Lettuce (''Lactuca sativa'') is an
annual plant
An annual plant is a plant that completes its life cycle, from germination to the production of seeds, within one growing season, and then dies. Globally, 6% of all plant species and 15% of herbaceous plants (excluding trees and shrubs) are ...
of the family
Asteraceae
Asteraceae () is a large family (biology), family of flowering plants that consists of over 32,000 known species in over 1,900 genera within the Order (biology), order Asterales. The number of species in Asteraceae is rivaled only by the Orchi ...
mostly grown as a
leaf vegetable
Leaf vegetables, also called leafy greens, vegetable greens, or simply greens, are plant leaves eaten as a vegetable, sometimes accompanied by their petioles and shoots, if tender. Leaf vegetables eaten raw in a salad can be called salad gre ...
. The leaves are most often used raw in
green salads, although lettuce is also seen in other kinds of food, such as sandwiches,
wraps and soups; it can also be grilled. Its stem and seeds are sometimes used;
celtuce
Celtuce () (''Lactuca sativa'' var. ''augustana'',Masatoshi Yamaguchi ''angustata'', or ''asparagina''), also called stem lettuce, celery lettuce, asparagus lettuce, or Chinese lettuce, is a cultivar of lettuce grown primarily for its thick s ...
(asparagus lettuce) is one variety grown for its stems, which are eaten either raw or cooked. In addition to its main use as a leafy green, it has also gathered religious and medicinal significance over centuries of human consumption. Europe and North America originally dominated the market for lettuce, but by the late 20th century the consumption of lettuce had spread throughout the world. , world production of lettuce (and
chicory) was 27 million
tonnes
The tonne ( or ; symbol: t) is a unit of mass equal to 1,000 kilograms. It is a non-SI unit accepted for use with SI. It is also referred to as a metric ton in the United States to distinguish it from the non-metric units of the s ...
, 53percent of which came from China.
Lettuce was originally farmed by the
ancient Egyptians
Ancient Egypt () was a cradle of civilization concentrated along the lower reaches of the Nile River in Northeast Africa. It emerged from prehistoric Egypt around 3150BC (according to conventional Egyptian chronology), when Upper and Lower ...
, who transformed it from a plant whose seeds were used to obtain oil into an important food crop raised for its succulent leaves and oil-rich seeds. Lettuce spread to the Greeks and Romans; the latter gave it the name , from which the English ''lettuce'' is derived. By 50 AD, many types were described, and lettuce appeared often in medieval writings, including several
herbal
A herbal is a book containing the names and descriptions of plants, usually with information on their medicinal, Herbal tonic, tonic, culinary, toxic, hallucinatory, aromatic, or Magic (paranormal), magical powers, and the legends associated wi ...
s. The 16th through 18th centuries saw the development of many
varieties in Europe, and by the mid-18th century, cultivars were described that can still be found in modern gardens.
Generally grown as a
hardy annual, lettuce is easily cultivated, although it requires relatively low temperatures to prevent it from flowering quickly. It can be plagued by numerous
nutrient deficiencies, as well as insect and mammal pests, and fungal and bacterial diseases. ''L. sativa''
crosses easily within the species and with some other species within the genus ''Lactuca''. Although this trait can be a problem to home gardeners who attempt to save seeds, biologists have used it to broaden the
gene pool
The gene pool is the set of all genes, or genetic information, in any population, usually of a particular species.
Description
A large gene pool indicates extensive genetic diversity, which is associated with robust populations that can survi ...
of cultivated lettuce varieties.
Lettuce is a rich source of
vitamin K
Vitamin K is a family of structurally similar, fat-soluble vitamers found in foods and marketed as dietary supplements. The human body requires vitamin K for post-translational modification, post-synthesis modification of certain proteins ...
and
vitamin A
Vitamin A is a fat-soluble vitamin that is an essential nutrient. The term "vitamin A" encompasses a group of chemically related organic compounds that includes retinol, retinyl esters, and several provitamin (precursor) carotenoids, most not ...
, and a moderate source of
folate
Folate, also known as vitamin B9 and folacin, is one of the B vitamins. Manufactured folic acid, which is converted into folate by the body, is used as a dietary supplement and in food fortification as it is more stable during processing and ...
and
iron
Iron is a chemical element; it has symbol Fe () and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, forming much of Earth's o ...
. Contaminated lettuce is often a source of bacterial, viral, and parasitic outbreaks in humans, including ''
E. coli'' and ''
Salmonella
''Salmonella'' is a genus of bacillus (shape), rod-shaped, (bacillus) Gram-negative bacteria of the family Enterobacteriaceae. The two known species of ''Salmonella'' are ''Salmonella enterica'' and ''Salmonella bongori''. ''S. enterica'' ...
''.
Taxonomy and etymology

''Lactuca sativa'' is a member of the ''
Lactuca'' (lettuce) genus and the
Asteraceae
Asteraceae () is a large family (biology), family of flowering plants that consists of over 32,000 known species in over 1,900 genera within the Order (biology), order Asterales. The number of species in Asteraceae is rivaled only by the Orchi ...
(
sunflower
The common sunflower (''Helianthus annuus'') is a species of large annual forb of the daisy family Asteraceae. The common sunflower is harvested for its edible oily seeds, which are often eaten as a snack food. They are also used in the pr ...
or
aster) family. The species was first described in 1753 by
Carl Linnaeus
Carl Linnaeus (23 May 1707 – 10 January 1778), also known after ennoblement in 1761 as Carl von Linné,#Blunt, Blunt (2004), p. 171. was a Swedish biologist and physician who formalised binomial nomenclature, the modern system of naming o ...
in the second volume of his ''
Species Plantarum
' (Latin for "The Species of Plants") is a book by Carl Linnaeus, originally published in 1753, which lists every species of plant known at the time, classified into genus, genera. It is the first work to consistently apply binomial nomenclature ...
