View source for Biology:Onion

{{short description|Bulbous vegetable, grown for food}}
{{Speciesbox
|taxon=Allium cepa
|parent=Allium subg. Cepa
|image=Mixed onions.jpg
|image_caption=A display of commercially-grown [[Biology:Bulb|bulb]]s, including red and yellow cultivars.
|authority=[[Biography:Carl Linnaeus|L.]]
|synonyms=
{{collapsible list|bullets=true
|title=<small>Species synonymy</small>
|''Allium angolense'' <small>Baker</small>
|''Allium aobanum'' <small>Araki</small>
|''Allium ascalonicum'' <small>auct.</small>
|''Allium ascalonicum'' var. ''condensum'' <small>Millán</small>
|''Allium ascalonicum'' var. ''fertile'' <small>Millán</small>
|''Allium ascalonicum'' f. ''rotterianum'' <small>Voss ex J.Becker</small>
|''Allium ascalonicum'' var. ''sterile'' <small>Millán</small>
|''Allium cepa'' var. ''aggregatum'' <small>G.Don</small>
|''Allium cepa'' var. ''anglicum'' <small>Alef.</small>
|''Allium cepa'' var. ''argenteum'' <small>Alef.</small>
|''Allium cepa'' var. ''bifolium'' <small>Alef.</small>
|''Allium cepa'' var. ''crinides'' <small>Alef.</small>
|''Allium cepa'' var. ''flandricum'' <small>Alef.</small>
|''Allium cepa'' var. ''globosum'' <small>Alef.</small>
|''Allium cepa'' var. ''hispanicum'' <small>Alef.</small>
|''Allium cepa'' var. ''jamesii'' <small>Alef.</small>
|''Allium cepa'' var. ''lisboanum'' <small>Alef.</small>
|''Allium cepa'' var. ''luteum'' <small>Alef.</small>
|''Allium cepa'' var. ''multiplicans'' <small>L.H.Bailey</small>
|''Allium cepa'' var. ''portanum'' <small>Alef.</small>
|''Allium cepa'' var. ''praecox'' <small>Alef.</small>
|''Allium cepa'' var. ''rosum'' <small>Alef.</small>
|''Allium cepa'' var. ''sanguineum'' <small>Alef</small>.
|''Allium cepa'' var. ''solaninum'' <small>Alef.</small>
|''Allium cepa'' var. ''tripolitanum'' <small>Alef.</small>
|''Allium cepa'' var. ''viviparum'' <small>(Metzg.) Alef.</small>
|''Allium cepaeum'' <small>St.-Lag.</small>
|''Allium commune'' <small>Noronha</small>
|''Allium cumaria'' <small>Buch.-Ham. ex Wall.</small>
|''Allium esculentum'' <small>Salisb.</small>
|''Allium napus'' <small>Pall. ex Kunth</small>
|''Allium nigritanum'' <small>A.Chev.</small>
|''Allium pauciflorum'' <small>Willd. ex Ledeb.</small>
|''Allium salota'' <small>Dostál</small>
|''Ascalonicum sativum'' <small>P.Renault</small>
|''Cepa alba'' <small>P.Renault</small>
|''Cepa esculenta'' <small>Gray</small>
|''Cepa pallens'' <small>P.Renault</small>
|''Cepa rubra'' <small>P.Renault</small>
|''Cepa vulgaris'' <small>Garsault</small>
|''Kepa esculenta'' <small>Raf.</small>
|''Porrum cepa'' <small>(L.) Rchb.</small>
}}
|synonyms_ref=<ref>{{ThePlantList |id=kew-295261 |taxon=Allium cepa |authority=L.}}</ref>
}}
An '''onion''' ('''''Allium cepa''''' L., from [[Social:Latin|Latin]] <nowiki/>''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 ''[[Biology:Allium|Allium]]''. The [[Biology:Shallot|shallot]] is a [[Biology:Variety (botany)|botanical variety]] of the onion which was classified as a separate species until 2011.<ref name="GRIN 2010-08-20">{{GRIN |access-date=2010-08-20}}</ref><ref name="fritsch2002">{{Cite book |last1=Fritsch |first1=Reinhard M. |title=Allium Crop Science: Recent Advances |last2=Friesen |first2=Nikolai |publisher=CABI Publishing |year=2002 |isbn=0-85199-510-1 |editor-last=Rabinowitch |editor-first=Haim D. |location=Wallingford, UK |chapter=Chapter 1: Evolution, Domestication, and Taxonomy |doi=10.1079/9780851995106.0005 |oclc=228168061 |s2cid=189956991 |editor-last2=Currah |editor-first2=Lesley}}</ref>{{rp|21}} Its close relatives include [[Biology:Garlic|garlic]], [[Biology:Scallion|scallion]], [[Biology:Leek|leek]], and chive.<ref name=Block2010>{{cite book |author=Block, E. |title=Garlic and Other Alliums: The Lore and the Science |url=https://books.google.com/books?id=6AB89RHV9ucC |publisher=Royal Society of Chemistry |year=2010 |isbn=978-0-85404-190-9 |access-date=30 June 2020 |archive-date=1 August 2020 |archive-url=https://web.archive.org/web/20200801202704/https://books.google.com/books?id=6AB89RHV9ucC |url-status=live }}</ref>

This genus also contains several other species variously referred to as onions and cultivated for food, such as the Japanese bunching onion ''[[Biology:Allium fistulosum|Allium fistulosum]]'', the [[Biology:Tree onion|tree onion]] ''Allium''&nbsp;× ''proliferum'', and the Canada onion ''[[Biology:Allium canadense|Allium canadense]]''. The name ''wild onion'' 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.<ref name="FNA">{{eFloras|1|200027457|Allium cepa |first1=Dale W. Jr. |last1=McNeal |first2=T. D. |last2=Jacobsen |volume=26}}</ref> The onion is most frequently a [[Biology:Biennial plant|biennial]] or a [[Biology:Perennial plant|perennial plant]], but is usually treated as an [[Biology:Annual plant|annual]] and harvested in its first growing season.

The onion plant has a fan of hollow, bluish-green leaves, and its [[Biology:Bulb|bulb]] 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 (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 [[Biology:Delia antiqua|onion fly]], the [[Biology:Ditylenchus dipsaci|onion eelworm]], and various fungi which can cause rotting. Some varieties of ''A. cepa'', such as [[Biology:Shallot|shallot]]s and potato onions, produce multiple bulbs.

