Broadcast the seeds and rake lightly to cover them, or sow seeds ¼ inch apart and as thinly as possible in rows 1½ feet apart. A small seed packet will plant a 100-foot row and produce some 80 heads, or about 50 pounds of leaf lettuce. Germination rate is over 80 percent.
Although lettuce is primarily considered a cool-season crop, it is possible to extend the harvest with some careful planning. If you’re a real lettuce lover, try the following schedule:
1. Start romaine, iceberg, or other head lettuces indoors 4 to 6 weeks before your last frost date, making three small sowings at weekly intervals. Set out the seedlings successively as soon as the ground is workable. At the same time, direct-seed leaf lettuce outdoors at 2-week intervals. If the soil temperature is at least 35°F, germination should take place in 6 to 12 days. If you plan to harvest only outer leaves as the plants grow, or harvest the crop for spring mix or mesclun, you can sow the entire loose-leaf crop at once. Choose cold-tolerant cultivars for spring planting (you can find them individually or as a “cool-season mix” or “winter mix” of cultivars from many seed companies).
2. As the weather warms, plant heat-resistant cultivars (you can find them individually or as a “warm-season mix” of cultivars from many seed companies). If you place them in shady areas and give them adequate water, they are less likely to bolt and go to seed during hot spells. You can also cover the lettuce bed with floating row cover to shade it, but leave the ends (and, preferably, the sides) open for air circulation. If the earth is very warm, try presprouting the seeds to get better germination. Place the seeds on wet blotting paper or mix them with a little damp peat moss and perlite; store in the refrigerator for 5 days before sowing.
3. In midsummer, switch back to head or romaine types, making successive sowings—again, in shady areas—for a fall harvest that can last until frost. In milder climates, cover immature heads with cloches to prolong the harvest; in cold-winter areas, transplant a few heads into pots and let them continue growing in a greenhouse or sunny window. You can also sow cold-tolerant loose-leaf lettuces as you did in spring, keeping the bed evenly moist and shaded until outdoor temperatures cool down. If you have a cold frame or a greenhouse with an in-ground bed, you can continue sowing lettuce, extending the season even further.