All five varieties of lettuce (stem, loose-leaf, butterhead, crisphead and romaine) produce seeds in a similar manner: by sending up a seed stalk in the center of the plant. Tiny flowers bloom from this seed stalk, and then they self-pollinate to produce seeds. Crisphead lettuces may need encouragement to send up the seed stalk, which you can provide by cutting them open on top. Leaves of lettuce can be eaten prior to seed stalk production, but will taste bitter after the seed stalk has emerged.
Observe your lettuces, taking special note of when the seed stalks appear. Cut a 2- to 3-inch-deep gash in the center of crisphead lettuces to encourage the seed stalk to emerge. Check daily to watch the flowers appear and see how the entire plant changes. Wait until you see a single seed appear in each flower before proceeding.
Remove the seed stalk from the lettuce plant using pruning shears. Do this carefully to avoid shaking the seeds loose.
Put a small paper bag over the top of the seed stalk, then turn it upside down. Bunch several seed stalks from the same variety of lettuce together inside a single paper bag for ease of harvesting.
Tie a piece of twine around the bottom of the bag, leaving the bottoms of the seed stalks sticking out.
Tie the bottoms of the seed stalks to a clothes hanger, so that the seed stalks are upside down with the paper bags upright.
Hang the clothes hanger in your house out of direct sunlight. Shake the paper bags gently as the seed stalks dry to dislodge the seeds. Allow the seed stalks to dry and shrivel completely before removing the paper bags. Discard seed stalks once seeds have been collected.
Store lettuce seeds in the paper bags in a cool, dry place, such as your refrigerator. Mark the name of each seed variety on the bag if you are saving seed from multiple varieties.