Lettuce is a popular, fast-growing leafy green that responds exceptionally well to the controlled conditions of a greenhouse environment. This method allows growers to extend the natural season for this cool-weather crop, enabling year-round production regardless of external weather fluctuations. A greenhouse provides protection from severe weather and manages pest pressure, allowing for precise control over the temperature and light that dictate lettuce quality. This control mitigates primary outdoor challenges, such as premature bolting, ensuring a consistent, high-quality harvest.
Selecting the System and Cultivar
The decision to grow lettuce in a greenhouse begins with selecting the appropriate cultivar and growing system. While outdoor varieties are bred for seasonal resistance, greenhouse cultivation focuses on heat tolerance and slow-bolting characteristics. Loose-leaf and butterhead types, such as ‘Jericho’ Romaine or ‘Muir’ Batavian, are preferred because they resist bolting and tip-burn better than crisphead varieties.
Growers generally choose between traditional soil or container culture and a hydroponic setup. Container growing in a peat-based medium is a simpler method suitable for smaller operations or beginners. Hydroponic systems, like Nutrient Film Technique (NFT) or Deep Water Culture (DWC), offer a higher yield potential and faster growth cycle by delivering nutrients directly to the roots. Although hydroponics requires more technical management, it eliminates soil-borne diseases and optimizes resource use, making it the choice for commercial-scale operations.
Mastering the Greenhouse Climate
Temperature management is crucial for preventing bolting, the physiological stress where the lettuce plant prematurely sends up a flower stalk and turns bitter. The ideal temperature range for vegetative growth is 60 to 68 degrees Fahrenheit (15–20°C) during the day and 45 to 55 degrees Fahrenheit (7–13°C) at night. Allowing daytime temperatures to exceed 75°F or nighttime temperatures above 60°F can trigger bolting.
Light is measured by the Daily Light Integral (DLI), which quantifies the total amount of photosynthetically active radiation received per day. Lettuce requires a DLI between 12 and 17 mol/m²/day for optimal growth, often necessitating supplemental lighting, especially during winter. The photoperiod, or light duration, should be maintained between 12 and 16 hours to support continuous vegetative growth.
Airflow and humidity control are necessary for preventing fungal diseases like downy mildew. Leafy greens thrive best when relative humidity (RH) is maintained within the 50 to 70 percent range. High humidity, especially above 95 percent RH combined with moderate temperatures (64-77°F), favors spore germination. Horizontal Airflow (HAF) fans should run continuously to circulate air, break up humid microclimates, and promote transpiration. Ventilation, either natural or mechanical, exchanges humid internal air with drier external air, lowering the overall RH.
Watering and Nutrient Management
The delivery of water and fertilizer is specific to the chosen growing system. For lettuce grown in containers or soil, the shallow root system requires consistent moisture. Watering frequency is determined by monitoring the growing medium, applying water when the top inch of soil feels dry.
Nutrient delivery in containers can be managed by incorporating a slow-release granular fertilizer (e.g., 8-4-4 NPK ratio) into the medium before planting. This initial feed can be supplemented every 10 to 14 days with a liquid feed having a higher nitrogen content (e.g., 20-10-10 or 16-6-4 NPK ratio) to promote leaf development. A target Electrical Conductivity (EC) of approximately 2.0 dS/m is optimal for container-grown lettuce.
Hydroponic systems require a balanced, water-soluble nutrient solution calibrated for the vegetative stage. The solution’s pH must be maintained between 5.5 and 6.5 to ensure all micronutrients remain available for plant uptake. The Electrical Conductivity (EC) is monitored to measure the concentration of dissolved nutrient salts, with a target range of 0.8 to 1.8 mS/cm for lettuce.
Maintenance and Continuous Harvesting
Routine monitoring is necessary to maintain a healthy greenhouse environment and prevent pest and disease outbreaks. Common greenhouse pests, including aphids and thrips, can rapidly proliferate. Non-chemical management relies on cultural practices, such as maintaining a clean, weed-free environment, and the introduction of biological controls like ladybugs or lacewings to predate on aphid colonies.
For downy mildew, management focuses on preventing the high-humidity conditions that favor its development by ensuring constant air movement and avoiding watering late in the day. Proper sanitation and the prompt removal of any diseased plant material also reduce the spread of pathogens.
The controlled environment allows for “cut-and-come-again” harvesting. With loose-leaf and butterhead varieties, only the mature outer leaves are removed, allowing the central growing point to continue producing foliage. This method extends the harvest period, providing a continuous supply of fresh leaves. For head lettuce, the entire head is harvested once it reaches marketable size, and a new seedling is immediately transplanted to maintain continuous production.