Onions are cultivated and used around the world. As a food item, they are usually served raw,{{cn|date=October 2023}} as a vegetable or part of a prepared savoury dish, but can also be eaten cooked 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==
[[File:ARS red onion.jpg|thumb|Roots, leaves and developing bulb]]
The onion plant (''Allium cepa''), also known as the bulb onion<ref name="GRIN Allium cepa var cepa">{{GRIN |''Allium cepa'' var. ''cepa'' |409114 |access-date=10 December 2017}}</ref> or common onion,{{r|fritsch2002|p=9–10}} is the most widely cultivated species of the genus ''[[Biology:Allium|Allium]].''<ref name="Eric Block 2010">Eric Block, "Garlic and Other ''Alliums'': The Lore and the Science" (Cambridge: Royal Society of Chemistry, 2010)</ref><ref>{{cite book |last=Brewster |first=James L. |title=Onions and other vegetable ''Alliums'' |edition=1st |year=1994 |publisher=CAB International |location=Wallingford, UK |isbn=978-0-85198-753-8 |page=16}}</ref> It was first officially [[Biology:Species description|described]] by [[Biography:Carl Linnaeus|Carl Linnaeus]] in his 1753 work ''[[Biology:Species Plantarum|Species Plantarum]]''.<ref>{{cite book |author=Linnaeus, Carolus |title=Species Plantarum |year=1753 |volume=1 |location=Stockholm |publisher=Laurentii Salvii |page=262 |url=https://www.biodiversitylibrary.org/page/358281 |language=la |access-date=21 February 2018 |archive-date=16 June 2018 |archive-url=https://web.archive.org/web/20180616061629/https://www.biodiversitylibrary.org/page/358281 |url-status=live }}</ref> 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.<ref>{{PLANTS |symbol=ALCE |taxon=Allium cepa |access-date=2013-03-31}}</ref><ref>{{ITIS |id=42720 |taxon=''Allium cepa'' L. |access-date=2013-04-01}}</ref>

''A. cepa'' is known exclusively from cultivation,<ref name="FNA" /> but related wild species occur in Central Asia and Iran. The most closely related species include ''[[Biology:Allium vavilovii|A. vavilovii]]'' from Turkmenistan and ''[[Biology:Allium asarense|A. asarense]]'' from [[Religion:Iran|Iran]].<ref name="prota">Grubben, G.J.H.; Denton, O.A. (2004) Plant Resources of Tropical Africa 2. Vegetables. PROTA Foundation, Wageningen; Backhuys, Leiden; CTA, Wageningen.</ref><ref>{{cite book |first1=Daniel |last1=Zohary |first2=Maria |last2=Hopf |title=Domestication of plants in the Old World |edition=Third |location=Oxford |publisher=Oxford University Press |year=2000 |page=198 |isbn=978-0-19-850357-6}}</ref>

The vast majority of [[Biology:Cultivar|cultivar]]s of ''A.&nbsp;cepa'' belong to the common onion group (''A.&nbsp;cepa''&nbsp;var.&nbsp;''cepa'') and are usually referred to simply as onions. The Aggregatum Group of cultivars (''A.&nbsp;cepa''&nbsp;var.&nbsp;''aggregatum'') includes both shallots and potato onions.{{r|fritsch2002|p=20–21}}

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.&nbsp;fistulosum''), Egyptian onion (''A. &nbsp;× proliferum''), and Canada onion (''A.&nbsp;canadense'').{{r|fritsch2002|p=9–10}}

''Cepa'' is commonly accepted as Latin for "onion" and has an affinity with [[Social:Ancient Greek|Ancient Greek]]: κάπια (''kápia'') and [[Social:Albanian language|Albanian]]: ''qepë'' and is ancestral to Aromanian: ''tseapã'', [[Social:Catalan language|Catalan]]: ''ceba'', Occitan: ''ceba'', [[Social:Portuguese language|Portuguese]]: ''cebola'', Spanish: ''cebolla'', Italian: ''cipolla'', and [[Social:Romanian language|Romanian]]: ''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 [[Biology:Biennial plant|biennial plant]] but is usually grown as an annual. Modern varieties typically grow to a height of {{convert|15|to|45|cm|0|abbr=on}}. 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 [[Biology:Bulb|bulb]]. From the underside of the plate, a bundle of [[Biology:Fibrous root system|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.<ref name=RHS />
[[File:Onion Flower Bangladesh.jpg|thumb|Inflorescence with flower buds]]
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 [[Biology:Bract|bract]] protecting a developing inflorescence. The [[Biology:Inflorescence|inflorescence]] takes the form of a rounded [[Biology:Umbel|umbel]] of white flowers with parts in sixes. The seeds are glossy black and triangular in cross-section.<ref name=RHS /> The average pH of an onion is around 5.5.<ref>{{Cite web |url=https://foodsafety.wisc.edu/business_food/files/Approximate_pH.pdf |title=Approximate pH Values of Common Foods and Ingredients |website=Wisconsin Food Safety and Health |archive-url=https://web.archive.org/web/20121224184605/http://www.foodsafety.wisc.edu/business_food/files/Approximate_pH.pdf |archive-date=2012-12-24 |url-status=dead |access-date=8 January 2018}}</ref>

[[File:Indian Onion.jpg|thumb|Indian onions]]

==History==

[[File:An onion, woodcut, 1547 Wellcome L0029214.jpg|thumb|Medieval woodcut print depicting an onion, from the Latin encyclopedia ''Hortus Sanitatis'' (1547)]]

The history of ancestral onion species is not well documented. Ancient records of onion use spans [[Place:Western Asia|western]] and eastern Asia, so the geographic origin of the onion is uncertain.<ref name="cumo">{{Cite book |url=https://books.google.com/books?id=WqfACQAAQBAJ&pg=PA248 |title=Onion. In: Foods that Changed History: How Foods Shaped Civilization from the Ancient World to the Present |last=Cumo |first=CE |publisher=ABC-CLIO LLC (American Bibliographic Center, CLIO Press) |year=2015 |pages=248–50 |isbn=9781440835377 |access-date=30 June 2020 |archive-date=4 February 2021 |archive-url=https://web.archive.org/web/20210204030543/https://books.google.com/books?id=WqfACQAAQBAJ&pg=PA248 |url-status=live }}</ref><ref name="ansari">{{Cite journal |last=Ansari |first=Naser Alemzadeh |year=2007 |title=Onion Cultivation and Production in Iran |url=http://www.globalsciencebooks.info/Online/GSBOnline/images/0712/MERJPSB_1(1&2)/MERJPSB_1(2)26-38o.pdf |journal=Middle Eastern and Russian Journal of Plant Science and Biotechnology |volume=1 |issue=2 |pages=26–38 |issn=1752-3907 |quote=Onion originated in Iran and its neighboring countries (Hanelt 1990). |via=Global Science Books |access-date=22 April 2022 |archive-date=28 August 2021 |archive-url=https://web.archive.org/web/20210828162151/http://www.globalsciencebooks.info/Online/GSBOnline/images/0712/MERJPSB_1%281%262%29/MERJPSB_1%282%2926-38o.pdf |url-status=live }}</ref> Yet, domestication likely took place in [[Place:West Asia|West]] or [[Place:Central Asia|Central Asia]].{{r|fritsch2002|p=20–21}}<ref name=NOA>{{cite web |url=https://www.onions-usa.org/all-about-onions/history-of-onions |title=History of onions |publisher=US National Onion Association, Greeley, CO |date=2011 |access-date=23 January 2017 |archive-date=24 June 2017 |archive-url=https://web.archive.org/web/20170624120840/https://www.onions-usa.org/all-about-onions/history-of-onions |url-status=live }}</ref> Onions have been variously described as having originated in [[Religion:Iran|Iran]], western {{wipe|Pakistan}} and [[Place:Central Asia|Central Asia]].<ref name=cumo/>{{r|NOA|p=1|quote=Many archaeologists, botanists, and food historians believe onions originated in central Asia. Other research suggests onions were first grown in Iran and West Pakistan.}}<ref name=ansari/><ref>{{Cite book |url=https://books.google.com/books?id=6AB89RHV9ucC |title=Garlic and Other Alliums: The Lore and the Science |last=Block |first=Eric |date=2010 |publisher=Royal Society of Chemistry |isbn=9780854041909 |language=en |pages=5–6 |access-date=30 June 2020 |archive-date=1 August 2020 |archive-url=https://web.archive.org/web/20200801202704/https://books.google.com/books?id=6AB89RHV9ucC |url-status=live }}</ref>

