Lettuce (Lactuca sativa) is a temperate annual or biennial plant of the daisy family Asteraceae. It is most often grown as a leaf vegetable. It is eaten either raw, notably in salads, sandwiches,hamburgers, tacos, and many other dishes, or cooked, as in Chinese cuisine in which the stem becomes just as important as the leaf. Both the English name and the Latin name of the genus are ultimately derived from lac, the Latin word for “milk”, referring to the plant’s milky juice. Mild in flavour, it has been described over the centuries as a cooling counterbalance to other ingredients in a salad.
The lettuce plant has a very short stem initially (a rosette growth habit), but when it gradually blooms, the stem and branches lengthen and produce many flower heads that look like those of dandelions, but smaller. This is referred to as bolting. When grown to eat, lettuce is harvested before it bolts.
Lettuce is grown commercially worldwide.
Lettuce plants should be grown in a light, sandy, fertile, humus-rich soil that will hold moisture in summer. A soil pH of 6.5 is preferred; lime may be added for this purpose. For best eating quality, water is essential; the plants prefer the soil to be moist at all times.
Lettuce plants prefer cool weather, ideally with day temperatures below 75 degrees Fahrenheit and night temperatures above 40 degrees Fahrenheit. Hot, sunny, or dry conditions may cause the plants to turn bitter and produce a flower shoot, a process known as bolting. Therefore, lettuce is often grown in the coolness of spring and autumn in the Northern Hemisphere; lettuce sown in summer is often grown in light shade. In addition, bolt-resistant summer cultivars of lettuce may be recommended as temperatures increase.
Lettuce can be direct sown in the garden, but lettuce plants are often started in cold frames or greenhouses, and the resulting seedlings transplanted to the garden or field. This allows an earlier start, or allows more efficient use of garden space, as the lettuce can be transplanted when growing rapidly, avoiding the use of garden space for germination of seeds.
As another way to allow an earlier crop in cold weather, lettuce is sometimes given glass protection, known as a cloche, or protected with spun material known as a floating row cover. In sufficiently mild-weather climates, these same protective devices (greenhouses, cold frames, cloches, row cover) may be used to protect lettuce throughout the winter, allowing harvest even in near-freezing or freezing weather.
Lettuce is often grown between rows of slower growing plants like brussel sprouts or broccoli. This is called a catch crop. It allows more efficient use of garden space, and also provides the lettuce with needed shade in warm weather.
There are six commonly recognised Cultivar Groups of lettuce which are ordered here by head formation and leaf structure; there are hundreds of cultivars of lettuce selected for leaf shape and colour, as well as extended field and shelf life, within each of these Cultivar Groups:
Butterhead (L. sativa var. capitata) forms loose heads. Its leaves have a buttery texture. Butterhead cultivars are most popular in Europe. Popular varieties include Boston, Bibb,Buttercrunch, and Tom Thumb.
Chinese lettuce (L. sativa var. asparagina) types generally have long, sword-shaped, non-head-forming leaves, with a bitter and robust flavour unlike Western types, for use in stir-frieddishes and stews. Chinese lettuce cultivars are divided into “stem-use” types (called celtuce in English), and “leaf-use” types such asyoumaicai or shengcai , respectively.
Crisphead, also called Iceberg, forms tight, dense heads that resemble cabbage. They are generally the mildest of the lettuces, valued more for their crunchy texture than for flavour. Cultivars of iceberg lettuce are the most familiar lettuces in the USA.
Looseleaf (L. sativa var. crispa) has tender, delicate, and mildly flavoured leaves. This group includes oak leaf and lollo rosso lettuces.
Romaine (L. sativa var. romana), also called Cos, grows in a long head of sturdy leaves with a firm rib down the center. Unlike most lettuces, it is tolerant of heat.
Summer Crisp, also called Batavian, forms moderately dense heads with a crunchy texture. This type is intermediate between iceberg and looseleaf types.
Some lettuces (especially iceberg) have been specifically bred to remove the bitterness from their leaves. These lettuces have high water content and so are less “nutritionally dense” than are the more bitter lettuces and those with darker leaves. While all lettuces contain antioxidants and vitamin K, romaine and looseleaf lettuce contain five to six times the vitamin C and five to ten times the vitamin A of iceberg. Romaine and butterhead lettuce are good sources of folate. Lettuce naturally absorbs and concentrates lithium.
Lettuce is a low calorie food and is a source of vitamin A and folic acid. Lactucarium (or “Lettuce Opium”) is a mild opiate-like substance that is contained in all types of lettuce.