An onion (''Allium cepa'' L., from Latin ''cepa'' meaning "onion"), also known as the bulb onion or common onion, is a vegetable that is the most widely cultivated species of the genus '' Allium''. The shallot is a botanical variety of the onion which was classified as a separate species until 2010. Its close relatives include

garlic Garlic (''Allium sativum'') is a species of bulbous flowering plant in the genus ''Allium''. Its close relatives include the onion, shallot, leek, chive, Allium fistulosum, Welsh onion and Allium chinense, Chinese onion. It is native to South A ...

, scallion,

leek The leek is a vegetable, a cultivar of ''Allium ampeloprasum'', the broadleaf wild leek ( syn. ''Allium porrum''). The edible part of the plant is a bundle of leaf sheaths that is sometimes erroneously called a stem or stalk. The genus ''Alli ...

, and chive. This genus also contains several other species variously referred to as onions and cultivated for food, such as the Japanese bunching onion (''Allium fistulosum''), the tree onion (''A.'' × ''proliferum''), and the Canada onion (''Allium canadense''). The name ''

wild onion Wild onion can refer to * any uncultivated species in the genus ''Allium'', especially: **''Allium bisceptrum'' ** ''Allium canadense'' ** ''Allium tricoccum'' ** ''Allium validum'' ** ''Allium vineale'' * ''Asphodelus tenuifolius'' * ''Cyperus bulb ...

'' is applied to a number of ''Allium'' species, but ''A. cepa'' is exclusively known from cultivation. Its ancestral wild original form is not known, although escapes from cultivation have become established in some regions. The onion is most frequently a biennial or a perennial plant, but is usually treated as an annual and harvested in its first growing season. The onion plant has a fan of hollow, bluish-green leaves and its

bulb In botany, a bulb is structurally a short stem with fleshy leaves or leaf basesBell, A.D. 1997. ''Plant form: an illustrated guide to flowering plant morphology''. Oxford University Press, Oxford, U.K. that function as food storage organs duri ...

at the base of the plant begins to swell when a certain day-length is reached. The bulbs are composed of shortened, compressed, underground stems surrounded by fleshy modified scale (leaves) that envelop a central bud at the tip of the stem. In the

autumn Autumn, also known as fall in American English and Canadian English, is one of the four temperate seasons on Earth. Outside the tropics, autumn marks the transition from summer to winter, in September ( Northern Hemisphere) or March ( Sou ...

(or in spring, in the case of overwintering onions), the foliage dies down and the outer layers of the bulb become more dry and brittle. The crop is harvested and dried and the onions are ready for use or storage. The crop is prone to attack by a number of pests and diseases, particularly the

onion fly ''Delia antiqua'', commonly known as the onion fly, is a cosmopolitan pest of crops. The larvae or maggots feed on onions, garlic, and other bulbous plants. Morphology and biology The onion fly has an ash-grey body and resembles a housefly. Th ...

, the onion eelworm, and various fungi which can cause rotting. Some varieties of ''A. cepa'', such as shallots and potato onions, produce multiple bulbs. Onions are cultivated and used around the world. As a food item, they are usually served cooked, as a vegetable or part of a prepared savoury dish, but can also be eaten raw or used to make pickles or chutneys. They are pungent when chopped and contain certain chemical substances which may irritate the eyes.

Taxonomy and etymology

The onion plant (''Allium cepa''), also known as the bulb onion or common onion, is the most widely cultivated species of the genus '' Allium.''Eric Block, "Garlic and Other ''Alliums'': The Lore and the Science" (Cambridge: Royal Society of Chemistry, 2010) It was first officially described by Carl Linnaeus in his 1753 work ''

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 genera. It is the first work to consistently apply binomial names and was the ...

''. A number of synonyms have appeared in its taxonomic history: *''Allium cepa'' var. ''aggregatum'' – G. Don *''Allium cepa'' var. ''bulbiferum'' – Regel *''Allium cepa'' var. ''cepa'' – Linnaeus *''Allium cepa'' var. ''multiplicans'' – L.H. Bailey *''Allium cepa'' var. ''proliferum'' – (Moench) Regel *''Allium cepa'' var. ''solaninum'' – Alef *''Allium cepa'' var. ''viviparum'' – (Metz) Mansf. ''A. cepa'' is known exclusively from cultivation, but related wild species occur in Central Asia and Iran. The most closely related species include '' A. vavilovii'' from Turkmenistan and '' A. asarense'' from Iran.Grubben, G.J.H.; Denton, O.A. (2004) Plant Resources of Tropical Africa 2. Vegetables. PROTA Foundation, Wageningen; Backhuys, Leiden; CTA, Wageningen. The vast majority of cultivars of ''A. cepa'' belong to the common onion group (''A. cepa'' var. ''cepa'') and are usually referred to simply as onions. The Aggregatum Group of cultivars (''A. cepa'' var. ''aggregatum'') includes both shallots and potato onions. The genus ''Allium'' also contains a number of other species variously referred to as onions and cultivated for food, such as the Japanese bunching onion (''A. fistulosum''), Egyptian onion (''A. × proliferum''), and Canada onion (''A. canadense''). ''Cepa'' is commonly accepted as Latin for "onion" and has an affinity with Ancient Greek: κάπια (''kápia'') and

