Lettuce that is intended for seed production can be started indoors or in a greenhouse approximately one month before the time for setting out. Growing a crop for seed rather than food means you need to take full advantage of the length of your season to be sure you get mature seed.
Starting the plants in individual cells permits you to transplant only plants that are strong and well formed and it allows you to space them accurately in the garden. They will also be less susceptible to insect damage if they have grown beyond the tenderest stage, and of course there will be no waste from thinning. Space the plants about a foot apart for seed production. This allows the plants to grow to their fullest dimensions before flowering and producing seed.
Lettuce is an inbreeding, or self-pollinating, species, and there is little danger of crossing between varieties, so you can plant more than one variety for seed in a given year. However, don’t plant two varieties right next to each other where the flowers might touch.
Grow the plants in full sun, in a garden bed of relatively fertile soil and ample moisture. For best quality seed you will need at least 12 plants from which to harvest. You won’t be able to eat any leaves from your seed plants, so be sure to have a separate planting for eating.
The plants will begin to bolt—this means they will produce a flower stalk—as the season warms and the days lengthen. Some varieties will grow tall enough so that they require staking to prevent the flower stalks from falling over. All lettuce plants produce yellow flowers. Once the flowers have faded, the seed capsule will form. Lettuce seed will spoil if it gets wet in its final stages of ripening. If you are having a wet season, pound stakes in at the corners of the bed and attach tarps to two corners so that it can be draped over the flowering plants if there’s danger of a rain storm. Uncover the plants again right after the rain stops so sun and fresh air can reach them.
Lettuce seed will ripen over a period of time and should be harvested daily if possible. The capsules holding the seeds will dry to the touch when the seed is ready. You can either pick individual seed capsules off the plants or, if most of the seed is mature, cut the heads and invert them in a bag or bucket. Swish the stems against the side of the container to release the seeds.
Cleaning lettuce seed is not the easiest since seed and chaff are almost the same size and weight. Rub seed heads between your palms to release seed and reduce the size of chaff particles, then separate them using sieves with different mesh sizes. Spread seed on a baking sheet with a rim and blow very gently to remove some of the chaff. Dry the seed thoroughly at room temperature out of sunlight before packing and labeling.
Lettuce seed will remain viable for at least three years (or up to eight) if stored in a cool, dark, dry place.