You can grow lettuce in a greenhouse when daytime temperatures do not exceed 70 degrees F and nighttime temperatures remain above 45 degrees (with some short-term fluctuations to 35 degrees). This can be accomplished during certain parts of the year in most parts of the U.S., but most locations will have temperatures out of the optimal growing range for part of the year. Artificial methods---such as heating or air coolers---will be needed to grow lettuce in a greenhouse year-round.
Choose lettuce plants that are adaptable to greenhouse production. Leaf or bibb lettuce plants grow best in greenhouses. Some examples of leaf and bibb lettuces are Grand Rapids, Ruby and Salina.
Seeds can be started indoors, in trays, or directly in the ground. If starting in containers, plant seeds in potting mix or potting soil. Lightly cover seeds and keep moist. Lettuce seeds should germinate within seven days. If starting seeds directly in the ground, sow seeds 1/4 inch deep and 1/2 inch apart.
Clear a well-drained area in the greenhouse before planting lettuce plants if you are planting plants directly into the ground instead of containers. The area should have the soil loosened to a depth of 4 to 6 inches. Also, spread a 1-inch layer of compost over the planting area along with the recommended amount of granulated organic fertilizer according to the fertilizer label. Mix it into the top 3 inches of soil and rake the planting area until it is smooth.
Plant the lettuce plants in a row 4 inches apart. If you are planting multiple rows, start planting in the row close to the greenhouse wall. Plant the next row 4 inches away from the first row in a checkerboard fashion, or with each new plant located between each plant in the previous row. Continue adding rows, working toward you or the access point until the area is filled.
If you directly seeded the lettuce into the planting area, thin the plants to 4 inches apart when they get their first set of true leaves.
Add water to the lettuce plants and keep them moist, but not wet, during the growing season. If the temperatures are low, you may not need to add water. Excess water on and around the plants can cause them to rot.
Harvest the plants at anytime by cutting the outer leaves first if you would like the plants to produce more leaves over a period of time. Otherwise, pull the plants up by the roots and cut off the root section below the leaves to harvest. You can replace the harvested plant with another lettuce plant if growing conditions are favorable. Most lettuce leaf and bibb lettuce plants are fully grown after 12 weeks, if grown under optimal conditions.