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.<ref>[http://www.gillsonions.com/history Onion History] {{Webarchive |url=https://web.archive.org/web/20170601002900/http://www.gillsonions.com/history |date=1 June 2017}} GillsOnions.com</ref><ref name=NOA/>{{Failed verification|date=June 2020}} Ancient Egyptians revered the onion bulb, viewing its spherical shape and concentric rings as symbols of eternal life.<ref name=NOA/> Onions were used in Egyptian burials, as evidenced by onion traces found in the eye sockets of Ramesses IV.<ref>{{cite journal |author1=Abdel-Maksouda, Gomaa |author2=El-Aminb, Abdel-Rahman |year=2011 |title=A review on the materials used during the mummification process in ancient Egypt |journal=Mediterranean Archaeology and Archaeometry |volume=11 |issue=2 |pages=129–150 |url=http://www.maajournal.com/Issues/2011/pdf/Maksoud.pdf |access-date=13 September 2013 |archive-date=8 March 2014 |archive-url=https://web.archive.org/web/20140308070536/http://www.maajournal.com/Issues/2011/pdf/Maksoud.pdf |url-status=live }}</ref>

Pliny the Elder of the first century AD wrote about the use of onions and cabbage in Pompeii. 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 [[Medicine:Dysentery|dysentery]]. Archaeologists unearthing Pompeii long after [[Earth:Eruption of Mount Vesuvius in 79 AD|its 79 AD volcanic burial]] have found gardens resembling those in Pliny's detailed narratives.<ref name=NOA/> 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.<ref name=NOA/>

In the [[Age of Discovery]], onions were taken to [[Earth:North America|North America]] by the first European settlers,<ref name=cumo/> who found close relatives of the plant such as ''[[Biology:Allium tricoccum|Allium tricoccum]]'' readily available and widely used in Native American gastronomy.<ref name=cumo/> According to diaries kept by certain first English colonists, the bulb onion was one of the first crops planted by the Pilgrims.<ref name="NOA" />

== Uses ==

=== Onion types and products ===

[[File:Frying onion.JPG|thumb|right|Sautéing onions]]
Common onions are normally available in three colour varieties:
*Yellow or brown onions are sweet, with many cultivars bred specifically to accentuate this sweetness, such as Vidalia, Walla Walla, Cévennes, and Bermuda.<ref>{{cite web |url=http://www.cooksinfo.com/bermuda-onions |title=Bermuda Onions |first=Randal |last=Oulton |date=2005-09-09 |website=cooksinfo.com |access-date=2017-11-25 |archive-date=11 February 2021 |archive-url=https://web.archive.org/web/20210211230456/https://www.cooksinfo.com/bermuda-onions |url-status=live }}</ref> Yellow onions turn a rich, dark brown when caramelised and are used to add a sweet flavour to various dishes, such as French onion soup.
*[[Biology:Red onion|Red or purple onion]]s, known for their sharp pungent flavour, are commonly cooked in many cuisines. They are also used raw and in grilling.
*White onions are mild in flavour; they have a golden colour when cooked and a particularly sweet flavour when sautéed.<ref>{{cite web |url=http://thecookingdish.com/0330/the-difference-between-yellow-onions-white-onions-and-red-onions/ |title=The Difference between Yellow, White, and Red Onions |author=Mower, Chris |website=The Cooking Dish |date=30 March 2009 |access-date=2013-03-24 |archive-date=1 May 2013 |archive-url=https://web.archive.org/web/20130501192552/http://thecookingdish.com/0330/the-difference-between-yellow-onions-white-onions-and-red-onions/ |url-status=live }}</ref><ref name=NOA/>
[[File:Tranby house 31 gnangarra.jpg|thumb|left|Jar of pickled onions]]
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 [[Biology:Allium fistulosum#Ambiguous names|spring onions]] or [[Biology:Scallion|scallion]]s. 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.<ref name="Thompson 1995 143">{{cite book |last=Thompson |first=Sylvia |year=1995 |title=The Kitchen Garden |publisher=Bantam Books |page=143}}</ref>

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 [[Biology:Pearl onion|pearl onion]]s which are a different species.<ref name="Thompson 1995 143" /> Pearl and boiler onions may be cooked as a vegetable rather than as an ingredient and pickler onions are often preserved in [[Chemistry:Vinegar|vinegar]] as a long-lasting relish.<ref>{{cite book |title=Home Preservation of Fruit and Vegetables |author1=Ministry of Agriculture |author2=Fisheries and Food |year=1968 |publisher=HMSO |page=107}}</ref>

Onions are available in fresh, frozen, canned, caramelised, pickled, and chopped forms. The dehydrated product is available as kibbled, 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.<ref>{{cite web |url=http://www.wisegeek.com/what-is-onion-powder.htm |title=What is onion powder |author=Smith, S. E. |year=2013 |website=WiseGeek |publisher=Conjecture Corporation |access-date=2013-04-02 |archive-date=8 March 2013 |archive-url=https://web.archive.org/web/20130308132918/http://www.wisegeek.com/what-is-onion-powder.htm |url-status=live }}</ref>

=== Culinary ===
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.<ref name="bbc">{{cite web |url=http://www.bbcgoodfood.com/content/knowhow/glossary/onion/ |title=Onion |website=GoodFood |publisher=BBC |access-date=2013-04-02 |archive-date=30 May 2013 |archive-url=https://web.archive.org/web/20130530032911/http://www.bbcgoodfood.com/content/knowhow/glossary/onion/ |url-status=live }}</ref> Their layered nature makes them easy to hollow out once cooked, facilitating stuffing them, as in Turkish sogan-dolma.

