Having a passive solar greenhouse — browse no electric heating required — you can develop head lettuce (Lactuca sativa L.) through the winter in frost-prone areas. In some frost-free areas, even a greenhouse is unneeded, but with the additional protection offers peace of mind through a cold spell. During warm spring, summer and fall months, it is too hot to develop this cool-season crop in a greenhouse.
Soil Amendments and Sun
Lettuce needs full sunlight, so put the greenhouse in a spot that gets at least six hours of direct sunlight every day. Fill the bed with loamy topsoil leaving 2 inches of space at the top, then add 1 to 2 inches of compost to the bed. Mix the compost thoroughly with the ground down 6 to 8 inches using a garden fork. Insert a balanced fertilizer — like a 10-10-10 formula — to the bed and blend it into the soil. Use 12 tablespoons for an 8-square-foot-bed.
Spacing, Thinning and Watering
Space head lettuce 12 inches apart. If you’re growing from seed, thin the seedlings when they get 3 to 4 inches tall to your 12-inch-spacing. You can use the thinned lettuce seedlings as new greens. Stagger head lettuce for greenhouse planting rather than creating proper rows to get more plants in the restricted space. Water the bed once per week or when the ground dries our 1 to 2 ins. If the greenhouse gets hot, the soil will dry out rapidly, so check daily in warm weather.
Keep the greenhouse between 50 and 70 degrees Fahrenheit during the afternoon. If it gets warm out, then open the doors so the air inside can cool off. Ideal night temperatures for lettuce are between 45 F and 55 F. Lettuce can survive brief cold spells at 35 F, especially when shielded in a greenhouse. A few days at 90 F will pressure lettuce but will not likely kill it. Keep the soil moist and circulate as much cool air as you can during warm spells.
Three weeks to a month into the growing season, give each mind lettuce plant a boost using 1 tablespoon of balanced 10-10-10 fertilizer. Scatter the fertilizer on the soil around every lettuce plant, then water until the ground is damp 5 to 6 inches deep. To avoid overwatering, schedule the mid-season fertilizer with one regular weekly watering. Weeds are rarely a problem in a greenhouse, but if any dip in, pull them manually. Lettuce has shallow roots, so weed carefully to avoid disturbing them.