''. Synonyms for ''L. sativa'' include ''Lactuca scariola'' ''sativa'',
''L. scariola'' ''integrata'' and ''L. scariola'' ''integrifolia''. ''L. scariola'' is itself a
synonym
A synonym is a word, morpheme, or phrase that means precisely or nearly the same as another word, morpheme, or phrase in a given language. For example, in the English language, the words ''begin'', ''start'', ''commence'', and ''initiate'' are a ...
for ''
L. serriola'', the common wild or prickly lettuce.
''L. sativa'' also has many identified taxonomic
groups,
subspecies
In Taxonomy (biology), biological classification, subspecies (: subspecies) is a rank below species, used for populations that live in different areas and vary in size, shape, or other physical characteristics (Morphology (biology), morpholog ...
and
varieties, which delineate the various
cultivar group
A Group (previously cultivar-groupInternational Code of Nomenclature for Cultivated Plants, 4th edition (1969), 5th edition (1980) and 6th edition (1995)) is a formal category in the '' International Code of Nomenclature for Cultivated Plants'' (' ...
s of domesticated lettuce. Lettuce is closely related to several ''Lactuca'' species from southwest Asia; the closest relationship is to ''L. serriola'', an aggressive
weed
A weed is a plant considered undesirable in a particular situation, growing where it conflicts with human preferences, needs, or goals.Harlan, J. R., & deWet, J. M. (1965). Some thoughts about weeds. ''Economic botany'', ''19''(1), 16-24. Pla ...
common in temperate and subtropical zones in much of the world.
The Romans referred to lettuce as ( meaning "
milk
Milk is a white liquid food produced by the mammary glands of lactating mammals. It is the primary source of nutrition for young mammals (including breastfeeding, breastfed human infants) before they are able to digestion, digest solid food. ...
" in
Latin
Latin ( or ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken by the Latins (Italic tribe), Latins in Latium (now known as Lazio), the lower Tiber area aroun ...
), an allusion to the white substance,
latex
Latex is an emulsion (stable dispersion) of polymer microparticles in water. Latices are found in nature, but synthetic latices are common as well.
In nature, latex is found as a wikt:milky, milky fluid, which is present in 10% of all floweri ...
, exuded by cut stems.
The name ''Lactuca'' has become the genus name, while (meaning "sown" or "cultivated") was added to create the species name.
[Katz and Weaver, p. 376.] The current word ''lettuce'', originally from
Middle English
Middle English (abbreviated to ME) is a form of the English language that was spoken after the Norman Conquest of 1066, until the late 15th century. The English language underwent distinct variations and developments following the Old English pe ...
, came from the
Old French
Old French (, , ; ) was the language spoken in most of the northern half of France approximately between the late 8th [2-4; we might wonder whether there's a point at which it's appropriate to talk of the beginnings of French, that is, when it wa ...
or , which derived from the Roman name. The name ''romaine'' came from the variety of lettuce grown in the Roman papal gardens, while , another term for
romaine lettuce
Romaine or cos lettuce (''Lactuca sativa'' L. var. ''longifolia'') is a variety of lettuce that grows in a tall head of sturdy dark green leaves with firm ribs down their centers. Unlike most lettuces, it is tolerant of heat. In North America, ...
, came from the earliest European seeds of the type from the Greek island of Kos, a center of lettuce farming in the Byzantine Empire, Byzantine period.
Description

Lettuce's native range spreads from the
Mediterranean
The Mediterranean Sea ( ) is a sea connected to the Atlantic Ocean, surrounded by the Mediterranean basin and almost completely enclosed by land: on the east by the Levant in West Asia, on the north by Anatolia in West Asia and Southern ...
to
Siberia
Siberia ( ; , ) is an extensive geographical region comprising all of North Asia, from the Ural Mountains in the west to the Pacific Ocean in the east. It has formed a part of the sovereign territory of Russia and its predecessor states ...
, although it has been transported to almost all areas of the world. Plants generally have a height and spread of . The leaves are colorful, mainly in the green and red color spectrums, with some
variegated varieties.
There are also a few varieties with yellow, gold or blue-teal leaves.
Lettuces have a wide range of shapes and textures, from the dense heads of the iceberg type to the notched, scalloped, frilly or ruffly leaves of leaf varieties.
Lettuce plants have a root system that includes a main
taproot
A taproot is a large, central, and dominant root from which other roots sprout laterally. Typically a taproot is somewhat straight and very thick, is tapering in shape, and grows directly downward. In some plants, such as the carrot, the taproot ...
and smaller secondary roots. Some varieties, especially those found in the United States and Western Europe, have long, narrow taproots and a small set of secondary roots. Longer taproots and more extensive secondary systems are found in varieties from Asia.
Depending on the variety and time of year, lettuce generally lives 65–130 days from planting to harvesting. Because lettuce that flowers (through the process known as "
bolting") becomes bitter and unsaleable, plants grown for consumption are rarely allowed to grow to maturity. Lettuce flowers more quickly in hot temperatures, while freezing temperatures cause slower growth and sometimes damage to outer leaves.
Once plants move past the edible stage, they develop flower stalks up to high with small yellow blossoms.
Like other members of the tribe
Cichorieae
The Cichorieae (also called Lactuceae) are a tribe in the plant family Asteraceae that includes 93 genera, more than 1,600 sexually reproductive species and more than 7,000 apomictic species. They are found primarily in temperate regions of the E ...
