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.
After about 3 to 4 weeks you can cut and harvest. Here is a nice closeup of another lettuce stump beginning to regrow with just a small amount of water indoors. Here we put 3 romaine lettuce stumps in 1/2 inch of water in a glass container and put on our kitchen window sill. (image credit here).