Onions pickled in vinegar are eaten as a snack around the world, and as a side serving in pubs and fish and chip shops throughout the {{wipe|United Kingdom}} and the Commonwealth. They are part of a traditional British pub's ploughman's lunch, usually served with crusty bread, English cheese, and ale.

Similar to [[Biology:Garlic|garlic]],<ref>{{Cite journal |last=Lukes |first=T. M. |date=1986-11-01 |title=Factors Governing the Greening of Garlic Puree |journal=Journal of Food Science |language=en |volume=51 |issue=6 |pages=1577 |doi=10.1111/j.1365-2621.1986.tb13869.x |issn=1750-3841}}</ref> onions can show an additional colour – pink-red – after cutting, an effect caused by reactions of [[Chemistry:Amino acid|amino acid]]s with sulfur compounds.<ref>{{Cite journal |last1=Lee |first1=Eun Jin |last2=Rezenom |first2=Yohannes H. |last3=Russell |first3=David H. |last4=Patil |first4=Bhimanagouda S. |last5=Yoo |first5=Kil Sun |date=2012-04-01 |title=Elucidation of chemical structures of pink-red pigments responsible for 'pinking' in macerated onion (''Allium cepa'' L.) using HPLC–DAD and tandem mass spectrometry |journal=Food Chemistry |volume=131 |issue=3 |pages=852–861 |doi=10.1016/j.foodchem.2011.09.059}}</ref>

=== Oils ===
Onion oil is authorised for use in the European Union for use as a pesticide against [[Biology:Carrot fly|carrot fly]] in [[Biology:Apiaceae|umbelliferous crops]] (carrots, parsnips, parsley, celery, celeriac).<ref>{{Cite web |last=European Commission |date=20 July 2018 |title=Final Review report for the basic substance Onion Oil finalised in the Standing Committee on Plants, Animals, Food and Feed at its meeting on 20 July 2018 in view of the approval of onion oil as basic substance in accordance with Regulation (EC) No 1107/2009 |url=https://mst.dk/media/171096/review-report-for-onion-oil-juli-2018.pdf |access-date=23 November 2021 |archive-date=23 November 2021 |archive-url=https://web.archive.org/web/20211123111251/https://mst.dk/media/171096/review-report-for-onion-oil-juli-2018.pdf |url-status=live }}</ref> Onions also contain one of the natural oils sometimes used in [[Chemistry:Hair oil|hair oil]].

=== Education ===
[[File:Cells from a red onion epidermal peel.tif|thumb|Onion [[Biology:Epidermis (botany)|epidermis]] cells are well known to be visible in true color with only minimal magnification, such as this red onion skin.]]
Onions have particularly large [[Biology:Cell|cells]] that are readily observed under low magnification. Forming a single layer of cells, the bulb epidermis is easy to separate for educational, experimental, and [[Biology:Plant breeding|breeding]] purposes.<ref>{{Cite journal |pmid=19684107 |year=2009 |last1=Suslov |first1=D |title=Onion epidermis as a new model to study the control of growth anisotropy in higher plants |journal=Journal of Experimental Botany |volume=60 |issue=14 |pages=4175–87 |last2=Verbelen |first2=J. P. |last3=Vissenberg |first3=K |doi=10.1093/jxb/erp251|doi-access=free}}</ref><ref>{{Cite journal |pmid=24416168 |year=2014 |last1=Xu |first1=K |title=A rapid, highly efficient and economical method of Agrobacterium-mediated in planta transient transformation in living onion epidermis |journal=PLOS One |volume=9 |issue=1 |pages=e83556 |last2=Huang |first2=X |last3=Wu |first3=M |last4=Wang |first4=Y |last5=Chang |first5=Y |last6=Liu |first6=K |last7=Zhang |first7=J |last8=Zhang |first8=Y |last9=Zhang |first9=F|last10=Yi|first10=L |last11=Li |first11=T |last12=Wang |first12=R |last13=Tan |first13=G |last14=Li |first14=C |doi=10.1371/journal.pone.0083556 |pmc=3885512 |bibcode=2014PLoSO...983556X|doi-access=free}}</ref> Onions are therefore commonly used in science education to teach the use of a [[Engineering:Microscope|microscope]] for observing cell structure.<ref name="McCabeO'Donnell2007">{{cite book |author1=Anne McCabe |author2=Mick O'Donnell |author3=Rachel Whittaker |title=Advances in Language and Education |url=https://books.google.com/books?id=UKLc_kfOJb8C&pg=PA35 |date=19 July 2007 |publisher=Bloomsbury Publishing |isbn=978-1-4411-0458-8 |page=35 |access-date=2 August 2015 |archive-date=26 September 2015 |archive-url=https://web.archive.org/web/20150926023847/https://books.google.com/books?id=UKLc_kfOJb8C&pg=PA35 |url-status=live }}</ref>

=== Dye ===
Onion skins can be boiled to make an orange-brown dye.<ref>{{cite web |author=Sam |url=http://www.allnaturaldyeing.com/onion-skin-dye/ |website=All Natural Dyeing |title=Onion Skin Dye (Yellow and Purple) |access-date=31 July 2020 |archive-date=6 August 2020 |archive-url=https://web.archive.org/web/20200806051713/http://www.allnaturaldyeing.com/onion-skin-dye/ |url-status=live }}</ref>

==Composition==
===Nutrients===
{{nutritional value |name=Raw onion bulbs
|water=89.11 g
|kJ=166
|protein=1.1 g
|fat=0.1 g
|carbs=9.34 g
|fibre=1.7 g
|sugars=4.24 g
|calcium_mg=23
|iron_mg=0.21
|magnesium_mg=10
|phosphorus_mg=29
|potassium_mg=146
|zinc_mg=0.17
|manganese_mg=0.129
|vitC_mg=7.4
|thiamin_mg=0.046
|riboflavin_mg=0.027
|niacin_mg=0.116
|pantothenic_mg=0.123
|vitB6_mg=0.12
|folate_ug=19
|opt1n=[[Chemistry:Fluoride|Fluoride]]
|opt1v=1.1 µg
|source_usda=1
|note=[https://fdc.nal.usda.gov/fdc-app.html#/food-details/170000/nutrients Link to USDA Database entry]
}}

