In head lettuces, as the plant grows the new leaves fold upon each other, forming a rounded head. Lettuces with the tightest heads, called crisphead lettuce, are harvested by cutting off the whole plant at the soil line with a sharp knife once the head fully forms. You can also just pull up the plant. Test for head firmness by gently pressing down on the top of the head with the back of your hand. Examples of crisphead lettuce are "Iceberg," "Great Lakes" and "Ithaca." Summer crisphead, or Batavian lettuce types, are more heat tolerant and feature crisp hearts surrounded by looser leaves. Progressively pick outer leaves or cut the entire plant when the head forms. Cultivars include reddish-green "Magenta" and green "Nevada."
Loose Head Lettuces
"Bibb" and "Buttercrunch" lettuces form looser heads. Harvest the outer leaves as the plant grows or let the plant grow until the lettuce is about 6 to 8 inches wide and forms the typical soft head framed by outer leaves. Cut the headed plant off at the soil line. The soft, mild-tasting inner leaves self-blanch and are yellowish in color.
Loose-Leaf and Cos Lettuces
Loose-leaf lettuces don't form heads and many cultivars, including highly colored leaf varieties, exist. Pick outermost leaves over the life of the plant by pinching them off with your fingers or by cutting them near the stem with scissors. You can also use a cut-and-come-again technique to make two or three loose-leaf lettuce harvests. Cut off all the leaves with a scissors about 1/2 to 1 inch above the soil and harvest again in two or three weeks. Cos or romaine lettuce forms an upright, loose, vertical head. Harvest outer leaves or cut off the entire head as for loose-leaf lettuce.
If you want baby greens for tender salads, plant a variety of loose-leaf and cos lettuces in a broadcast fashion. Include some colorful loose-leaf varieties such as bright red "New Red Fire," dark red "Ruby" and purple-black "Midnight Ruffles." "Freckles" is a cos lettuce with the green leaves splashed with burgundy. When the seedlings are about 4 inches tall, cut off 3 inches of the leaves with a scissors, leaving an inch to resprout. Harvest baby greens in the evening if you're going to store them for more than a day.
Cleaning and Storing Lettuce
Once picked, lettuce leaves wilt quickly. Process the leaves or plants within 15 minutes of picking if possible. Wash them thoroughly in a sink or large container filled with cool water. Put leaves in a colander and wash them again. Remove water with a salad spinner or pat leaves dry between clean towels. Store lettuce in a plastic bag in the refrigerator for up to two weeks. Add a paper towel or two to the bag to absorb any extra moisture. Avoid putting lettuce with fruits such as apples, bananas or pears. They release ethylene gas, which causes brown spots on the lettuce.