, lettuce
inflorescence
In botany, an inflorescence is a group or cluster of flowers arranged on a plant's Plant stem, stem that is composed of a main branch or a system of branches. An inflorescence is categorized on the basis of the arrangement of flowers on a mai ...
s (also known as
flower heads
A pseudanthium (; : pseudanthia) is an inflorescence that resembles a flower. The word is sometimes used for other structures that are neither a true flower nor a true inflorescence. Examples of pseudanthia include flower heads, composite flowers ...
or capitula) are composed of multiple
floret
This glossary of botanical terms is a list of definitions of terms and concepts relevant to botany and plants in general. Terms of plant morphology are included here as well as at the more specific Glossary of plant morphology and Glossary ...
s, each with a modified
calyx called a
pappus (which becomes the feathery "parachute" of the fruit), a corolla of five petals fused into a
ligule
A ligule (from "strap", variant of ''lingula'', from ''lingua'' "tongue") is a thin outgrowth at the junction of leaf
A leaf (: leaves) is a principal appendage of the plant stem, stem of a vascular plant, usually borne laterally above g ...
or strap, and the reproductive parts. These include fused
anther
The stamen (: stamina or stamens) is a part consisting of the male reproductive organs of a flower. Collectively, the stamens form the androecium., p. 10
Morphology and terminology
A stamen typically consists of a stalk called the filament ...
s that form a tube which surrounds a
style
Style, or styles may refer to:
Film and television
* ''Style'' (2001 film), a Hindi film starring Sharman Joshi, Riya Sen, Sahil Khan and Shilpi Mudgal
* ''Style'' (2002 film), a Tamil drama film
* ''Style'' (2004 film), a Burmese film
* '' ...
and
bipartite stigma. As the anthers shed
pollen
Pollen is a powdery substance produced by most types of flowers of seed plants for the purpose of sexual reproduction. It consists of pollen grains (highly reduced Gametophyte#Heterospory, microgametophytes), which produce male gametes (sperm ...
, the style elongates to allow the stigmas, now coated with pollen, to emerge from the tube.
The ovaries form compressed,
obovate
The following terms are used to describe leaf morphology in the description and taxonomy of plants. Leaves may be simple (that is, the leaf blade or 'lamina' is undivided) or compound (that is, the leaf blade is divided into two or more leaflets) ...
(teardrop-shaped) dry fruits that do not open at maturity, measuring 3 to 4 mm long. The fruits have 5–7 ribs on each side and are tipped by two rows of small white hairs. The pappus remains at the top of each fruit as a
dispersal structure. Each fruit contains one seed, which can be white, yellow, gray or brown depending on the variety of lettuce.
The domestication of lettuce over the centuries has resulted in several changes through
selective breeding
Selective breeding (also called artificial selection) is the process by which humans use animal breeding and plant breeding to selectively develop particular phenotypic traits (characteristics) by choosing which typically animal or plant m ...
: delayed bolting, larger seeds, larger leaves and heads, better taste and texture, a lower
latex
Latex is an emulsion (stable dispersion) of polymer microparticles in water. Latices are found in nature, but synthetic latices are common as well.
In nature, latex is found as a wikt:milky, milky fluid, which is present in 10% of all floweri ...
content, and different leaf shapes and colors. Work in these areas continues through the present day.
[Davey, et al., pp. 222–225.] Scientific research into the
genetic modification
Genetic engineering, also called genetic modification or genetic manipulation, is the modification and manipulation of an organism's genes using technology. It is a set of technologies used to change the genetic makeup of cells, including th ...
of lettuce is ongoing, with over 85 field trials taking place between 1992 and 2005 in the European Union and the United States to test modifications allowing greater
herbicide
Herbicides (, ), also commonly known as weed killers, are substances used to control undesired plants, also known as weeds.EPA. February 201Pesticides Industry. Sales and Usage 2006 and 2007: Market Estimates. Summary in press releasMain page f ...
tolerance, greater resistance to insects and fungi and slower bolting patterns. However, genetically modified lettuce is not currently used in commercial agriculture.
History

DNA analysis of 445 types of lettuce indicates that lettuce was first domesticated from its
wild ancestor near the
Caucasus
The Caucasus () or Caucasia (), is a region spanning Eastern Europe and Western Asia. It is situated between the Black Sea and the Caspian Sea, comprising parts of Southern Russia, Georgia, Armenia, and Azerbaijan. The Caucasus Mountains, i ...
, where
seed shattering was first selected out of the
cultivar
A cultivar is a kind of Horticulture, cultivated plant that people have selected for desired phenotypic trait, traits and which retains those traits when Plant propagation, propagated. Methods used to propagate cultivars include division, root a ...
. At this time, the lettuce plant was only suitable for harvesting its
seeds
In botany, a seed is a plant structure containing an embryo and stored nutrients in a protective coat called a ''testa''. More generally, the term "seed" means anything that can be sown, which may include seed and husk or tuber. Seeds are the ...
, which could be pressed to extract
oil, likely used for cooking, among other purposes.
From there, lettuce was likely transported to the
Near East
The Near East () is a transcontinental region around the Eastern Mediterranean encompassing the historical Fertile Crescent, the Levant, Anatolia, Egypt, Mesopotamia, and coastal areas of the Arabian Peninsula. The term was invented in the 20th ...
and then to
ancient Egypt
Ancient Egypt () was a cradle of civilization concentrated along the lower reaches of the Nile River in Northeast Africa. It emerged from prehistoric Egypt around 3150BC (according to conventional Egyptian chronology), when Upper and Lower E ...
,
where the first depictions of lettuce cultivation can be found as early as 2680 BC.
Like the early lettuce from the Caucasus, this lettuce was grown to produce cooking oil from its seeds.
Lettuce was considered a sacred plant of the reproduction god
Min, and was carried during his festivals and placed near his images. The plant was thought to help the god "perform the sexual act untiringly". Its use in religious ceremonies resulted in the creation of many images in tombs and wall paintings. The cultivated variety appears to have been about tall and resembled a large version of the modern
romaine lettuce
Romaine or cos lettuce (''Lactuca sativa'' L. var. ''longifolia'') is a variety of lettuce that grows in a tall head of sturdy dark green leaves with firm ribs down their centers. Unlike most lettuces, it is tolerant of heat. In North America, ...
. These upright lettuces were developed by the Egyptians and passed to the Greeks, who in turn shared them with the Romans. Around 50 AD, Roman agriculturalist
Columella
Lucius Junius Moderatus Columella (, Arabic: ) was a prominent Roman writer on agriculture in the Roman Empire.