Albanian Albanian may refer to: *Pertaining to Albania in Southeast Europe; in particular: **Albanians, an ethnic group native to the Balkans **Albanian language **Albanian culture **Demographics of Albania, includes other ethnic groups within the country ...

: ''qepë'' and is ancestral to Aromanian: ''tseapã'',

Catalan Catalan may refer to: Catalonia From, or related to Catalonia: * Catalan language, a Romance language * Catalans, an ethnic group formed by the people from, or with origins in, Northern or southern Catalonia Places * 13178 Catalan, asteroid #1 ...

: ''ceba'',

Occitan Occitan may refer to: * Something of, from, or related to the Occitania territory in parts of France, Italy, Monaco and Spain. * Something of, from, or related to the Occitania administrative region of France. * Occitan language Occitan (; o ...

: ''ceba'', Portuguese: ''cebola'', Spanish: ''cebolla'', Italian: ''cipolla'', and

Romanian Romanian may refer to: *anything of, from, or related to the country and nation of Romania **Romanians, an ethnic group **Romanian language, a Romance language ***Romanian dialects, variants of the Romanian language **Romanian cuisine, traditional ...

: ''ceapă''. The English word " chive" is also derived from the Old French ''cive'', which derived from ''cepa''.

Description

The onion has been grown and selectively bred in cultivation for at least 7,000 years. It is a

biennial plant A biennial plant is a flowering plant that, generally in a temperate climate, takes two years to complete its biological life cycle. Life cycle In its first year, the biennal plant undergoes primary growth, during which its vegetative structures ...

but is usually grown as an annual. Modern varieties typically grow to a height of . The leaves are yellowish- to bluish green and grow alternately in a flattened, fan-shaped swathe. They are fleshy, hollow, and cylindrical, with one flattened side. They are at their broadest about a quarter of the way up, beyond which they taper to blunt tips. The base of each leaf is a flattened, usually white sheath that grows out of the basal plate of a

. From the underside of the plate, a bundle of fibrous roots extends for a short way into the soil. As the onion matures, food reserves accumulate in the leaf bases, and the bulb of the onion swells. Onion Flower Bangladesh

In the autumn, the leaves die back, and the outer scales of the bulb become dry and brittle, so the crop is normally harvested. If left in the soil over winter, the growing point in the middle of the bulb begins to develop in the spring. New leaves appear, and a long, stout, hollow stem expands, topped by a bract protecting a developing inflorescence. The inflorescence takes the form of a rounded umbel of white flowers with parts in sixes. The seeds are glossy black and triangular in cross-section. The average pH of an onion is around 5.5.

History

An onion, woodcut, 1547 Wellcome L0029214

The history of ancestral onion species is not well documented. Ancient records of onion use spans western and eastern Asia, so the geographic origin of the onion is uncertain. Yet, domestication likely took place in West or Central Asia. Onions have been variously described as having originated in Iran, western Pakistan and Central Asia. Traces of onions recovered from Bronze Age settlements in China suggest that onions were used as far back as 5000 BC, not only for their flavour, but also for the bulb's durability in storage and transport. Ancient Egyptians revered the onion bulb, viewing its spherical shape and concentric rings as symbols of eternal life. Onions were used in Egyptian burials, as evidenced by onion traces found in the eye sockets of Ramesses IV. Pliny the Elder of the first century AD wrote about the use of onions and cabbage in

Pompeii Pompeii (, ) was an ancient city located in what is now the ''comune'' of Pompei near Naples in the Campania region of Italy. Pompeii, along with Herculaneum and many villas in the surrounding area (e.g. at Boscoreale, Stabiae), was buried ...

