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How to Grow Lettuce
Mature lettuce does not stay at its best for long; for a steady supply of high quality lettuce you must sow it at regular intervals. For most people, six to twelve heads of lettuce every two weeks is more than ample. If you have plenty of garden space, direct sowing your seeds is the easiest approach. If space is limited, you may want to transplant small seedlings, spotting them between other plants that require more time to mature such as broccoli and cabbage. This technique of interplanting makes the best use of precious garden space.
Lettuce seeds may be sown directly in the garden when the soil temperature reaches 50° F. Plant two or three seeds at regular intervals, every 6-12" depending on the mature size of the lettuce you are growing. When the seeds emerge thin the small plants so there is one head in each location.
Lettuce is always best if picked and used at once. For heading types such as butterhead, crisphead and romaine, harvest when the heads are firm and mature. The outer leaves of looseleaf lettuce may be cut when they are mature, leaving the inner leaves to be harvested at a later date. Whether you cut individual leaves or entire heads, the leaves should be glossy. If they lose their gleam this is a precursor to bolting, when the plant is preparing to go to seed and the leaves turn bitter. If you see milky sap exuded when you cut the leaves, this is another sign that the lettuce is past its prime.
For the best summer lettuce, carefully choose your varieties and the garden location. Romaine may be the most heat tolerant type of lettuce, although with care you can find good choices in every category. Some of the varieties adapted to summer production are Red Sails, Black Seeded Simpson, Summer Bibb, Buttercrunch, and Summer Crispheads such as Nevada and Reines des Glaces. Sow these in a location that is well irrigated and out of direct afternoon sun, and you are on your way to delicious salads all summer long.