Lettuce generally takes 45 to 55 days to mature after planting the seeds. Heading varieties of lettuce, such as romaine, take longer to mature. Butterhead and looseleaf lettuce leaves can be harvested at essentially any time in the growth process.
Lettuce grows best when the soil has a pH of 6.0 to 6.8 and is rich in nitrogen and potassium. The soil should drain well, but it should hold some moisture.
If lettuce is harvested later, its leaves taste bitterer. The best time of day to harvest lettuce is in the morning, since the leaves have not yet been exposed to the sun.