Lettuce (Lactuca sativa) is actually a member of the aster/sunflower family Asteraceae. It is easily cultivated, and grows best in full sun, in loose, nitrogen-rich soils with a pH between 6.0 and 6.8. Heat generally prompts lettuce to bolt (go to flower, then seed) so temperatures of 60 to 65 °F are preferred, and as low as 45 °F being tolerated. Temperatures above 80 °F will generally result in poor or non-existent germination of lettuce seeds. After harvest, lettuce – which is about 95% water – lasts the longest when kept at 33 to 34 °F. It is imperative to store it correctly because if it spoils, it becomes a source of bacterial, viral and parasitic outbreaks in humans, including E. coli and Salmonella.
There are several types of lettuce, but the 3 most common are Leaf, Crisphead and Cos/Romaine. Variants of these include Butterhead and Batavian. Each of these 5 cultivars contains numerous varieties.
1. Leaf: Also known as loose-leaf, cutting or bunching lettuce. It is used mainly for salads, and this cultivar is the most widely planted.
2. Crisphead: Better known as "Iceberg" lettuce. This type is very heat-sensitive and was originally adapted for growth in the northern U.S. Most supermarket produce sections sell more Head lettuce than other types. It ships well, but is low in flavor and nutritional content, being composed of even more water than other lettuce types.
3. Cos/Romaine: Forms long, upright “heads”. Used mainly for Caesar salads, but also for sandwiches and other fresh salads.
4. Butterhead: Also known as Boston or Bibb lettuce. A head-type but with a loose arrangement of leaves. Has sweet flavor and tender texture.
5. Batavian: Also called Summercrisp or French Crisp. It is midway between the crisphead and leaf types. These lettuces tend to be larger yet well-flavored, and with some bolt-resistance.
Plants generally have a height and spread of 6 to 12 inches. Leaf types are especially colorful, mainly in the green and red color spectrums, and some are even variegated. There are also a few varieties with yellow, gold or blue-teal leaves. Lettuces also have a wide range of shapes and textures, from the dense heads of the iceberg type to the notched, scalloped, or frilly leaves of leaf types.
Depending on the variety and time of year, lettuce generally lives 65 to 130 days from planting to harvesting. Some varieties can be over-wintered (even in relatively cold climates) under a layer of straw, while older heirloom varieties are often grown in cold frames. Leaf and Cos varieties are generally planted directly into the garden in thick rows. Heading varieties are commonly started in peat strips, then transplanted to the garden after developing several leaves. When lettuce plants are spaced further apart, they receive more sunlight which improves color and the concentration of nutrients in the leaves. Pale to white lettuce, such as the centers in some iceberg lettuce, contain very few nutrients.