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Lettuce: belong to the aster family of leafy greens packed with calcium, potassium, vitamin C, and folate. These plants originated in the Mediterranean region of southern Europe, eastern Asia and northern Africa.
Soil: have your soil tested. The optimal pH range is between 6.0 and 6.8. Apply phosphorus (P) and potassium (K) accordingly. Improve your soil by adding well-rotted manure or compost in spring or fall. The soil should be well drained and moisture retentive. Till the soil deeply, and then smooth the surface.
Fertilize: use a complete fertilizer such as a 10-10-10, at the rate of 2 pounds per 100 square feet. When the plants are about 4 inches tall, apply a side-dressing of the same fertilizer at a rate of 1 pound for each 25 feet of row.
Types: lettuce varieties include: non-heading: loose-leaf, including oakleaf and lolla; soft heads: butterhead and bibb; ruffled heads: French crisp, summer crisp, or Batavia; tall, compact heads: cos or romaine; dense, solid head: crisphead or iceberg.
Starting seed indoors: lettuce seed is very small and difficult to see. Starting seeds indoors gives you the chance to handle the seed in controlled conditions while it is still cold outside, as well as during the warm summer. Indoor air temperatures, easily managed fertility and water, and bright lights on a timer will produce strong plants.
Seedlings: sow seeds in the spring as soon as the soil is workable. For a fall crop, sow seeds about 3 months before the average first fall frost date. Plant seeds 1/4 to 1/2 inch deep, in rows 18 to 30 inches apart. Thin the seedlings when they are still small. Clip off unwanted plants.
Transplanting: purchase transplants from a garden center or grow your own by planting seeds indoors in early April or about 4 weeks before transplanting.
For fall crops, plant seeds in July. Harden seedlings by reducing water and temperature for 2 to 3 days before transplanting.
Watering: is important to grow salad crops, as they have small, shallow root systems.
One inch of rainfall per week is adequate. If your soil is sandy, it is better to water more often than once a week. Lettuce will be crisper if you water it often in the days prior to harvesting.
Control weeds: frequent, shallow cultivation with a hoe or hand tool will eliminate weeds. Scratch the soil just deeply enough to cut the weeds off below the surface of the soil. Mulching with grass clippings, weed-free straw, or other organic material to a depth of 3 to 4 inches can help prevent weed growth.
Harvest: single leaves as soon as they reach a usable size. If you harvest individual leaves at the “baby” stage, leaving most of the small leaves on the plant, multiple harvests are possible. You can also remove the whole plant by cutting it off at or just above the soil surface.
Source: https://extension.umn.edu/vegetables/growing-lettuce-endive-and-radicchio#harvest-445264