. He documented Roman beliefs about the onion's ability to improve ocular ailments, aid in sleep, and heal everything from oral sores and toothaches to dog bites, lumbago, and even dysentery. Archaeologists unearthing Pompeii long after its 79 AD volcanic burial have found gardens resembling those in Pliny's detailed narratives. According to texts collected in the fifth/sixth century AD under the authorial aegis of "Apicius" (said to have been a gourmet), onions were used in many Roman recipes. In the Age of Discovery, onions were taken to

North America North America is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and the Car ...

by the first European settlers, who found close relatives of the plant such as '' Allium tricoccum'' readily available and widely used in Native American gastronomy. According to diaries kept by certain first English colonists, the bulb onion was one of the first crops planted by the Pilgrim fathers.

Uses

Onion types and products

Common onions are normally available in three colour varieties: * Yellow or brown onions are sweeter and are the onions of choice for everyday use in

European cuisine European cuisine comprises the cuisines of Europe "European Cuisine."Vidalia, Walla Walla, Cévennes, "Bermuda," etc.). Yellow onions turn a rich, dark brown when caramelised and give French onion soup its sweet flavour. * Red or purple onions are known for their sharp pungent flavor and are the onions of choice for everyday use in Asian cuisine. They are also used raw and in

grilling Grilling is a form of cooking that involves dry heat applied to the surface of food, commonly from above, below or from the side. Grilling usually involves a significant amount of direct, radiant heat, and tends to be used for cooking meat a ...

. * White onions are traditional in classic

Mexican cuisine Mexican cuisine consists of the cooking cuisines and traditions of the modern country of Mexico. Its earliest roots lie in Mesoamerican cuisine. Its ingredients and methods begin with the first agricultural communities such as the Olmec and M ...

, and are milder in flavor; they have a golden colour when cooked and a particularly sweet flavour when sautéed. Tranby house 31 gnangarra

While the large, mature onion bulb is most often eaten, onions can be eaten at immature stages. Young plants may be harvested before bulbing occurs and used whole as spring onions or scallions. When an onion is harvested after bulbing has begun, but the onion is not yet mature, the plants are sometimes referred to as "summer" onions. Additionally, onions may be bred and grown to mature at smaller sizes. Depending on the mature size and the purpose for which the onion is used, these may be referred to as pearl, boiler, or pickler onions, but differ from true

pearl onion The pearl onion (''Allium ampeloprasum'' var. ''sectivum'' or ''A. ampeloprasum'' 'Pearl-Onion Group'), also known as button, baby or silverskin onions in the UK, is a close relative of the leek (''A. ampeloprasum'' var. ''porrum''), and may be d ...

s which are a different species. Pearl and boiler onions may be cooked as a vegetable rather than as an ingredient and pickler onions are often preserved in vinegar as a long-lasting relish. Onions are available in fresh, frozen, canned, caramelised, pickled, and chopped forms. The dehydrated product is available as

kibbled A mill is a device, often a structure, machine or kitchen appliance, that breaks solid materials into smaller pieces by grinding, crushing, or cutting. Such comminution is an important unit operation in many processes. There are many different ...

, sliced, ring, minced, chopped, granulated, and powder forms. Onion powder is a seasoning widely used when the fresh ingredient is not available. It is made from finely ground, dehydrated onions, mainly the pungent varieties of bulb onions, and has a strong odour. Being dehydrated, it has a long shelf life and is available in several varieties: yellow, red, and white.

Culinary uses

Onions are commonly chopped and used as an ingredient in various hearty warm dishes, and may also be used as a main ingredient in their own right, for example in French onion soup, creamed onions, and onion chutney. They are versatile and can be baked, boiled, braised, grilled, fried, roasted, sautéed, or eaten raw in salads. Their layered nature makes them easy to hollow out once cooked, facilitating stuffing them, as in Turkish

sogan-dolma Dolma (Turkish for “stuffed”) is a family of stuffed dishes associated with Ottoman cuisine, and common in modern national cuisines of regions and countries that once were part of the Ottoman Empire. Some types of dolma are made with whol ...

. Onions pickled in vinegar are eaten as a

snack A snack is a small portion of food generally eaten between meals. Snacks come in a variety of forms including packaged snack foods and other processed foods, as well as items made from fresh ingredients at home. Traditionally, snacks are p ...

around the world, and as a side serving in pubs and

fish and chip shops A fish and chip shop, sometimes referred to as a chip shop, is a (often fast food) restaurant that specialises in selling fish and chips. Usually, fish and chip shops provide takeaway service, although some have seating facilities. Fish and c ...

throughout the United Kingdom and the Commonwealth. They are part of a traditional British pub's

ploughman's lunch A ploughman's lunch is an English cold meal based around bread, cheese, and fresh or pickled onions.Hessayon, ''The new vegetable and herb expert'', 2014, p. 73 Additional items can be added such as ham, green salad, hard boiled eggs, and ...

