Lettuce is pretty flexible when it comes to starting from seed. Lettuce seeds germinate in a wide temperature range -- 32 to 86 degrees Fahrenheit -- but you have most success at around 75 F. They sprout in almost any well-draining medium, but do best in soilless mixes. Lettuce seeds grow in almost any type of container -- foil food trays, foam cooler-type trays and, of course, seed trays -- as long as you give them enough warmth, light and space. Starting lettuce in trays gives you a jump on the growing season and gives you more time to prepare the garden.
Mix 1 part bleach and 9 parts water and soak the trays in the mixture for 30 minutes. Remove the trays and allow them to air dry.
Poke or drill one 1/4-inch hole for every square inch of area in the bottom of the tray, using a skewer or drill for the task. You can use a skewer for foil or foam trays, but you'll need a small drill to go through most plastic trays. For example, if you have a tray that measures 18 inches by 24 inches, poke 36 small, evenly spaced holes in the bottom of it. You shouldn’t have to poke holes in seedling trays with individual cells.
Fill a basket or bucket with equal parts sterilized, coarse river sand, sphagnum peat moss and perlite. Although the amount of growing medium you need varies with the number and size of the containers, always mix enough to fill the planting trays to the top and use the same ratio of ingredients. Moisten the growing medium with just enough water to make mixing easier. Mix the growing medium with your hands.
Fill the trays with growing medium until it reaches 3/4 inch from the tops. If you're using a tray with individual cells, fill each cell 3/4 inch from the top.
Wrap the tray filled with growing medium with clear plastic film and place it inside another tray. The second tray collects water when it drains from the growing medium.
Place the trays on a seedling heat mat or a regular heating pad set to medium. Insert a soil thermometer through the plastic film and into the growing medium. Leave the tray on the mat for two or three hours and check the temperature. When the growing medium reaches between 68 and 77 F, adjust the heat so it stays in that range. If after two or three hours the temperature reads above or below the ideal range, adjust the heat until it holds between 68 and 75 F. Remove the plastic film.
Sprinkle the lettuce seeds on top of the growing medium, so you have one seed per 1 square inch. Lettuce seeds are so small exact spacing is difficult, so just spread them as evenly as you can.
Sprinkle just enough growing medium over the seeds to cover, about 1/16 to 1/8 inch. Water the seedlings until water drains from the tray. Wrap the tray with plastic film.
Spray the growing medium a few times with water from a spray bottle if you see the medium drying out during the germination period. The seeds should germinate and sprout within one week. Remove the plastic film as soon as you seed the seedlings emerge.
Place the tray of seedlings 2 to 4 inches below a T-8 fluorescent shop light with two bulbs.
Water the seedlings enough to keep the growing medium moist, or about every other day with 1/2 inch or water, until the first set of true leaves emerge, about two or three weeks. You can also spray the growing medium several times a day with water from a spray bottle.