Horticulture/Lettuce

Lettuce is a temperate annual or biennial, grown as a leaf vegetable. In Western countries, it is typically eaten cold and raw, but in other places such as China, lettuce is typically eaten cooked and the uses of the stem are as important as use of the leaf.

Description
The lettuce plant develops as a rosette, but when it blooms, the stem lengthens and branches, and it produces many flower heads that look like those of dandelions, but smaller. This is called bolting. When grown to eat, lettuce is harvested before it bolts.

Growing Conditions
Lettuces are generally considered a cool season crop, but can be grown to a young stage in summer if given some protection from the sun. They can also be grown in cold or hot frames through the winter in most temperate climates.

Lettuces do best in a rich, fast-draining soil. Adequate and consistent watering is required for good growth and taste.

Seeds can be sown in situ or in flats throughout the year, but seeds will not germinate at temperatures above 25C. Plants can be started in a cool area for summer plantings, but will require some shelter from sun and heat in warm climates. When starting in flats, sow 2 seeds in each cell, using small cells if planting out, large cells if growing in the flats for a bench crop. Thin to one plant per cell once germination takes place, planting out as soon as the roots reach the wall.

Varieties
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, also called Boston or Bibb, forms loose heads and has a buttery texture. Butterhead cultivars are most popular in Europe.
 * Chinese lettuce types generally have long, sword-shaped, non-head-forming leaves, with a bitter and robust flavour unlike Western types, appropriate for use in stir-fried dishes and stews. Chinese lettuce cultivars are divided into "stem-use" types (called celtuce in English), and "leaf-use" types such as youmaicai or shengcai (生菜).
 * 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 has tender, delicate, and mildly flavoured leaves.
 * Romaine, also called Cos, is a head-forming type with elongated leaves.
 * Summer Crisp, also called Batavian, forms moderately dense heads with a crunchy texture; this type is intermediate between iceberg and loose leaf types.

Some lettuces (especially iceberg) have been specifically bred to remove the bitterness from their leaves. These lettuces have a high water content with very little nutrient value. The more bitter lettuces and the ones with pigmented leaves contain antioxidants.

Uses
The wild predecessor of our modern lettuce, ../Lactuca serriola/, can still be seen all over Europe and the more temperate parts of Asia. It is likely that it originated on the Mediterranean rim on rocky wasteland or woodland clearings. This ancient wild relative of the modern lettuce contains lactucarium, a narcotic similar to opium. The Ancient Romans took advantage of this property by eating lettuce at the end of a meal to induce sleep.

In earlier times the Egyptians held a similar view of the lettuce. However, as well as a hypnotic or an aid to sleep, the plant was also linked with male virility. It is thought these Egyptian plants were closely linked with the modern day Cos lettuce and could have originated on the Turkish coast opposite the island of Kos.

It is certain that these ancient civilizations saw the plant as both an appetite stimulant and an aid to sleep. In ancient Greece this led to confusion whether to eat the plant at the beginning or the end of a meal. The physician Galen, from Pergamon, would eat the plant to allow restful sleep and allow him to study without 'mental churnings' the following day. Somewhat contrary to this, a century earlier, Rufus of Ephesos declared the opposite; claiming lettuce 'fogged the memory and prevented clear thought'.

Maintenance
Lettuces can be planted out as soon as the soil is workable. If sowing directly into the garden, make sure the bed is as weed-free as possible. If there are weed issues, it may be better to plant first in flats, then transplant to the field once the plants are large enough to be easily distinguished from weed seedlings.

Lettuces may "bolt" (send up a flower stalk) in hot weather. The foliage becomes bitter when the plant bolts.

Monitor for slugs, keep thinned to ensure good air circulation.

Propagation
Lettuces are propagated by seed only. Lettuces are mostly self-fertile (except for F1 hybrids, etc.), and often come true from seed.

Harvest
Harvest any time before bolting. Harvested leaves and heads should be chilled by dipping them in ice water, then kept refrigerated until ready to use.

Fungal Diseases

 * Damping off
 * Bottom Rot (Rhizoctinia)
 * Leaf drop (Sclerotinia)

Slugs and Snails
Slugs and snails can be a serious problem.

Arthropod Pests

 * Aphids
 * Thrips
 * Cutworms
 * Flea Beetles
 * Aster leafhopper

Vertebrate Pests

 * Rabbits
 * Deer

Environmental Issues

 * Tipburn: A browning of the leaf tips may occur during periods of high temperatures if the leaves are wet and then allowed to dry too rapidly. Growing in a shaded area or under shadecloth during the hot season can help, as can misting.
 * Bolting: As the days get long and temperatures rise, lettuces will "bolt", sending up a tall stalk on which flowers and seeds will be produced. The greens become bitter once the plant begins to bolt, so the plant should either be pulled out or simply allowed to go to seed.