Depending on its variety, lettuce seed can germinate when the soil temperature is 35 to 80 degrees Fahrenheit. The ideal soil temperature for germination though, is 70 to 75 F. In cooler temperatures, germination takes longer. When temperatures are warmer than 70 to 75 F, lettuce seed may not germinate. To germinate the seeds however, the best temperatures are between 70-85. At these temps with the good seed and proper light I can get 10% germination in 18 hours and 98% germination in 48 hours. 4) Light is the most overlooked part of growing lettuce. Since lettuce is photodorment it will not germinate in the dark.