, usually served with crusty

bread Bread is a staple food prepared from a dough of flour (usually wheat) and water, usually by baking. Throughout recorded history and around the world, it has been an important part of many cultures' diet. It is one of the oldest human-made f ...

, English

cheese Cheese is a dairy product produced in wide ranges of flavors, textures, and forms by coagulation of the milk protein casein. It comprises proteins and fat from milk, usually the milk of cows, buffalo, goats, or sheep. During production, ...

, and

ale Ale is a Type of beer, type of beer brewed using a Warm fermentation, warm fermentation method, resulting in a sweet, full-bodied and fruity taste. Historically, the term referred to a drink brewed without hops. As with most beers, ale typicall ...

. Similar to

, onions can show an additional colour – pink-red – after cutting, an effect caused by reactions of amino acids with sulfur compounds.

Non-culinary uses

Onion oil is authorised for use in the European Union and United Kingdom for use as a pesticide against

carrot fly The carrot fly (''Chamaepsila rosae'') is a pest of gardens and farms, and mainly affects the crop of carrots, but can also attack parsnips, parsley and celery. It is a member of the family Psilidae (order Diptera). Larvae Crop damage is caused ...

in umbelliferous crops (carrots, parsnips, parsley, celery, celeriac). Onions also contain one of the natural oils sometimes used in

hair oil Hair oil is an oil-based cosmetic product intended to improve the condition of hair. Various types of oils may be included in hair oil products. These often purport to aid with hair growth, dryness, or damage. History Ancient Egyptians paid spec ...

. Onions have particularly large

cells Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Locations * Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery w ...

that are readily observed under low magnification. Forming a single layer of cells, the bulb epidermis is easy to separate for educational, experimental, and

breeding Breeding is sexual reproduction that produces offspring, usually animals or plants. It can only occur between a male and a female animal or plant. Breeding may refer to: * Animal husbandry, through selected specimens such as dogs, horses, and rab ...

purposes. Onions are therefore commonly used in science education to teach the use of a microscope for observing cell structure. Onions are toxic to dogs, cats, guinea pigs, and many other animals. Onion skins can be boiled to make an orange-brown dye. Onions contain quercetin, a plant pigment which has

antioxidant Antioxidants are compounds that inhibit oxidation, a chemical reaction that can produce free radicals. This can lead to polymerization and other chain reactions. They are frequently added to industrial products, such as fuels and lubricant ...

properties '' in vitro''. The results of a Japanese study suggest that onions help in reducing oxidative stress caused by diabetes. Another study conducted in

China China, officially the People's Republic of China (PRC), is a country in East Asia. It is the world's most populous country, with a population exceeding 1.4 billion, slightly ahead of India. China spans the equivalent of five time zones and ...

concluded that onion juice reduces oxidative stress and can be recommended to patients dealing with bone problems like

osteoporosis Osteoporosis is a systemic skeletal disorder characterized by low bone mass, micro-architectural deterioration of bone tissue leading to bone fragility, and consequent increase in fracture risk. It is the most common reason for a broken bone ...

Composition

Nutrients

Most onion cultivars are about 89% water, 9% carbohydrates (including 4% sugar and 2%

dietary fibre Dietary fiber (in British English fibre) 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 composition, and can be grouped generally by the ...

), 1% protein, and negligible fat (table). Onions contain low amounts of

essential nutrient A nutrient is a substance used by an organism to survive, grow, and reproduce. The requirement for dietary nutrient intake applies to animals, plants, fungi, and protists. Nutrients can be incorporated into cells for metabolic purposes or excret ...

s and have an energy value of 166 kJ (40

kilocalorie The calorie is a unit of energy. For historical reasons, two main definitions of "calorie" are in wide use. The large calorie, food calorie, or kilogram calorie was originally defined as the amount of heat needed to raise the temperature of on ...

s) in a 100 g (3.5 oz) amount. Onions contribute savoury flavour to dishes without contributing significant caloric content.