Most onion cultivars are about 89% water, 9% [[Chemistry:Carbohydrate|carbohydrate]]s (including 4% sugar and 2% dietary fibre), 1% protein, and negligible [[Chemistry:Fat|fat]] (table). Onions contain low amounts of essential nutrients and have an energy value of 166 kJ (40 kilocalories) in a 100&nbsp;g (3.5&nbsp;oz) amount. Onions contribute savoury flavour to dishes without contributing significant caloric content.<ref name=NOA/>

===Phytochemicals===
Considerable differences exist between onion varieties in [[Chemistry:Phytochemical|phytochemical]] content, particularly for [[Chemistry:Polyphenol|polyphenol]]s, with shallots having the highest level, six times the amount found in Vidalia onions.<ref name="jafc" /> Yellow onions have the highest total [[Chemistry:Flavonoid|flavonoid]] content, an amount 11 times higher than in white onions.<ref name="jafc" /> Red onions have considerable content of [[Chemistry:Anthocyanin|anthocyanin]] pigments, with at least 25 different compounds identified representing 10% of total flavonoid content.<ref name="jafc" />

Onion polyphenols are under [[Basic research|basic research]] to determine their possible biological properties in humans.<ref name="jafc">{{Cite journal |pmid=17997520 |year=2007 |last1=Slimestad |first1=R |title=Onions: A source of unique dietary flavonoids |journal=Journal of Agricultural and Food Chemistry |volume=55 |issue=25 |pages=10067–80 |last2=Fossen |first2=T |last3=Vågen |first3=I. M. |doi=10.1021/jf0712503}}</ref><ref>{{cite journal |author1=Olsson, M.E. |author2=Gustavsson, K.E. |author3=Vågen, I.M. |name-list-style=amp |year=2010 |title=Quercetin and isorhamnetin in sweet and red cultivars of onion (''Allium cepa'' L.) at harvest, after field curing, heat treatment, and storage |journal=Journal of Agricultural and Food Chemistry |volume=58 |issue=4 |pages=2323–2330 |doi=10.1021/jf9027014 |pmid=20099844}}</ref>

===Allergic reactions===
Some people suffer from [[Medicine:Allergy|allergic reactions]] after handling onions.<ref>{{cite journal |pmid=25381903 |year=2014 |last1=Cantisani |first1=C |title=Unusual food allergy: Alioidea allergic reactions overview |journal=Recent Patents on Inflammation & Allergy Drug Discovery |volume=8 |issue=3 |pages=178–84 |last2=Visconti |first2=B |last3=Paolino |first3=G |last4=Frascani |first4=F |last5=Tofani |first5=S |last6=Fazia |first6=G |last7=Calvieri |first7=S |doi=10.2174/1872213X08666141107170159}}</ref> Symptoms can include contact [[Medicine:Dermatitis|dermatitis]], intense itching, [[Medicine:Rhinitis|rhinoconjunctivitis]], blurred vision, bronchial [[Medicine:Asthma|asthma]], sweating, and [[Medicine:Anaphylaxis|anaphylaxis]]. Allergic reactions may not occur when eating cooked onions, possibly due to the [[Biology:Denaturation (biochemistry)|denaturing]] of the proteins from cooking.<ref>{{cite journal |author1=Arochena, L. |author2=Gámez, C. |author3=del Pozo, V. |author4=Fernández-Nieto, M. |year=2012 |title=Cutaneous allergy at the supermarket |journal=Journal of Investigational Allergology and Clinical Immunology |volume=22 |issue=6 |pages=441–442 |pmid=23101191}}</ref>

===Eye irritation===
[[File:Cortando cebolla.jpg|thumb|left|Cut onions emit certain compounds which cause the [[Medicine:Lacrimal gland|lacrimal gland]]s in the eyes to become irritated, releasing tears.]]

Freshly cut onions often cause a stinging sensation in the eyes of people nearby, and often uncontrollable [[Biology:Tears|tears]]. This is caused by the release of a [[Chemistry:Volatility|volatile liquid]], ''syn''-propanethial-S-oxide and its [[Chemistry:Aerosol|aerosol]], which stimulates nerves in the eye.<ref name="Eric Block 2010" /> This gas is produced by a chain of reactions which serve as a [[Biology:Anti-predator adaptation|defence mechanism]]: chopping an onion causes damage to [[Biology:Cell|cells]] which releases [[Biology:Enzymes|enzymes]] called [[Biology:Alliinase|alliinase]]s. These break down [[Chemistry:Amino acid|amino acid]] [[Chemistry:Sulfoxide|sulfoxide]]s and generate [[Chemistry:Sulfenic acid|sulfenic acid]]s. A specific sulfenic acid, 1-propenesulfenic acid, is rapidly acted on by a second enzyme, the [[Medicine:Lacrimal gland|lacrimatory]] factor synthase (LFS), producing the ''syn''-propanethial-S-oxide.<ref name="Eric Block 2010" /> This gas diffuses through the air and soon reaches the eyes, where it activates sensory neurons. [[Medicine:Lacrimal gland|Lacrimal gland]]s produce [[Biology:Tears|tears]] to dilute and flush out the irritant.<ref name="Sciam">{{cite web |last=Scott |first=Thomas |title=What is the chemical process that causes my eyes to tear when I peel an onion! |website=Ask the Experts: Chemistry |publisher=Scientific American |url=http://www.sciam.com/article.cfm?id=what-is-the-chemical-proc |access-date=2007-04-28 |archive-date=22 April 2022 |archive-url=https://web.archive.org/web/20220422190206/https://www.scientificamerican.com/article/what-is-the-chemical-proc/ |url-status=live }}</ref>

Eye irritation can be avoided by cutting onions under running water or submerged in a basin of water.<ref name="Sciam" /> 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.<ref name=FAQ>{{cite web |url=http://www.onions-usa.org/about/faq.php#cooking |title=FAQ |publisher=National Onion Association |access-date=2013-03-28 |archive-url=https://web.archive.org/web/20110318030002/http://www.onions-usa.org/about/faq.php#cooking |archive-date=18 March 2011 |url-status=dead}}</ref>