His in twelve volumes has been completely preserved and forms an important source on Roman agriculture and ancient Roman cuisin ...
described several lettuce varieties – some of which may have been ancestors of today's lettuces.
The plant was eventually selectively bred into a plant grown for its edible leaves.
The long leaves in Egyptian depictions suggest that it may have been grown for its leaves, which would make it the first lettuce cultivar grown for this purpose.
However, genome wide analysis suggests the traits needed for cultivation as a leafy vegetable, like the loss of bitterness and thorns, evolved much later, from around 500 BC in Southern Europe. Lettuce cultivars radiated more rapidly from this point, with
oilseed lettuce likely being brought by the ancient Greeks from Egypt to Italy, where it was modified into
''cos'' lettuce and cultivated for its leaves. From there, it was brought north to Central Europe, where it was modified into
butterhead lettuce and other varieties.
Lettuce appears in many medieval writings, especially as a medicinal herb.
Hildegard of Bingen
Hildegard of Bingen Benedictines, OSB (, ; ; 17 September 1179), also known as the Sibyl of the Rhine, was a German Benedictines, Benedictine abbess and polymath active as a writer, composer, philosopher, Christian mysticism, mystic, visiona ...
mentioned it in her writings on medicinal herbs between 1098 and 1179, and many early
herbal
A herbal is a book containing the names and descriptions of plants, usually with information on their medicinal, Herbal tonic, tonic, culinary, toxic, hallucinatory, aromatic, or Magic (paranormal), magical powers, and the legends associated wi ...
s also describe its uses. In 1586,
Joachim Camerarius provided descriptions of the three basic modern lettuces – head lettuce, loose-leaf lettuce, and romaine (or cos) lettuce.
Lettuce was first brought to the Americas from Europe by
Christopher Columbus
Christopher Columbus (; between 25 August and 31 October 1451 – 20 May 1506) was an Italians, Italian explorer and navigator from the Republic of Genoa who completed Voyages of Christopher Columbus, four Spanish-based voyages across the At ...
in the late 15th century.
Between the late 16th century and the early 18th century, many varieties were developed in Europe, particularly Holland. Books published in the mid-18th and early 19th centuries describe several varieties found in gardens today.
Due to its short lifespan after harvest, lettuce was originally sold relatively close to where it was grown. The early 20th century saw the development of new packing, storage and shipping technologies that improved the lifespan and transportability of lettuce and resulted in a significant increase in availability. During the 1950s, lettuce production was revolutionized with the development of
vacuum cooling, which allowed field cooling and packing of lettuce, replacing the previously used method of ice-cooling in packing houses outside the fields.
Lettuce is easy to grow, and as such has been a significant source of sales for many
seed companies. Tracing the history of many varieties is complicated by the practice of many companies, particularly in the US, of changing a variety's name from year to year. This practice is conducted for several reasons, the most prominent being to boost sales by promoting a "new" variety, or to prevent customers from knowing that the variety had been developed by a competing seed company. Documentation from the late 19th century shows between 65 and 140 distinct varieties of lettuce, depending on the amount of variation allowed between types – a distinct difference from the 1,100 named lettuce varieties on the market at the time. Names also often changed significantly from country to country. Although most lettuce grown today is used as a vegetable, a minor amount is used in the production of
tobacco-free cigarettes; however, domestic lettuce's wild relatives produce a leaf that visually more closely resembles tobacco.
[Katz and Weaver, p. 377.]
Cultivation

A
hardy annual, some varieties of lettuce can be
overwintered even in relatively cold climates under a layer of straw, and older,
heirloom varieties are often grown in
cold frame
In agriculture and gardening, a cold frame is a transparent-roofed enclosure, built low to the ground, used to protect plants from adverse weather, primarily excessive cold or wet. The transparent top admits sunlight and prevents heat escape via ...
s.
[Weaver, pp. 172–173.] Lettuces meant for the cutting of individual leaves are generally planted straight into the garden in thick rows. Heading varieties of lettuces are commonly started in flats, then
transplanted to individual spots, usually apart, in the garden after developing several leaves. Lettuce spaced farther apart receives more sunlight, which improves color and nutrient quantities in the leaves. Pale to white lettuce, such as the centers in some iceberg lettuce, contain few nutrients.

Lettuce grows best in full sun in loose,
nitrogen
Nitrogen is a chemical element; it has Symbol (chemistry), symbol N and atomic number 7. Nitrogen is a Nonmetal (chemistry), nonmetal and the lightest member of pnictogen, group 15 of the periodic table, often called the Pnictogen, pnictogens. ...
-rich soils with a
pH of between 6.0 and 6.8. Heat generally prompts lettuce to
bolt, with most varieties growing poorly above ; cool temperatures prompt better performance, with being preferred and as low as being tolerated.
[Bradley, p. 129.] Plants in hot areas that are provided partial shade during the hottest part of the day will bolt more slowly. Temperatures above will generally result in poor or non-existent germination of lettuce seeds.
After harvest, lettuce lasts the longest when kept at and 96 percent humidity. The high water content of lettuce (94.9 percent) creates problems when attempting to preserve the plant – it cannot be successfully frozen, canned or dried and must be eaten fresh.
In spite of its high water content, traditionally grown lettuce has a low
water footprint
A water footprint shows the extent of water use in relation to Consumption (economics), consumption by people. The water footprint of an individual, community, or business is defined as the total volume of fresh water used to produce the goods an ...
, with of water required for each kilogram of lettuce produced. Hydroponic growing methods can reduce this water consumption by nearly two orders of magnitude.
Lettuce varieties will
cross
A cross is a religious symbol consisting of two Intersection (set theory), intersecting Line (geometry), lines, usually perpendicular to each other. The lines usually run vertically and horizontally. A cross of oblique lines, in the shape of t ...
with each other, making spacing of between varieties necessary to prevent contamination when saving seeds. Lettuce will also cross with ''
Lactuca serriola'' (wild lettuce), with the resulting seeds often producing a plant with tough, bitter leaves.
