Lettuce is a cool-season crop and is best grown in either spring or fall when temperatures do not go to extremes. However, even though lettuce likes to grow during cool, damp days, lettuce seed germinates best in temperature around 70 degrees F.
This means you should start early spring plantings in the garden with seedlings, rather than seed. If lettuce seed is put in the ground while it is still cold and wet, the seed will simply rot. You can always start seed indoors and then transplant the seedlings when all danger of frost has passed.
Lettuce seeds need light to germinate, so barely cover the seed with soil and keep it moist. Lettuce is a quick grower. It is ready to transplant when several sets of leaves have developed. Don’t let the seedlings get too large before planting them out, or they will bolt to seed the first chance they get.
Seeds or Plants?
Lettuce is so easy to grow it can be started indoors for early transplants or sown directly in the garden. In fact, doing both is recommended to get maximum production. Start some lettuce seeds indoors in peat pots a few weeks before the last frost date in your area.
Provide the seedlings with plenty of sunlight or keep them under artificial lighting until ready to move into the garden. Transplant the seedlings as soon as the soil can be worked in the spring. If a hard freeze threatens, protect the seedlings with a cloche or row cover. Reserve a number of lettuce seedlings to fill empty spaces in the garden as the season progresses.
To sow lettuce directly in the garden, simply plant the seeds about 1/4 inch deep, tamp them down, and water. It's that simple! Space the sowings according to packet directions that are based on the size of the mature lettuce. For example, a crisphead may require a square foot of garden space. As many as nine plants of a small leaf lettuce variety can grow in the same space.
Keep in mind that lettuce seeds won't germinate in soil that is 80 degrees F. or warmer, so there's no sense in sowing directly in the garden in the summer. Resort to starting heat-tolerant varieties indoors and moving the lettuce seedlings into the garden, preferably under partial shade, after they've developed a few true leaves.
Cultivating Lettuce
Lettuce is ideal for succession planting. Sow seeds every two weeks for production throughout the season, starting with early lettuce varieties, using heat-tolerant varieties as your main crop, and then switching to fall crops late in the summer. Or, if you prefer, use lettuce in successions with other crops. For example, plant lettuce in the spring, followed by bush beans in the summer, followed by lettuce again in the fall.
The key to lettuce production is supplying moderate but almost constant water, especially during hot weather. Unless there is regular rainfall, lettuce must be watered deeply at least once a week- more frequently during periods of drought. Mulch with a layer of compost or clean straw to help the soil retain moisture. A drip-irrigation system is ideal.
Harvesting Lettuce
You can harvest cut and come again types as soon as the outer leaves reach about 6 inches long. If you are growing head lettuce, be sure to harvest before the head starts to elongate. That means it's ready to bolt and the flavor will suffer. And be forewarned, maturing to a head takes time and therefore makes it more difficult to grow without bolting than the loose-leaf varieties.
For the longest harvest, direct seed or transplant every 7-10 days. When direct seeding, seeds can either be broadcast and planted in wide.