To define a number range, you have to create a new number interval: 1  Specify an alphanumeric key with up to 2 characters. 2  Specify the limits of the number interval.  Mark the number range if it is planned for external number assignment.
3. Trim back the entire plant when it reaches 3 to 6 inches high, or approximately every 10 days. Cut back to within 1 to 2 inches of the ground. Most loose-leaf lettuce varieties grow back from a severe trim to produce more leaves.
Growing Head Lettuce. 1  1. Choose a head lettuce variety for indoor planting. Head lettuces take longer to mature. 2  2. Prepare seed trays. You can start your lettuce seeds in store-bought seed trays or make your own out of an old egg carton, box or newspaper. 3  3. Sow the seeds 4-6 weeks before the last spring frost.