Phytochemicals

Considerable differences exist between onion varieties in

phytochemical Phytochemicals are chemical compounds produced by plants, generally to help them resist fungi, bacteria and plant virus infections, and also consumption by insects and other animals. The name comes . Some phytochemicals have been used as poisons ...

content, particularly for polyphenols, with shallots having the highest level, six times the amount found in

Vidalia onion A Vidalia onion ( or ) is one of several varieties of sweet onion grown in a production area defined by law of the U.S. state of Georgia since 1986 and the United States Code of Federal Regulations (CFR). Varieties include the hybrid Yellow Gr ...

s. Yellow onions have the highest total

flavonoid Flavonoids (or bioflavonoids; from the Latin word ''flavus'', meaning yellow, their color in nature) are a class of polyphenolic secondary metabolites found in plants, and thus commonly consumed in the diets of humans. Chemically, flavonoids ...

content, an amount 11 times higher than in white onions. Red onions have considerable content of

anthocyanin Anthocyanins (), also called anthocyans, are water-soluble vacuolar pigments that, depending on their pH, may appear red, purple, blue, or black. In 1835, the German pharmacist Ludwig Clamor Marquart gave the name Anthokyan to a chemical compo ...

pigments, with at least 25 different compounds identified representing 10% of total flavonoid content. Onion polyphenols are under basic research to determine their possible biological properties in humans. Some people suffer from allergic reactions after handling onions. Symptoms can include contact dermatitis, intense itching,

rhinoconjunctivitis Rhinitis, also known as coryza, is irritation and inflammation of the mucous membrane inside the nose. Common symptoms are a stuffy nose, runny nose, sneezing, and post-nasal drip. The inflammation is caused by viruses, bacteria, irritants or ...

, blurred vision, bronchial asthma, sweating, and anaphylaxis. Allergic reactions may not occur when eating cooked onions, possibly due to the denaturing of the proteins from cooking.

Eye irritation

Freshly cut onions often cause a stinging sensation in the eyes of people nearby, and often uncontrollable tears. This is caused by the release of a volatile liquid, ''syn''-propanethial-S-oxide and its

aerosol An aerosol is a suspension (chemistry), suspension of fine solid particles or liquid Drop (liquid), droplets in air or another gas. Aerosols can be natural or Human impact on the environment, anthropogenic. Examples of natural aerosols are fog o ...

, which stimulates nerves in the eye. This gas is produced by a chain of reactions which serve as a defence mechanism: chopping an onion causes damage to

which releases

enzymes Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different molecule ...

called alliinases. These break down amino acid

sulfoxide In organic chemistry, a sulfoxide, also called a sulphoxide, is an organosulfur compound containing a sulfinyl () functional group attached to two carbon atoms. It is a polar functional group. Sulfoxides are oxidized derivatives of sulfides. E ...

s and generate

sulfenic acid In chemistry, a sulfenic acid is an organosulfur compound and oxoacid with the general formula . It is the first member of the family of organosulfur oxoacids, which also include sulfinic acids () and sulfonic acids (), respectively. The base ...

s. A specific sulfenic acid, 1-propenesulfenic acid, is rapidly acted on by a second enzyme, the lacrimatory factor synthase (LFS), producing the ''syn''-propanethial-S-oxide. This gas diffuses through the air and soon reaches the eyes, where it activates sensory neurons. Lacrimal glands produce tears to dilute and flush out the irritant. Eye irritation can be avoided by cutting onions under running water or submerged in a basin of water. Leaving the root end intact also reduces irritation as the onion base has a higher concentration of sulphur compounds than the rest of the bulb. Refrigerating the onions before use reduces the enzyme reaction rate and using a fan can blow the gas away from the eyes. The more often one chops onions, the less one experiences eye irritation. The amount of sulfenic acids and lacrimal factor released and the irritation effect differs among ''Allium'' species. In 2008, the New Zealand Institute for Crop and Food Research created "no tears" onions by genetic modification to prevent the synthesis of lachrymatory factor synthase in onions. One study suggests that consumers prefer the flavor of onions with lower LFS content. Since the process impedes sulfur ingestion by the plant, some find LFS− onions inferior in flavor. A method for efficiently differentiating LFS− and LFS+ onions has been developed based on

mass spectrometry Mass spectrometry (MS) is an analytical technique that is used to measure the mass-to-charge ratio of ions. The results are presented as a ''mass spectrum'', a plot of intensity as a function of the mass-to-charge ratio. Mass spectrometry is use ...

