Lettuce grows best in cool weather and sunny locations. Spring, mid-summer, and early fall are the times of year to plant lettuce, but you can grow lettuce in the summer even in warm regions if you choose heat-tolerant and bolt-resistant varieties.
There are lettuce cultivars that are ready for picking in 45 days and others that take twice as long. Lettuces fall on a continuum from leaf lettuces that do not form heads to loose-heading butterheads to tight-headed crispheads.
Growing lettuce is not difficult; here are some easy-growing tips:
Site. Grow lettuce in full sun in cool conditions. A site that receives four to six hours of direct sun each day should work. Lettuce requires afternoon shade in warm regions or hot weather.
Temperature. Lettuce does best in cool weather, temperatures between 60° and 65°F. To grow lettuce where temperatures are higher (especially 80°F or greater), make sure your planting bed gets afternoon shade.
Warm-weather growing. Plant heat-resistant lettuce cultivars where the weather is warm. Shade and water will help prevent bolting.
Seed starting. The optimal temperature to germinate lettuce seed is 65°F. To germinate lettuce seed in warmer temperatures, sow seed in a shallow trench on a cool morning, water and then cover the row with a board to protect seeds from sun. Lift the board every morning to check for germination; remove when seedling start to break the ground. Where soil is very warm you can pre-sprout seeds to get better germination or start seed on a cool garage or garden shed floor.
Soil. Lettuce will grow in average soil but sandy loam is optimal. Prepare the garden bed by tilling or digging to 4 inches deep, clear away large clumps of soil or organic matter, and work in an inch or more of compost. Plant lettuce in raised beds where the soil is very heavy or slow to drain.
Transplanting and spacing. Thin or transplant lettuce seedlings from 7 to 8 inches apart for butterhead varieties, 12 to 15 inches apart for looseleaf cultivars, and 12 to 16 inches apart for romaine and crisphead varieties. Protect seedlings from cold with floating row covers or in warm weather with shade cloth set over a frame.
Water. Lettuce requires moist but not soggy soil. Make sure the crop gets at least 1 inch of water a week; that is slightly more than ½ gallon of water for each plant. Too much water can wash nutrients from the soil and cause lettuce leaves to yellow; add a 15-10-10 fertilizer to green up plants.
Disease prevention. To help prevent fungal diseases water lettuce in the morning, especially in cool weather. Water early to ensure moisture evaporates from leaves during the course of the day and are dry by evening.
Mulch. Once plants are 4 to 6 inches tall, apply a 2-inch mulch of aged-compost around plants to conserve soil moisture, suffocate weeds, and keep leaves clean. Be sure the mulch is pulled an inch or two back from the crown of each plant. Mulching near the crown can cause the plant to rot.
Feeding. Lettuce and other leafy crops need plenty of nitrogen. Feed lettuce with manure tea or fish emulsion once or twice during the growing season to ensure quick and steady growth.
Bolting. Bolting (going to seed) will leave lettuce bitter tasting. When lettuce starts to elongate, bolting is just a day or two away. To keep this from happening, pinch off the top center of the plant or wound the roots slightly with a sharp spade by piercing the soil and roots from an angle. This will slow leaf growth and bolting.
Harvest. Lettuce is the crispest if picked in the morning. Looseleaf varieties can be harvested cut-and-come again trimming leaves from the outside first. Heading types should be just firm at harvest; press each head with the back of your hand to test for firmness. Use a sharp knife to cut heads below the lowest leaves, or pull plants out by the roots.
Succession planting. Make successive plantings every three weeks to ensure a constant supply of fresh lettuce.
Varieties to plant. Loosehead varieties called butterhhead or Boston include ‘Buttercrunch,’ ‘Bibb,’ and ‘Gem’; these will mature in 60 to 75 days. Crisphead varieties which form cabbage-like heads include ‘Iceberg,’ ‘Great Lakes,’ and ‘Ithaca’ which mature in about 75 days. Looseleaf varieties which are suited for cut-and-come again harvests include ‘Oakleaf,’ ‘Salad Bowl,’ and ‘Red Ruby’ which mature in 40 to 60 days. Romaine or cos varieties–with long narrow leaves and heads–include ‘Rosalita,’ ‘Apollo,’ and ‘Ballon’ which mature in 75 or more days.
More tips at How to Grow Lettuce.