Fresh lettuce can be grown year-round in greenhouses. It grows well in cooler weather but may develop spindly growth in higher temperatures. The time from planting to harvesting ranges from 10 to 15 weeks, depending on the variety of lettuce you grow. There are different varieties of lettuce for each growing season. Winter varieties are the slowest to grow but are hardy enough to sustain a light frost. Summer varieties may need ventilation to keep leaf temperatures cool.
Plant lettuce seeds in seed trays. Sow the seeds 1.25 cm (1/2 inch) deep, provide sun or artificial light and maintain a cool temperature, around 15.5 degrees C (60F). Lettuce grows better in cooler temperatures and may not germinate at 27 degrees C (80F) or above.
Prepare the greenhouse's soil for the seedlings. Lettuce needs nutrient-rich soil that drains well while retaining moisture. Add compost and fibrous material to your soil, if necessary.
Transplant the seedlings into the prepared soil once the seedlings are four weeks old. The seedlings should be spaced at least 15 cm (6 inches) apart.
Use a soaker hose to water the newly planted seedlings. Soaker hoses keep the moisture below the surface of the soil and deter moisture-seeking pests in greenhouses.
Feed the lettuce each week with a nitrogen supplement. Nitrogen supplements are available at garden centres. Follow the label's directions for proper dilution ratios.
Provide ventilation for lettuce plants that receive southern sun exposure in the greenhouse. Strong sun exposure can cause temperatures to rise and cause leaf scorch.
Plant the correct varieties of lettuce during each season. Get advice from your local garden centre or the Royal Horticultural Society.