head formation requires slightly more space per plant than loose-leaf types. Tipburn on inner leaves is possible in warm, still conditions; improve airflow and maintain calcium levels. For commercial growers, romaine hearts (trimmed, bagged inner portions) are a premium product ($3-5 per head). The food safety advantage of hydroponic romaine (no soil contact, no irrigation water contamination) is a genuine selling point in markets that have experienced recall anxiety from Salinas Valley outbreaks.
Plan a setup with Romaine lettuce
Verified against: rhs-uk, university-of-arizona-ccac. Last reviewed 2026-05-15.