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 [[Biology:Genetically modified organism|genetic modification]] to prevent the synthesis of lachrymatory factor synthase in onions.<ref>{{cite web |url=https://www.sciencedaily.com/releases/2008/02/080202115345.htm |title=Tearless Onion Created in Lab Using Gene Silencing |website=ScienceDaily |date=5 February 2008 |access-date=23 November 2016 |archive-date=24 November 2016 |archive-url=https://web.archive.org/web/20161124025149/https://www.sciencedaily.com/releases/2008/02/080202115345.htm |url-status=live }}</ref> One study suggests that consumers prefer the flavour of onions with lower LFS content.<ref>{{Cite journal |last1=Kim |first1=Ha-Yeon |last2=Jackson |first2=Daniel |last3=Adhikari |first3=Koushik |last4=Riner |first4=Cliff |last5=Sanchez-Brambila |first5=Gabriela |date=2017-10-01 |title=Relationship Between Consumer Acceptability and Pungency-Related Flavor Compounds of Vidalia Onions |journal=Journal of Food Science |language=en |volume=82 |issue=10 |pages=2396–2402 |doi=10.1111/1750-3841.13915 |pmid=28898424 |issn=1750-3841}}</ref> Since the process impedes sulfur ingestion by the plant, some find LFS− onions inferior in flavour.<ref>{{Cite journal |last1=Eady |first1=Colin C. |last2=Kamoi |first2=Takahiro |last3=Kato |first3=Masahiro |last4=Porter |first4=Noel G. |last5=Davis |first5=Sheree |last6=Shaw |first6=Martin |last7=Kamoi |first7=Akiko |last8=Imai |first8=Shinsuke |date=2008-08-01 |title=Silencing Onion Lachrymatory Factor Synthase Causes a Significant Change in the Sulfur Secondary Metabolite Profile |journal=Plant Physiology |language=en |volume=147 |issue=4 |pages=2096–2106 |doi=10.1104/pp.108.123273 |issn=0032-0889 |pmid=18583530 |pmc=2492635}}</ref>

A method for efficiently differentiating LFS− and LFS+ onions has been developed based on [[Physics:Mass spectrometry|mass spectrometry]], with potential application in high-volume production;<ref>{{Cite journal |last1=Joyce |first1=Nigel I. |last2=Eady |first2=Colin C. |last3=Silcock |first3=Patrick |last4=Perry |first4=Nigel B. |last5=van Klink |first5=John W. |date=January 2013 |title=Fast Phenotyping of LFS-Silenced (Tearless) Onions by Desorption Electrospray Ionization Mass Spectrometry (DESI-MS) |journal=Journal of Agricultural and Food Chemistry |volume=61 |issue=7 |pages=1449–1456 |doi=10.1021/jf304444s |pmid=23350988}}</ref> [[Gas chromatography|gas chromatography]] is also used to measure lachrymatory factor in onions.<ref>{{Cite journal |last1=Tewari |first1=Gyanendra M. |last2=Bandyopadhyay |first2=Chiranjib. |date=1975-07-01 |title=Quantitative evaluation of lachrymatory factor in onion by thin-layer chromatography |journal=Journal of Agricultural and Food Chemistry |volume=23 |issue=4 |pages=645–647 |doi=10.1021/jf60200a044 |issn=0021-8561}}</ref><ref>{{Cite journal |last1=Schmidt |first1=Norman E. |last2=Santiago |first2=Leanne M. |last3=Eason |first3=H. Donald |last4=Dafford |first4=Kurtus A. |last5=Grooms |first5=Chris A. |last6=Link |first6=Tammy E. |last7=Manning |first7=Dana T. |last8=Cooper |first8=Sylina D. |last9=Keith |first9=R. Chad |date=1996-01-01 |title=Rapid Extraction Method of Quantitating the Lachrymatory Factor of Onion Using Gas Chromatography |journal=Journal of Agricultural and Food Chemistry |volume=44 |issue=9 |pages=2690–2693 |doi=10.1021/jf950686s |issn=0021-8561}}</ref> In early 2018, [[Company:Bayer|Bayer]] released the first crop yield of commercially available LFS-silenced onions under the name "Sunions".<ref name=":0">{{Cite news |url=https://www.npr.org/sections/thesalt/2018/02/08/583703262/stop-crying-tear-free-onions-are-here |title=Stop Crying! Tear-Free Onions Are Here |last=Danovich |first=Tove |date=8 February 2018 |publisher=NPR |access-date=13 April 2018 |archive-date=17 April 2018 |archive-url=https://web.archive.org/web/20180417025118/https://www.npr.org/sections/thesalt/2018/02/08/583703262/stop-crying-tear-free-onions-are-here |url-status=live }}</ref> They were the product of 30 years of cross-breeding; genetic modification was not employed.<ref name=":0" /><ref>{{Cite news |url=http://www.latimes.com/food/sns-dailymeal-1863539-healthy-eating-tearless-onions-121917-20171219-story.html |title=Onions that don't make you cry are finally here |last=Van Hare |first=Holly |date=19 December 2017 |newspaper=Los Angeles Times |access-date=15 April 2018 |archive-date=13 April 2018 |archive-url=https://web.archive.org/web/20180413034721/http://www.latimes.com/food/sns-dailymeal-1863539-healthy-eating-tearless-onions-121917-20171219-story.html |url-status=live }}</ref>

[[Biology:Petiveria|Guinea hen weed]] and [[Biology:Allium siculum|honey garlic]] contain a similar lachrymatory factor.<ref>{{Cite news |url=https://www.nytimes.com/2017/09/05/science/onions-crying-chemicals.html |title=Why Onions Make You Cry |last=Klein |first=Joanna |date=5 September 2017 |work=The New York Times |access-date=16 April 2018 |archive-date=17 April 2018 |archive-url=https://web.archive.org/web/20180417025055/https://www.nytimes.com/2017/09/05/science/onions-crying-chemicals.html |url-status=live }}</ref> Synthetic onion lachrymatory factor has been used in a study related to tear production,<ref>{{cite journal |doi=10.1007/s10384-009-0786-0 |volume=54 |title=Using synthesized onion lachrymatory factor to measure age-related decreases in reflex-tear secretion and ocular-surface sensation |year=2010 |journal=Japanese Journal of Ophthalmology |pages=215–220 |last1=Higashihara |first1=Hisayo |last2=Yokoi |first2=Norihiko |last3=Aoyagi |first3=Morihiro |last4=Tsuge |first4=Nobuaki |last5=Imai |first5=Shinsuke |last6=Kinoshita |first6=Shigeru |s2cid=23549173 |issue=3 |pmid=20577855}}</ref> and has been proposed as a nonlethal deterrent against thieves and intruders.<ref>{{Cite patent |country=US |number=9482496B1 |status=patent |title=Wall-mounted nonlethal device for defending against intruders |pubdate=2015-06-01 |gdate=2016-11-01 |pridate=2015-06-01 |inventor=James Anthony Rocchi, Thomas John Stewart |invent1=James Anthony Rocchi |invent2=Thomas John Stewart |assign1=Fighting Chance Systems Inc |url=https://patents.google.com/patent/US9482496B1/en }} {{Webarchive|url=https://web.archive.org/web/20210318041651/https://patents.google.com/patent/US9482496B1/en |date=18 March 2021 }}</ref><ref>{{cite patent |country=US |number=9890561B2 |status=patent |title=Pressurized chemical theft deterrent device |pubdate=2017-03-03 |gdate=2018-02-13 |pridate=2016-03-04 |inventor=Yves Perrenoud, Daniel Idzkowski |invent1=Yves Perrenoud |invent2=Daniel Idzkowski |assign1=Skunklock Inc |url=https://patents.google.com/patent/US9890561B2/en }} {{Webarchive|url=https://web.archive.org/web/20200113141617/https://patents.google.com/patent/US9890561B2/en |date=13 January 2020 }}</ref>
[[File:Onion mother plants (bulbs) for production seeds.jpg|thumb|Onion bulbs used for planting and seeds]]