Celtuce
Celtuce () (''Lactuca sativa'' var. ''augustana'',Masatoshi Yamaguchi ''angustata'', or ''asparagina''), also called stem lettuce, celery lettuce, asparagus lettuce, or Chinese lettuce, is a cultivar of lettuce grown primarily for its thick s ...
, a lettuce variety grown primarily in Asia for its stems, crosses easily with lettuces grown for their leaves.
[Weaver, pp. 175–176.] This propensity for crossing, however, has led to breeding programs using closely related species in ''Lactuca'', such as ''L. serriola'', ''
L. saligna'', and ''
L. virosa'', to broaden the available
gene pool
The gene pool is the set of all genes, or genetic information, in any population, usually of a particular species.
Description
A large gene pool indicates extensive genetic diversity, which is associated with robust populations that can survi ...
. Starting in the 1990s, such programs began to include more distantly related species such as ''
L. tatarica''.



Seeds keep best when stored in cool conditions, and, unless stored
cryogenically, remain viable the longest when stored at ; they are relatively short lived in storage.
At room temperature, lettuce seeds remain viable for only a few months. However, when newly harvested lettuce seed is stored cryogenically, this life increases to a
half-life Half-life is a mathematical and scientific description of exponential or gradual decay.
Half-life, half life or halflife may also refer to:
Film
* Half-Life (film), ''Half-Life'' (film), a 2008 independent film by Jennifer Phang
* ''Half Life: ...
of 500 years for vaporized nitrogen and 3,400 years for liquid nitrogen; this advantage is lost if seeds are not frozen promptly after harvesting.
Cultivars (varieties)
There are several types and
cultivar
A cultivar is a kind of Horticulture, cultivated plant that people have selected for desired phenotypic trait, traits and which retains those traits when Plant propagation, propagated. Methods used to propagate cultivars include division, root a ...
s of lettuce. Categorization may sometimes refer to "leaf" versus "head", but there are seven main cultivar groups of lettuce, each including many varieties:
# Leaf—Also known as looseleaf, cutting or bunching lettuce,
this type has loosely bunched leaves and is the most widely planted. It is used mainly for salads.
#*
Red leaf lettuce—A group of lettuce types with red leaves.
#
Romaine/Cos—Used mainly for salads and sandwiches, this type forms long, upright heads.
This is the most often used lettuce in
Caesar salad
A Caesar salad (also spelled Cesar, César and Cesare), also known as Caesar's salad, is a green salad of romaine lettuce and croutons dressed with lemon juice (or lime juice), olive oil, eggs, Worcestershire sauce, anchovies, garlic, Dijon ...
s.
#* Little Gem—a dwarf, compact romaine lettuce, popular in the UK.
# Iceberg/Crisphead—The most popular type in the United States. Iceberg lettuce is very heat-sensitive and was originally developed in 1894 for growth in the northern United States by
Burpee Seeds and Plants. It gets its name from the way it was transported in crushed ice, where the heads of lettuce looked like
iceberg
An iceberg is a piece of fresh water ice more than long that has broken off a glacier or an ice shelf and is floating freely in open water. Smaller chunks of floating glacially derived ice are called "growlers" or "bergy bits". Much of an i ...
s. Today, it ships well, but is low in flavor and nutritional content, being composed of even more water than other lettuce types.
# Butterhead—Also known as Boston or Bibb lettuce,
and traditionally in the
UK as "round lettuce", this type is a head lettuce with a loose arrangement of leaves, known for its sweet flavor and tender texture.
# Summercrisp—Also called Batavian or French crisp, this lettuce is midway between the crisphead and leaf types. These lettuces tend to be larger, bolt-resistant and well-flavored.
#
Celtuce
Celtuce () (''Lactuca sativa'' var. ''augustana'',Masatoshi Yamaguchi ''angustata'', or ''asparagina''), also called stem lettuce, celery lettuce, asparagus lettuce, or Chinese lettuce, is a cultivar of lettuce grown primarily for its thick s ...
/Stem—This type is grown for its seedstalk, rather than its leaves, and is used in Asian cooking, primarily Chinese, as well as stewed and creamed dishes.
# Oilseed—This type is grown for its seeds, which are pressed to extract an oil mainly used for cooking. It has few leaves, bolts quickly and produces seeds around 50 percent larger than other types of lettuce.
The four main types in the Western world have been looseleaf, romaine, crisphead, and butterhead, with the others being intermediary or more exotic. The butterhead and crisphead types are sometimes known together as "cabbage" lettuce, because their heads are shorter, flatter, and more cabbage-like than romaine lettuces.
Cultivation problems

Soil nutrient deficiencies can cause a variety of plant problems that range from malformed plants to a lack of head growth.
Many insects are attracted to lettuce, including
cutworm
Cutworms are moth larvae that hide under litter or soil during the day, coming out in the dark to feed on plants. A larva typically attacks the first part of the plant it encounters, namely the stem, often of a seedling, and consequently cuts it ...
s, which cut seedlings off at the soil line;
wireworm
Elateridae or click beetles (or "typical click beetles" to distinguish them from the related families Cerophytidae and Eucnemidae, which are also capable of clicking) are a family of beetles. Other names include elaters, snapping beetles, sp ...
s and
nematode
The nematodes ( or ; ; ), roundworms or eelworms constitute the phylum Nematoda. Species in the phylum inhabit a broad range of environments. Most species are free-living, feeding on microorganisms, but many are parasitic. Parasitic worms (h ...
s, which cause yellow, stunted plants;
tarnished plant bugs and
aphid
Aphids are small sap-sucking insects in the Taxonomic rank, family Aphididae. Common names include greenfly and blackfly, although individuals within a species can vary widely in color. The group includes the fluffy white Eriosomatinae, woolly ...
s, which cause yellow, distorted leaves;
leafhopper
Leafhopper is the common name for any species from the family (biology), family Cicadellidae: based on the type genus ''Cicadella''. These minute insects, colloquially known as hoppers, are plant feeders that suck plant sap from grass, shrubs, or ...
s, which cause stunted growth and pale leaves;
thrips, which turn leaves gray-green or silver;
leafminers, which create tunnels within the leaves;
flea beetle
The flea beetle is any small, jumping beetle of the leaf beetle family (Chrysomelidae) that is part of the tribe (biology), tribe Alticini, which is part of the subfamily Galerucinae. Historically the flea beetles were classified as their own ...
s, which cut small holes in leaves and
caterpillar
Caterpillars ( ) are the larval stage of members of the order Lepidoptera (the insect order comprising butterflies and moths).
