Harvest all types of lettuce almost any time during growth.
• Harvest loose-headed leaf lettuce when the leaves are large enough to use—just 2 to 3 inches (5-7 cm) long.
• Harvest Bibb and Butterhead types of lettuce—with large ruffly outer leaves surrounding a soft, folded heart—when the leaves begin to cup inward to form a loose head or wait until they form a rosette at full size—6 to 8 inches (15-20 cm) across.
• Harvest Romaine lettuce, also called Cos lettuce, when the leaves have elongated, formed midribs, and overlapped to form a fairly tight head–about 6 to 8 inches tall.
• Harvest Crisphead lettuce—such as Iceberg with a solid, tight, spherical head–when heads are medium-size (6 to 8 inches/15-20 cm), round, and firm.
Pick all lettuce before a seed stalk forms.
If you don’t want to pull or cut the whole plant at once then snip or break away older leaves—whatever size—from the outside allowing younger leaves at the center (called the central bud) to continue growing for later use. This type of harvest is called “cut-and-come-again.”
Young leaves will be the tastiest and most nutritious. Baby leaves are best for tender salads.
Lettuce grows best and tastes sweetest when harvested in cool weather. When the weather gets too warm–daytime temperatures greater than 75°F/24°C and nighttime temperatures above 60°F/16°C–lettuce will send up a seed stalk (called bolting) and leaves will become bitter. If hot weather is predicted it is better to harvest the crop and store it in the refrigerator than watch it go to seed and become inedible. (Bitter lettuce leaves can be used as a tangy accent in salads.)
Heading lettuces often do not make heads in warm regions.
All types of lettuce will be ready to harvest about 45 to 60 days after planting, longer if planted in autumn or winter.
How to Store Lettuce
Store lettuce cold and moist, 32°-40°F (0°-5°C) and 95 percent relative humidity. Place lettuce in the refrigerator in a perforated plastic bag in the vegetable crisper section. Crisphead lettuce can be stored for two weeks under optimal conditions. Leaf and Bibb will store as long as four weeks if the leaves are dry when bagged.
Lettuce that is stored to cold or too long will develop brown spots on the midrib and leaf yellowing.
More tips: How to Grow Lettuce