Growing guidelines: When the seedlings have four leaves, thin head or romaine lettuce to 12 to 16 inches apart. Do the same for leaf lettuce unless you plan to harvest entire plants instead of leaves; in that case, 4-inch spacing is adequate. Thin butterheads to 3 to 5 inches apart.
Lettuce is 90 percent water and has very shallow roots, so keep the soil surface moist but not soggy. Make sure the crop gets at least 1 inch of water a week from rain or irrigation; otherwise, leaves can be thin and bitter, and the plants might bolt to seed. To help prevent disease, try to water on sunny mornings, so the leaves can dry by evening. After a good rain or watering, apply a thick layer of mulch to conserve moisture, suffocate weeds around the easily damaged roots, and keep lettuce leaves free of dirt. To promote quick growth, side-dress with compost or fish emulsion once or twice during the growing season.
Just before bolting, lettuce plants start to elongate and form a bitter sap. To keep this from happening, pinch off the top center of the plant. Pull up and discard any plant that goes to seed. If you are a seed saver, wait and save seeds from the last plants to bolt, since quickness to bolt is a bad trait. Seed savers should also be aware that different lettuce cultivars can cross with each other and with wild lettuce, so next year’s plants may not come true from your saved seed.
Soggy soil and crowded plants can encourage bottom rot, a disease that turns lettuce plants black and foul-smelling. There are only a few varieties that are resistant to this disease, which has spores that can persist for years in the soil. If you’ve had problems with bottom rot, choose varieties with an upright form rather than a spreading form, and try planting your lettuce plants on 4-inch-tall ridges of soil. Be careful to avoid wetting the leaves when you water.
Gray mold makes grayish green or dark brown spots on lower leaves and is usually brought on by damp, overcast weather. Injured seedlings are particularly vulnerable. Pull up any infected plant and dispose of it far from the garden.
Harvesting: Pick lettuce in the morning to preserve the crispness it acquires overnight. Watch your crop closely, as mature plants deteriorate quickly. To test the firmness of heading types, press down gently on lettuce hearts with the back of your hand; don’t pinch them, as this can bruise the hearts. Use a sharp knife to cut heads below the lowest leaves, or pull plants out by the roots. Harvest leaf types as needed. Lettuce tastes best when eaten fresh but will keep up to 2 weeks when refrigerated.