As with most common names, the application of the word is arbitrary, since the larvae of sawflies (suborder ...
s,
slug
Slug, or land slug, is a common name for any apparently shell-less Terrestrial mollusc, terrestrial gastropod mollusc. The word ''slug'' is also often used as part of the common name of any gastropod mollusc that has no shell, a very reduced ...
s and
snail
A snail is a shelled gastropod. The name is most often applied to land snails, terrestrial molluscs, terrestrial pulmonate gastropod molluscs. However, the common name ''snail'' is also used for most of the members of the molluscan class Gas ...
s, which cut large holes in leaves. For example, the larvae of the
ghost moth
The ghost moth or ghost swift (''Hepialus humuli'') is a moth of the family Hepialidae. It is common throughout Europe, except for the far south-east.
Female ghost moths are larger than males, and exhibit sexual dimorphism with their differenc ...
is a common pest of lettuce plants.
Mammals, including
rabbit
Rabbits are small mammals in the family Leporidae (which also includes the hares), which is in the order Lagomorpha (which also includes pikas). They are familiar throughout the world as a small herbivore, a prey animal, a domesticated ...
s and
groundhog
The groundhog (''Marmota monax''), also known as the woodchuck, is a rodent of the family Sciuridae, belonging to the group of large ground squirrels known as marmots.
A lowland creature of North America, it is found through much of the Easte ...
s, also eat the plants.
[Bradley, pp. 129–132.] Lettuce contains several defensive compounds, including
sesquiterpene lactone
Sesquiterpene lactones (SLs) are a class of sesquiterpenoids that contain a lactone ring. They are most often found in plants of the family Asteraceae (daisies, asters). Other plant families with SLs are Umbelliferae (celery, parsley, carrots) ...
s, and other natural
phenolics such as
flavonol and
glycosides
In chemistry, a glycoside is a molecule in which a sugar is bound to another functional group via a glycosidic bond. Glycosides play numerous important roles in living organisms. Many plants store chemicals in the form of inactive glycosides. ...
, which help to protect it against pests. Certain varieties contain more than others, and some selective breeding and genetic modification studies have focused on using this trait to identify and produce commercial varieties with increased pest resistance.
Lettuce also suffers from several
viral diseases
A viral disease (or viral infection) occurs when an organism's body is invaded by pathogenic viruses, and infectious virus particles (virions) attach to and enter susceptible cells.
Examples include the common cold, gastroenteritis, COVID-19, ...
, including
big vein, which causes yellow, distorted leaves, and
mosaic virus, which is spread by aphids and causes stunted plant growth and deformed leaves.
Aster yellows are a disease-causing bacteria carried by leafhoppers, which causes deformed leaves. Fungal diseases include
powdery mildew
Powdery mildew is a fungus, fungal disease that affects a wide range of plants. Powdery mildew diseases are caused by many different species of Ascomycota, ascomycete fungi in the order Erysiphales. Powdery mildew is one of the easier plant disea ...
and
downy mildew
Downy mildew refers to any of several types of oomycete microbes that are obligate parasites of plants. Downy mildews exclusively belong to the Peronosporaceae family. In commercial agriculture, they are a particular problem for growers of c ...
, which cause leaves to mold and die and
bottom rot,
lettuce drop and
gray mold
''Botrytis cinerea'' is a necrotrophic fungus that affects many plant species, although its most notable hosts may be List of grape varieties, wine grapes. In viticulture, it is commonly known as "botrytis bunch rot"; in horticulture, it is usual ...
, which cause entire plants to rot and collapse.
Bacterial diseases include ''
Botrytis cinerea
''Botrytis cinerea'' is a necrotrophic fungus that affects many plant species, although its most notable hosts may be wine grapes. In viticulture, it is commonly known as "botrytis bunch rot"; in horticulture, it is usually called "grey mould" ...
'', for which
UV-C
Ultraviolet radiation, also known as simply UV, is electromagnetic radiation of wavelengths of 10–400 nanometers, shorter than that of visible light, but longer than X-rays. UV radiation is present in sunlight and constitutes about 10% of the ...
treatments may be used: Vàsquez et al. 2017 find that
phenylalanine ammonia-lyase
The enzyme phenylalanine ammonia lyase (EC 4.3.1.24) catalysis, catalyzes the conversion of L-phenylalanine to ammonia and cinnamic acid, ''trans''-cinnamic acid.:
:L-phenylalanine = ''trans''-cinnamate + NH3
Phenylalanine ammonia lyase (PAL) i ...
activity, phenolic production, and ''B. cinerea''
resistance are increased by UV-C.
Crowding lettuce tends to attract pests and diseases.
Weeds can also be an issue, as cultivated lettuce is generally not competitive with them, especially when directly seeded into the ground. Transplanted lettuce (started in flats and later moved to growing beds) is generally more competitive initially, but can still be crowded later in the season, causing misshapen lettuce and lower yields. Weeds also act as homes for insects and disease and can make harvesting more difficult. Herbicides are often used to control weeds in commercial production. However, this has led to the development of
herbicide-resistant weeds in lettuce cultivation.