, with potential application in high-volume production;

gas chromatography Gas chromatography (GC) is a common type of chromatography used in analytical chemistry for separating and analyzing compounds that can be vaporized without decomposition. Typical uses of GC include testing the purity of a particular substance, ...

is also used to measure lachrymatory factor in onions. In early 2018,

Bayer Bayer AG (, commonly pronounced ; ) is a German multinational corporation, multinational pharmaceutical and biotechnology company and one of the largest pharmaceutical companies in the world. Headquartered in Leverkusen, Bayer's areas of busi ...

released the first crop yield of commercially available LFS-silenced onions under the name "Sunions." They were the product of 30 years of cross-breeding; genetic modification was not employed. Guinea hen weed and honey garlic contain a similar lachrymatory factor. Synthetic onion lachrymatory factor has been used in a study related to tear production, and has been proposed as a nonlethal deterrent against thieves and intruders. Onion mother plants (bulbs) for production seeds

Onion mother plants (bulbs) for production seeds

Cultivation

Onions are best cultivated in fertile soils that are well-drained. Sandy loams are good as they are low in sulphur, while clayey soils usually have a high sulphur content and produce pungent bulbs. Onions require a high level of nutrients in the soil. Phosphorus is often present in sufficient quantities, but may be applied before planting because of its low level of availability in cold soils. Nitrogen and potash can be applied at regular intervals during the growing season, the last application of nitrogen being at least four weeks before harvesting. Bulbing onions are day-length sensitive; their bulbs begin growing only after the number of daylight hours has surpassed some minimal quantity. Most traditional European onions are referred to as "long-day" onions, producing bulbs only after 14 hours or more of daylight occurs. Southern European and North African varieties are often known as "intermediate-day" types, requiring only 12–13 hours of daylight to stimulate bulb formation. "Short-day" onions, which have been developed in more recent times, are planted in mild-winter areas in the autumn and form bulbs in the early spring, and require only 11–12 hours of daylight to stimulate bulb formation. Onions are a cool-weather crop and can be grown in

USDA zones A hardiness zone is a geographic area defined as having a certain average annual minimum temperature, a factor relevant to the survival of many plants. In some systems other statistics are included in the calculations. The original and most wide ...

3 to 9. Hot temperatures or other stressful conditions cause them to "

bolt The BOLT Browser was a web browser for mobile phones including feature phones and smartphones that can run Java ME applications. The BOLT Browser was offered free of charge to consumers and by license to mobile network operators and handset manuf ...

", meaning that a flower stem begins to grow. Onions may be grown from seeds or from partially grown bulbs called "sets". Because onion seeds are short-lived, fresh seeds germinate more effectively when sown in shallow rows, or "drills," with each drill 12" to 18" apart. In suitable climates, certain cultivars can be sown in late summer and autumn to overwinter in the ground and produce early crops the following year. Onion bulbs are produced by sowing seeds in a dense pattern in early summer, then harvested in the autumn when the bulbs are still small, followed by drying and storage. These bulbs planted the following spring grow into mature bulbs later in the growing season. Certain cultivars used for growing and storing bulbs may not have such good storage characteristics as those grown directly from seed. Routine care during the growing season involves keeping the rows free of competing weeds, especially when the plants are young. The plants are shallow-rooted and do not need much water when established. Bulbing usually takes place after 12 to 18 weeks. The bulbs can be gathered when needed to eat fresh, but if they will be stored, they are harvested after the leaves have died back naturally. In dry weather, they can be left on the surface of the soil for a few days for drying, then placed in nets, roped into strings, or laid in layers in shallow boxes. They are stored effectively in a well-ventilated, cool place.

Pests and diseases

Onions suffer from a number of plant disorders. The most serious for the home gardener are likely to be the onion fly, stem and bulb eelworm, white rot, and neck rot. Diseases affecting the foliage include rust and smut, downy mildew, and white tip disease. The bulbs may be affected by splitting, white rot, and neck rot. Shanking is a condition in which the central leaves turn yellow and the inner part of the bulb collapses into an unpleasant-smelling slime. Most of these disorders are best treated by removing and burning affected plants. The larvae of the onion leaf miner or leek moth (''

Acrolepiopsis assectella The leek moth or onion leaf miner (''Acrolepiopsis assectella'') is a species of moth of family Acrolepiidae (formerly Glyphipterigidae) and the genus '' Acrolepiopsis''. The species is native to Europe and Siberia, but is also found in North A ...

'') sometimes attack the foliage and may burrow down into the bulb. The onion fly (''

Delia antiqua ''Delia antiqua'', commonly known as the onion fly, is a cosmopolitan pest of crops. The larvae or maggots feed on onions, garlic, and other bulbous plants. Morphology and biology The onion fly has an ash-grey body and resembles a housefly. Th ...

