Lettuce has stem lettuce (lettuce) and leaf lettuce, leaf lettuce and ball lettuce and loose leaf lettuce. Lettuce seed germination of the lowest temperature 4 ℃, but the time required is longer, the most suitable temperature of 15 ~ 20 ℃, only 3 ~ 4 days, the temperature of 30 ℃ or more germination is blocked. Seedling stage of the adaptability of temperature, the growth of suitable temperature 12 ~ 20 ℃, and can tolerate -5 ~ -6 ℃ low temperature, but the temperature is too high, especially the surface temperature up to 30 ℃ when the seedling stems are easy to burn and collapse, so, autumn lettuce in the high-temperature season should be shaded to reduce the temperature of seedling.

Stem lettuce and ball lettuce are generally transplanted using seedlings, while loose-leaf lettuce is often live or with hole tray seedlings. The sowing seasons of lettuce in major regions of China are shown in Table 3.

Table 3 Sowing seasons of lettuce in different regions