Production
In 2022, world production of lettuce (report combined with
chicory) was 27 million tonnes, with
China
China, officially the People's Republic of China (PRC), is a country in East Asia. With population of China, a population exceeding 1.4 billion, it is the list of countries by population (United Nations), second-most populous country after ...
alone producing 55% of the total (table).
Lettuce is the only member of the genus ''Lactuca'' to be grown commercially. Although China is the top world producer of lettuce, the majority of the crop is consumed domestically.
Markets
Western Europe and North America were the original major markets for large-scale lettuce production. By the late 1900s, Asia, South America, Australia and Africa became more substantial markets. Different locations tended to prefer different types of lettuce, with butterhead prevailing in northern Europe and Great Britain, romaine in the Mediterranean and stem lettuce in China and Egypt. By the late 20th century, the preferred types began to change, with crisphead, especially iceberg, lettuce becoming the dominant type in northern Europe and Great Britain and more popular in western Europe. In the US, no one type predominated until the early 20th century, when crisphead lettuces began gaining popularity. After the 1940s, with the development of iceberg lettuce, 95 percent of the lettuce grown and consumed in the US was crisphead lettuce. By the end of the century, other types began to regain popularity and eventually made up over 30 percent of production.
[Katz and Weaver, p. 378.] Stem lettuce was first developed in China, where it remains primarily cultivated.

In the early 21st century, bagged salad products increased in the lettuce market, especially in the US where innovative packaging and shipping methods prolonged freshness.
In the United States in 2022, lettuce was the main vegetable ingredient in salads, and was the most consumed among leaf vegetables; its market was about 20% of all vegetables, with Romaine and iceberg having about equal sales. Some 85% of the lettuce consumed in the United States in 2022 was produced domestically.[
]
Uses
Culinary
As described around 50 AD, lettuce leaves were often cooked and served by the Romans with an oil-and-vinegar dressing; however, smaller leaves were sometimes eaten raw. During the 81–96 AD reign of Domitian
Domitian ( ; ; 24 October 51 – 18 September 96) was Roman emperor from 81 to 96. The son of Vespasian and the younger brother of Titus, his two predecessors on the throne, he was the last member of the Flavian dynasty. Described as "a r ...
, the tradition of serving a lettuce salad
A salad is a dish consisting of mixed ingredients, frequently vegetables. They are typically served chilled or at room temperature, though some can be served warm. Condiments called '' salad dressings'', which exist in a variety of flavors, a ...
before a meal began. Post-Roman Europe continued the tradition of poaching
Poaching is the illegal hunting or capturing of wild animals, usually associated with land use rights.
Poaching was once performed by impoverished peasants for subsistence purposes and to supplement meager diets. It was set against the huntin ...
lettuce, mainly with large romaine types, as well as the method of pouring a hot oil and vinegar mixture over the leaves.[Weaver, pp. 170–172.]
Today, the majority of lettuce is grown for its leaves, although one type is grown for its stem and one for its seeds, which are made into an oil. Most lettuce is used in salads, either alone or with other greens, vegetables, meats and cheeses. Romaine lettuce is often used for Caesar salad
A Caesar salad (also spelled Cesar, César and Cesare), also known as Caesar's salad, is a green salad of romaine lettuce and croutons dressed with lemon juice (or lime juice), olive oil, eggs, Worcestershire sauce, anchovies, garlic, Dijon ...
s. Lettuce leaves can also be found in soups, sandwiches and wraps, while the stems are eaten both raw and cooked.
The consumption of lettuce in China developed differently from in Western countries, due to health risks and cultural aversion to eating raw leaves; Chinese "salads" are composed of cooked vegetables and are served hot or cold. Lettuce is also used in a larger variety of dishes than in Western countries, contributing to a range of dishes including bean curd and meat dishes, soups and stir-frys plain or with other vegetables. Stem lettuce, widely consumed in China, is eaten either raw or cooked, the latter primarily in soups and stir-frys. Lettuce is also used as a primary ingredient in the preparation of lettuce soup.
Nutrition
Raw iceberg lettuce is 96% water, 3% carbohydrate
A carbohydrate () is a biomolecule composed of carbon (C), hydrogen (H), and oxygen (O) atoms. The typical hydrogen-to-oxygen atomic ratio is 2:1, analogous to that of water, and is represented by the empirical formula (where ''m'' and ''n'' ...
s, and contains negligible protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...
and fat
In nutrition science, nutrition, biology, and chemistry, fat usually means any ester of fatty acids, or a mixture of such chemical compound, compounds, most commonly those that occur in living beings or in food.
The term often refers specif ...
(table). In a reference amount of , iceberg lettuce supplies 14 calorie
The calorie is a unit of energy that originated from the caloric theory of heat. The large calorie, food calorie, dietary calorie, kilocalorie, or kilogram calorie is defined as the amount of heat needed to raise the temperature of one liter o ...
s and is a rich source (20% or more of the Daily Value
In the U.S. and Canada, the Reference Daily Intake (RDI) is used in nutrition labeling on food and dietary supplement products to indicate the daily intake level of a nutrient that is considered to be sufficient to meet the requirements of 97� ...
, DV) of vitamin K
Vitamin K is a family of structurally similar, fat-soluble vitamers found in foods and marketed as dietary supplements. The human body requires vitamin K for post-translational modification, post-synthesis modification of certain proteins ...
(20% DV), with no other micronutrient
Micronutrients are essential chemicals required by organisms in small quantities to perform various biogeochemical processes and regulate physiological functions of cells and organs. By enabling these processes, micronutrients support the heal ...
s in significant content (table).
In lettuce varieties with dark green leaves, such as romaine (also called ''cos''), vitamin A
Vitamin A is a fat-soluble vitamin that is an essential nutrient. The term "vitamin A" encompasses a group of chemically related organic compounds that includes retinol, retinyl esters, and several provitamin (precursor) carotenoids, most not ...
contents are appreciable due to the presence of the provitamin A
A provitamin is a substance that may be converted within the body to a vitamin. The term previtamin is a synonym.
