How to Start Lettuce from Seed
Starting indoors and transplanting is my favorite method for growing lettuce. You can buy seedlings or try sowing seeds directly in the garden, but I've found this is cheaper and has the highest success rate. Growing seedlings indoors (or on a porch) in a small container gives you more control over spacing later and makes it easier to keep the tender young plants well watered.
If you do buy lettuce seedlings, smaller is better. The big ones with lots of leaves are more likely to go to seed quickly with the stress of transplanting.
You can buy hundreds of lettuce seeds for just a dollar or two and they remain viable for a few years. If using older seeds, sow more than you think you'll need to allow for diminished germination.
For my first batch of lettuce I chose two butterhead types and a romaine. Butterhead lettuces do best in the cool temps of spring and fall. I start lettuce in my basement under lights, but it could be done in a sunny window or on a porch if the temps warm enough (above 50 degrees or so).
Put about 2 inches of sterile soil mix in a container and lay the seeds on top. Crumble a 1/2 inch layer of soil over the top and wet with a spray bottle. Lettuce seeds don't need to be buried very deep. Cover with a clear lid or piece of plastic until the first leaves appear to keep the moisture inside.
I raised my lights to get the picture, but they'll do best if you keep the leaves about 2 inches from the lights. About a week before you want to transplant, start keeping the seedlings outdoors in a semi shaded spot. This "hardening off" period helps the leaves get used to the stronger sunlight. I prefer to wait until frost is unlikely but lettuce can usually survive a light frost.
To transplant, dig out a chunk of seedlings and gently tease them apart. Well watered seedlings are easier to separate. I space my lettuces pretty close together, about 4 inches apart. As they grow, I'll harvest every other plant, allowing the remaining lettuce heads more room to grow.
For the first week or two after transplanting, lettuce seedlings will probably look sad and wilted. If you keep up with the watering, the plants will start sprouting new fresh leaves and you'll have nice heads in no time. Red or speckled varieties won't have much color under grow lights but will color up quickly after transplanting.
When I've finished transplanting the first batch of lettuce, I try to remember to sow the next round at home. Even if you harvest a few leaves at a time rather than cutting a whole head, most lettuces will start to get bitter after a few weeks at maturity in warm weather. If you eat lots of lettuce, you may want to start fresh seedlings every 2-3 weeks. My second lettuce planting will have more leaf and romaine types that are happy in the warmer temperatures of the summer.