Crisphead varieties, the iceberg types common at supermarkets all over the country, are adapted to northern conditions and require the most care. In areas without long, cool seasons, they generally are grown from transplants, started early and moved to the garden as soon as the soil can be worked. They are extremely sensitive to heat and must mature before the first hot spell of summer to achieve high-quality heads. If an unseasonably early heat wave hits before they have matured, they almost certainly fail. In many locations, crisphead lettuce plants started in late summer to mature in the cooler weather of fall have a much better chance of success.