'') lays eggs on the leaves and stems and on the ground close to onion, shallot, leek, and garlic plants. The fly is attracted to the crop by the smell of damaged tissue and is liable to occur after thinning. Plants grown from sets are less prone to attack. The larvae tunnel into the bulbs and the foliage wilts and turns yellow. The bulbs are disfigured and rot, especially in wet weather. Control measures may include crop rotation, the use of seed dressings, early sowing or planting, and the removal of infested plants. The onion eelworm ('' Ditylenchus dipsaci''), a tiny parasitic soil-living

nematode The nematodes ( or grc-gre, Νηματώδη; la, Nematoda) or roundworms constitute the phylum Nematoda (also called Nemathelminthes), with plant-Parasitism, parasitic nematodes also known as eelworms. They are a diverse animal phylum inhab ...

, causes swollen, distorted foliage. Young plants are killed and older ones produce soft bulbs. No cure is known and affected plants should be uprooted and burned. The site should not be used for growing onions again for several years and should also be avoided for growing

carrot The carrot ('' Daucus carota'' subsp. ''sativus'') is a root vegetable, typically orange in color, though purple, black, red, white, and yellow cultivars exist, all of which are domesticated forms of the wild carrot, ''Daucus carota'', nat ...

s, parsnips, and

bean A bean is the seed of several plants in the family Fabaceae, which are used as vegetables for human or animal food. They can be cooked in many different ways, including boiling, frying, and baking, and are used in many traditional dishes th ...

s, which are also susceptible to the eelworm. White rot of onions, leeks, and garlic is caused by the soil-borne fungus ''

Sclerotium cepivorum ''Stromatinia cepivora'' is a fungus in the division Ascomycota. It is the teleomorph of ''Sclerotium cepivorum'', the cause of white rot in onions, garlic, and leeks. The infective sclerotia remain viable in the soil for many years and are s ...

''. As the roots rot, the foliage turns yellow and wilts. The bases of the bulbs are attacked and become covered by a fluffy white mass of mycelia, which later produces small, globular black structures called sclerotia. These resting structures remain in the soil to reinfect a future crop. No cure for this fungal disease exists, so affected plants should be removed and destroyed and the ground used for unrelated crops in subsequent years. Neck rot is a fungal disease affecting onions in storage. It is caused by '' Botrytis allii'', which attacks the neck and upper parts of the bulb, causing a grey mould to develop. The symptoms often first occur where the bulb has been damaged and spread down the affected scales. Large quantities of

spore In biology, a spore is a unit of sexual or asexual reproduction that may be adapted for dispersal and for survival, often for extended periods of time, in unfavourable conditions. Spores form part of the life cycles of many plants, algae, f ...

s are produced and crust-like sclerotia may also develop. In time, a dry rot sets in and the bulb becomes a dry, mummified structure. This disease may be present throughout the growing period, but only manifests itself when the bulb is in storage. Antifungal seed dressings are available and the disease can be minimised by preventing physical damage to the bulbs at harvesting, careful drying and curing of the mature onions, and correct storage in a cool, dry place with plenty of circulating air.

Production

In 2020, world production of onions and shallots (as green produce) was 4.5 million tonnes, led by the People's Republic of China with 20% of the world total, and Mali,

Japan Japan ( ja, 日本, or , and formally , ''Nihonkoku'') is an island country in East Asia. It is situated in the northwest Pacific Ocean, and is bordered on the west by the Sea of Japan, while extending from the Sea of Okhotsk in the north ...

, and South Korea as secondary producers.

Storage

In the home

Cooking onions and sweet onions are better stored at

room temperature Colloquially, "room temperature" is a range of air temperatures that most people prefer for indoor settings. It feels comfortable to a person when they are wearing typical indoor clothing. Human comfort can extend beyond this range depending on ...

, optimally in a single layer, in large mesh bags in a dry, cool, dark, well-ventilated location. In this environment, cooking onions have a shelf life of three to four weeks and sweet onions one to two weeks. Cooking onions will absorb odours from apples and pears. Also, they draw moisture from vegetables with which they are stored which may cause them to

decay Decay may refer to: Science and technology * Bit decay, in computing * Software decay, in computing * Distance decay, in geography * Decay time (fall time), in electronics Biology * Decomposition of organic matter * Tooth decay (dental caries) ...

. Sweet onions have a greater water and sugar content than cooking onions. This makes them sweeter and milder tasting, but reduces their shelf life. Sweet onions can be stored refrigerated; they have a shelf life of around 1 month. Irrespective of type, any cut pieces of onion are best tightly wrapped, stored away from other produce, and used within two to three days.