=== Toxicity to animals ===
Onions are toxic to dogs, cats, [[Biology:Guinea pig|guinea pig]]s, and many other animals.<ref name="Cope2005">{{cite journal |title=''Allium'' species poisoning in dogs and cats |journal=Veterinary Medicine |date=August 2005 |volume=100 |issue=8 |pages=562–566 |url=https://www.aspcapro.org/sites/default/files/c-vetm0805_562-566.pdf |author=Cope, R.B. |issn=8750-7943 |access-date=22 July 2022 }}</ref><ref name="Salgado2011">{{cite journal |title=''Allium'' species poisoning in dogs and cats |journal=Journal of Venomous Animals and Toxins Including Tropical Diseases |year=2011 |volume=17 |issue=1 |pages=4–11 |author=Salgado, B.S. |issn=1678-9199 |doi=10.1590/S1678-91992011000100002 |last2=Monteiro |first2=L.N. |last3=Rocha |first3=N.S. |doi-access=free|hdl=11449/12942 |hdl-access=free }}</ref>

==Cultivation==
[[File:Field with onions.jpg|thumb|Large-scale onion 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 [[Chemistry:Soil#Nutrients|nutrients]] in the soil. [[Chemistry:Phosphorus|Phosphorus]] is often present in sufficient quantities, but may be applied before planting because of its low level of availability in cold soils. [[Chemistry:Nitrogen|Nitrogen]] and [[Chemistry:Potash|potash]] can be applied at regular intervals during the growing season, the last application of nitrogen being at least four weeks before harvesting.<ref>{{cite web |url=http://www.caes.uga.edu/publications/pubDetail.cfm?pk_id=7749 |title=2007 Onion Production Guide |author=Boyhan, George E.; Kelley, W. Terry (eds.) |year=2007 |website=Production Guides |publisher=University of Georgia: College of Agricultural and Environmental Sciences |access-date=2013-09-14 |archive-date=11 October 2013 |archive-url=https://web.archive.org/web/20131011180348/http://www.caes.uga.edu/Publications/pubDetail.cfm?pk_id=7749 |url-status=live }}</ref>

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.<ref>{{cite web |url=http://extension.oregonstate.edu/gardening/onion-bulb-formation-strongly-linked-day-length |title=Onion bulb formation is strongly linked with day length |author=Savonen, Carol |date=2006-07-13 |publisher=Oregon State University Extension Service |access-date=2013-09-14 |archive-date=9 November 2013 |archive-url=https://web.archive.org/web/20131109131226/http://extension.oregonstate.edu/gardening/onion-bulb-formation-strongly-linked-day-length |url-status=live }}</ref> Onions are a cool-weather crop and can be grown in [[Hardiness zone|USDA zones]] 3&nbsp;to&nbsp;9.<ref name=almanac /> Hot temperatures or other stressful conditions cause them to "bolt", meaning that a flower stem begins to grow.<ref>{{cite web |url=http://www.gardeningknowhow.com/edible/vegetables/onion/onion-bolting.htm |title=What is Onion Bolting and how to Keep an Onion from Bolting |author=Rhoades, Jackie |website=Gardening Know How |access-date=2013-03-27 |archive-date=1 May 2013 |archive-url=https://web.archive.org/web/20130501001809/http://www.gardeningknowhow.com/edible/vegetables/onion/onion-bolting.htm |url-status=live }}</ref>
[[File:"Oregon Onion Market" "Higher Prices" "Dealer" "I Guess We Had Better Let Go, Boys." 3 cent market cartoon in The Oregon Daily Journal., February 19, 1905, page 22.jpg|left|thumb|1905 cartoon about high market prices for onions]]
Onions may be grown from seeds or from partially grown [[Biology:Bulb|bulbs called "sets"]] or [[Biology:Starter bulb|starter bulb]]s. 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.<ref name=almanac>{{cite web |url=http://www.almanac.com/plant/onions |title=Onions: Planting, Growing and Harvesting Onion Plants |website=The Old Farmer's Almanac |access-date=2013-03-27 |archive-date=5 November 2012 |archive-url=https://web.archive.org/web/20121105125106/http://www.almanac.com/plant/onions |url-status=live }}</ref><ref>{{cite web |url=http://www.ars.usda.gov/Aboutus/docs.htm?docid=6463 |title=Onion production |date=2011-02-23 |publisher=USDA: Agricultural Research Service |access-date=2013-03-27 |archive-date=11 April 2014 |archive-url=https://web.archive.org/web/20140411135334/http://www.ars.usda.gov/Aboutus/docs.htm?docid=6463 |url-status=live }}</ref> 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.<ref name=RHS />

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.<ref name=PFAF>{{PFAF |access-date=2013-03-22}}</ref> Certain cultivars used for growing and storing bulbs may not have such good storage characteristics as those grown directly from seed.<ref name=RHS>{{cite book |title=The Royal Horticultural Society Encyclopedia of Gardening |editor-last=Brickell |editor-first=Christopher |year=1992 |publisher=Dorling Kindersley |isbn=978-0-86318-979-1 |page=345}}</ref>

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.<ref name=RHS />

===Pests and diseases===
[[File:Uienvlieg maden.jpg|thumb|Larvae of the onion fly]]

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.<ref name=Hessayon>{{cite book |title=Be your own Vegetable Doctor |last=Hessayon |first=D.G. |year=1978 |publisher=[[Company:Pan Britannica Industries|Pan Britannica Industries]] |isbn=978-0-903505-08-6 |pages=22–23}}</ref> The larvae of the onion leaf miner or leek moth (''Acrolepiopsis assectella'') sometimes attack the foliage and may burrow down into the bulb.<ref>{{cite journal |author=Landry, Jean-François |s2cid=86748199 |year=2007 |title=Taxonomic review of the leek moth genus ''Acrolepiopsis'' (Lepidoptera: Acrolepiidae) in North America |journal=The Canadian Entomologist |volume=139 |issue=3 |pages=319–353 |doi=10.4039/n06-098}}</ref>

The onion fly (''[[Biology:Delia antiqua|Delia antiqua]]'') 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.<ref>{{cite web |url=http://www.agroatlas.ru/en/content/pests/Delia_antiqua/ |title=''Delia antiqua'' (Meigen): Onion Fly |website=Interactive Agricultural Ecological Atlas of Russia and Neighboring Countries |access-date=2013-03-29 |archive-date=14 May 2013 |archive-url=https://web.archive.org/web/20130514095654/http://www.agroatlas.ru/en/content/pests/Delia_antiqua/ |url-status=live }}</ref>

