Lettuce is a cool-season vegetable. It is usually grown in spring, but it can also be successfully grown as a fall crop. Saving your own seeds is an inexpensive way to plant your favorite type of lettuce each year. Save seeds from spring-planted lettuce, as fall crops often freeze and die before they have a chance to produce seeds. If you plant lettuce in both the spring and fall, save enough seed from your spring crop for both your fall garden and for replanting the next spring.
Harvest only the outer leaves from the lettuce plants you plan on using for seed stock. Leave the inner leaves in place so the plant continues to grow.
Check the plant every few days for signs of bolting once the weather begins to warm in summer. Bolting is when the lettuce plant sends up a tall flower stalk from the center of the leaves.
Cut off the flower stalk at ground level with a sharp knife once the flowers have withered and the stalk is mature. Mature flower stalks have an orb of white fluff at the top, similar to a dandelion.
Pluck the fluff off the top of the flower stalk. Rub the fluff between your forefinger and thumb so the seeds separate from the fluff. Work over a bowl so the seeds fall into the bowl instead of getting lost on the floor.
Label an envelope or jar with the type of lettuce and the year harvested. Place the seeds inside and seal it closed. Store in a cool, dry place until you are ready to replant.