Lettuce is a Cool Season Vegetable (Salad Greens, Lactuca sativa from Asteraceae Family). Not native to North America - Mediterranean origin. Was cultivated by Egyptians 6,500 years ago.
With the variety of colors, shapes and flavors available, your salads may never be the same. Lettuce is adapatable to many growing conditions, but likes it cool - around 60 F to 65 F. Grow in spring and fall, and slow bolting by shading summer crops.
Read more at Doctor's Garden.