The term "provitamin" is used when it is desirable to label a substance with little or no vitamin activity, but which can be converte ...
compound, beta-carotene. Dark green varieties of lettuce also contain moderate amounts of calcium
Calcium is a chemical element; it has symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to it ...
and iron
Iron is a chemical element; it has symbol Fe () and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, forming much of Earth's o ...
. The edible spine and ribs of the lettuce plant supply dietary fiber
Dietary fiber (fibre in English in the Commonwealth of Nations, Commonwealth English) or roughage is the portion of plant-derived food that cannot be completely broken down by human digestive enzymes. Dietary fibers are diverse in chemical co ...
, while micronutrients are contained in the leaf portion.
Food-borne illness
Food-borne pathogen
In biology, a pathogen (, "suffering", "passion" and , "producer of"), in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a Germ theory of d ...
s that can survive on lettuce include ''Listeria monocytogenes
''Listeria monocytogenes'' is the species of pathogenic bacteria that causes the infection listeriosis. It is a facultative anaerobic bacterium, capable of surviving in the presence or absence of oxygen. It can grow and reproduce inside the ho ...
'', the causative agent of listeriosis
Listeriosis is a bacterial infection most commonly caused by '' Listeria monocytogenes'', although '' L. ivanovii'' and '' L. grayi'' have been reported in certain cases. Listeriosis can cause severe illness, including severe sepsis, me ...
, which multiplies in storage. However, despite high levels of bacteria being found on ready-to-eat lettuce products, a 2008 study found no incidents of food-borne illness related to listeriosis, possibly due to the product's short shelf life
Shelf life is the length of time that a commodity may be stored without becoming unfit for use, consumption, or sale. In other words, it might refer to whether a commodity should no longer be on a pantry shelf (unfit for use), or no longer on a s ...
, indigenous microflora competing with the ''Listeria'' bacteria or inhibition of bacteria to cause listeriosis.
Other bacteria found on lettuce include '' Aeromonas'' species, which have not been linked to any outbreaks; ''Campylobacter
''Campylobacter'' is a type of bacteria that can cause a diarrheal disease in people. Its name means "curved bacteria", as the germ typically appears in a comma or "s" shape. According to its scientific classification, it is a genus of gram-negat ...
'' species, which cause campylobacteriosis; and '' Yersinia intermedia'' and '' Yersinia kristensenii'' (species of ''Yersinia
''Yersinia'' is a genus of bacteria in the family Yersiniaceae. ''Yersinia'' species are Gram-negative, coccobacilli bacteria, a few micrometers long and fractions of a micrometer in diameter, and are facultative anaerobes. Some members of '' ...
''), which have been found mainly in lettuce. ''Salmonella
''Salmonella'' is a genus of bacillus (shape), rod-shaped, (bacillus) Gram-negative bacteria of the family Enterobacteriaceae. The two known species of ''Salmonella'' are ''Salmonella enterica'' and ''Salmonella bongori''. ''S. enterica'' ...
'' bacteria, including the uncommon ''Salmonella braenderup'' type, have also caused outbreaks traced to contaminated lettuce. Viruses, including hepatitis A
Hepatitis A is an infectious liver disease caused by Hepatitis A virus (HAV); it is a type of viral hepatitis. Many cases have few or no symptoms, especially in the young. The time between infection and symptoms, in those who develop them, is ...
, calicivirus and a Norwalk-like strain, have been found in lettuce. The vegetable has also been linked to outbreaks of parasitic infestations, including ''Giardia lamblia
''Giardia duodenalis'', also known as ''Giardia intestinalis'' and ''Giardia lamblia'', is a flagellated Parasitism, parasitic protozoan microorganism of the genus ''Giardia'' that colonizes the small intestine, causing a diarrheal condition kn ...
''.
Lettuce has been linked to numerous outbreaks of the bacteria ''E.coli'' O157:H7 and ''Shigella
''Shigella'' is a genus of bacteria that is Gram negative, facultatively anaerobic, non–spore-forming, nonmotile, rod shaped, and is genetically nested within ''Escherichia''. The genus is named after Kiyoshi Shiga, who discovered it in 1 ...
''; the plants were most likely contaminated through contact with animal or human feces. A 2007 study determined that the vacuum cooling method, especially prevalent in the California lettuce industry, increased the uptake and survival rates of ''E. coli'' O157:H7. Scientific experiments using treated municipal wastewater
Sewage (or domestic sewage, domestic wastewater, municipal wastewater) is a type of wastewater that is produced by a community of people. It is typically transported through a sewer system. Sewage consists of wastewater discharged from residen ...
as irrigation for romaine lettuce have shown that the contamination levels of foliage, leachate, and soil with ''E. coli'' and bacteriophage AP205 (used by researchers as a surrogate for enteric viruses), respectively, were directly correlated with the presence of these organisms in the irrigation water.
Due to the increase in food demand, the use of treated wastewater effluent for irrigation and animal or human excreta (i.e., manure or biosolids) as soil amendments is increasing. As such, so are the outbreaks of food-borne illnesses. Due to the overuse of antibiotics in farming, the number of pathogens resistant to antibiotics is increasing, one of these being ''AR E.coli'', which has been found on lettuce irrigated with wastewater.
Pathogens found on lettuce are not specific to lettuce (though some ''E. coli'' strains have affinity for Romaine). But, unlike other vegetables which tend to be cooked, lettuce is eaten raw, thus food-borne outbreaks associated with it are more frequent and affect a larger number of people.
References
Cited literature
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External links
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Crops
Crops originating from Egypt
sativa
Sativa, sativus, and sativum are Latin botanical adjectives meaning '' cultivated''. It is often associated botanically with plants that promote good health and used to designate certain seed-grown domestic crops.
Usage
''Sativa'' (ending in -a) ...