Plant more seeds. In a separate area of the garden, you can plant another batch of seeds that will come up later than the seedlings so you'll have a longer lettuce growing season. Broadcast the seeds over tilled soil, then scatter about 1⁄2 inch (1.3 cm) of soil on top of them. One seed packet will cover about 100 feet (30.5 m).
It's best to plant loose leaf varieties from seed. They'll be coming up later in the season, and since they're more heat-resistant than iceberg and romaine, they'll be less likely to wilt in the summer heat.
Water the seed bed thoroughly after planting