Varieties

Common onion group (var. ''cepa'')

Most of the diversity within ''A. cepa'' occurs within this group, the most economically important ''Allium'' crop. Plants within this group form large single bulbs, and are grown from seed or seed-grown sets. The majority of cultivated varieties grown for dry bulbs, salad onions, and pickling onions belong to this group. The range of diversity found among these cultivars includes variation in photoperiod (length of day that triggers bulbing), storage life, flavour, and skin colour. Common onions range from the pungent varieties used for dried soups and onion powder to the mild and hearty

sweet onion A sweet onion is a variety of onion that is not pungent. Their mildness is attributable to their low sulfur content and high water content when compared to other onion varieties. Origins in the United States United States sweet onions origin ...

s, such as the Vidalia from

Georgia, USA Georgia is a state in the Southeastern region of the United States, bordered to the north by Tennessee and North Carolina; to the northeast by South Carolina; to the southeast by the Atlantic Ocean; to the south by Florida; and to the west by ...

, or Walla Walla from Washington that can be sliced and eaten raw on a sandwich. Cipollatropea

Aggregatum group (var. ''aggregatum'')

This group contains shallots and potato onions, also referred to as multiplier onions. The bulbs are smaller than those of common onions, and a single plant forms an aggregate cluster of several bulbs from a master. They are propagated almost exclusively from daughter bulbs, although reproduction from seed is possible. Shallots are the most important subgroup within this group and comprise the only cultivars cultivated commercially. They form aggregate clusters of small, narrowly ovoid to pear-shaped bulbs. Potato onions differ from shallots in forming larger bulbs with fewer bulbs per cluster, and having a flattened (onion-like) shape. Intermediate forms exist. I'itoi onion is a prolific multiplier onion cultivated in the

Baboquivari Peak Wilderness The Baboquivari Peak Wilderness or La Bestia is a wilderness area in the U.S. state of Arizona. It is located in the Baboquivari Mountains southwest of Tucson, Arizona. It is administered by the Bureau of Land Management. The United States Cong ...

, Arizona area. This small-bulb type has a shallot-like flavour and is easy to grow and ideal for hot, dry climates. Bulbs are separated, and planted in the fall below the surface and apart. Bulbs will multiply into clumps and can be harvested throughout the cooler months. Tops die back in the heat of summer and may return with heavy rains; bulbs can remain in the ground or be harvested and stored in a cool dry place for planting in the fall. The plants rarely flower; propagation is by division.

Hybrids with ''A. cepa'' parentage

A number of hybrids are cultivated that have ''A. cepa'' parentage, such as the

diploid Ploidy () is the number of complete sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and pseudoautosomal genes. Sets of chromosomes refer to the number of maternal and paternal chromosome copies, respectively ...

tree onion or Egyptian onion (''A.'' ×''proliferum''), and the triploid onion (''A.'' ×''cornutum''). The tree onion or Egyptian onion produces bulblets in the umbel instead of flowers, and is now known to be a

hybrid Hybrid may refer to: Science * Hybrid (biology), an offspring resulting from cross-breeding ** Hybrid grape, grape varieties produced by cross-breeding two ''Vitis'' species ** Hybridity, the property of a hybrid plant which is a union of two dif ...

of ''A. cepa'' and ''A. fistulosum''. It has previously been treated as a variety of ''A. cepa'', for example ''A. cepa'' var. ''proliferum'', ''A. cepa'' var. ''bulbiferum'', and ''A. cepa'' var. ''viviparum''. It has been grown for centuries in Japan and China for use as a salad onion. The triploid onion is a hybrid species with three sets of chromosomes, two sets from ''A. cepa'' and the third set from an unknown parent. Various clones of the triploid onion are grown locally in different regions, such as 'Ljutika' in Croatia, and 'Pran', 'Poonch', and 'Srinagar' in the India-Kashmir region. 'Pran' is grown extensively in the northern Indian provinces of Jammu and Kashmir. There are very small genetic differences between 'Pran' and the Croatian clone 'Ljutika', implying a

monophyletic In cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor (or more precisely an ancestral population) and all of its lineal descendants. Monophyletic gro ...

origin for this species. Some authors have used the name ''A. cepa'' var. ''viviparum'' (Metzg.) Alef. for the triploid onion, but this name has also been applied to the Egyptian onion. The only name unambiguously connected with the triploid onion is ''A.'' ×''cornutum''. Spring onions or salad onions may be grown from the Welsh onion (''A. fistulosum''), as well as from ''A. cepa''. Young plants of ''A. fistulosum'' and ''A. cepa'' look very similar, but may be distinguished by their leaves, which are circular in cross-section in ''A. fistulosum'' rather than flattened on one side.

References

External links

* *
PROTAbase on ''Allium cepa''
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