The onion eelworm (''[[Biology:Ditylenchus dipsaci|Ditylenchus dipsaci]]''), a tiny parasitic soil-living [[Biology:Nematode|nematode]], 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 [[Biology:Carrot|carrot]]s, [[Biology:Parsnip|parsnip]]s, and [[Biology:Bean|bean]]s, which are also susceptible to the eelworm.<ref>{{cite web |url=http://www.gardenaction.co.uk/techniques/pests/onion_eelworm.htm |title=Onion Eelworm (''Ditylenchus dipsaci'') |year=2011 |website=GardenAction |access-date=2013-03-29 |archive-date=3 February 2013 |archive-url=https://web.archive.org/web/20130203011252/http://www.gardenaction.co.uk/techniques/pests/onion_eelworm.htm |url-status=live }}</ref>

White rot of onions, leeks, and garlic is caused by the soil-borne fungus ''[[Biology:Stromatinia cepivora|Sclerotium cepivorum]]''. 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 [[Biology:Mycelium|mycelia]], which later produces small, globular black structures called [[Biology:Sclerotium|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.<ref>{{cite web |url=https://www.rhs.org.uk/advice/profile?pid=226 |title=Onion white rot |website=RHS Gardening |publisher=Royal Horticultural Society |access-date=2013-03-29 |archive-date=4 August 2018 |archive-url=https://web.archive.org/web/20180804231507/https://www.rhs.org.uk/advice/profile?pid=226 |url-status=live }}</ref>

Neck rot is a fungal disease affecting onions in storage. It is caused by ''[[Biology:Botrytis allii|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 [[Biology:Spore|spore]]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.<ref>{{cite web |url=https://www.rhs.org.uk/advice/profile?PID=747 |title=Onion neck rot |website=RHS Gardening |publisher=Royal Horticultural Society |access-date=2013-03-29 |archive-date=4 August 2018 |archive-url=https://web.archive.org/web/20180804231835/https://www.rhs.org.uk/advice/profile?PID=747 |url-status=live }}</ref>
{|class="wikitable" style="float:right; clear:left; width:14em; text-align:center;"
! colspan=2|Production of onions and shallots (green) in 2021
|-
! style="background:#ddf; width:75%;"|Country
! style="background:#ddf; width:25%;"|<small>[[Tonne|tonne]]s</small>
|-
|{{CHN}}||869,838
|-
|{{MLI}}||684,653
|-
|{{JPN}}||512,933
|-
|{{KOR}}||424,457
|-
|'''World''' ||'''4,562,530'''
|-
|colspan=2|<small>Source: UN Food and Agriculture Organization<ref name="faostat">{{cite web |url=http://www.fao.org/faostat/en/#data/QC |title=Production of onions and shallots (green) in 2021: Crops/World Regions/Production Quantity from pick lists |publisher=United Nations Food and Agriculture Organization, Statistics Division (FAOSTAT) |date=2023 |access-date=23 April 2023 }}</ref></small>
|}

==Production==

[[File:Onion grading at the Centre for Agroecology, Water and Resilience (CAWR), Coventry University.webm|thumb|Onion grading at the Centre for Agroecology, Water and Resilience, England, using a manually-operated sorter]]

In 2021, world production of onions and shallots (as green produce) was 4.6 million [[Tonne|tonne]]s, led by {{wipe|China}} with 19% of the world total, with Mali, {{wipe|Japan}}, and South Korea as secondary producers.<ref name=faostat/>

==Storage==

===In the home===

Cooking onions and sweet onions are better stored at [[Physics:Room temperature|room temperature]], 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 [[Medicine:Decomposition|decay]].<ref name=almanac /><ref name=Jauron>{{cite web |url=http://www.extension.iastate.edu/news/2009/jul/062201.htm |title=Harvesting and storing onions |author=Jauron, Richard |date=2009-07-27 |publisher=Iowa State University Extension |access-date=2013-03-28 |archive-date=18 May 2013 |archive-url=https://web.archive.org/web/20130518151139/http://www.extension.iastate.edu/news/2009/jul/062201.htm |url-status=live }}</ref>

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.<ref name=FAQ />

==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, [[Biology:Allium fistulosum#Ambiguous names|salad onions]], and pickling onions belong to this group.{{r|fritsch2002|p=20–21}} The range of diversity found among these cultivars includes variation in photoperiod (length of day that triggers bulbing), storage life, flavour, and skin colour.<ref>{{cite book |last=Brewster |first=James L. |title=Onions and other vegetable Alliums |edition=1st |year=1994 |publisher=CAB International |location=Wallingford, UK |isbn=978-0-85198-753-8 |page=5}}</ref> Common onions range from the pungent varieties used for dried soups and onion powder to the mild and hearty sweet onions, such as the Vidalia from Georgia, US, or Walla Walla from {{wipe|Washington (state)}} that can be sliced and eaten raw on a sandwich.
[[File:Cipollatropea.jpg|thumb|''Rossa di Tropea'' onions for sale in Italy]]

===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.{{r|fritsch2002|p=20–21}}

I'itoi onion is a prolific multiplier onion cultivated in the Baboquivari Peak Wilderness, 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 {{convert|1|in|mm|order=flip|0|abbr=on}} below the surface and {{convert|12|in|mm|order=flip|sigfig=1|abbr=on}} 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.<ref>{{cite web |url=http://www.slowfoodusa.org/index.php/programs/ark_product_detail/iitoi_onion/ |title=I'Itoi Onion |year=2010 |website=Ark of Taste |publisher=Slow Food USA |access-date=2013-03-25 |archive-date=23 August 2013 |archive-url=https://web.archive.org/web/20130823061009/http://www.slowfoodusa.org/index.php/programs/ark_product_detail/iitoi_onion |url-status=dead}}</ref>

====Hybrids with ''A. cepa'' parentage====
A number of hybrids are cultivated that have ''A. cepa'' parentage, such as the diploid [[Biology:Tree onion|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 [[Biology:Hybrid|hybrid]] 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''.<ref>{{GRIN |''Allium'' × ''proliferum'' |404736 |access-date=21 February 2011}}</ref>{{r|fritsch2002|p=19}} It has been grown for centuries in Japan and China for use as a salad onion.<ref>{{cite book |last=Brewster |first=James L. |title=Onions and other vegetable Alliums |edition=1st |year=1994 |publisher=CAB International |location=Wallingford, UK |isbn=978-0-85198-753-8 |page=15}}</ref>{{r|fritsch2002|p=9–10}}

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.{{r|fritsch2002|p=19}} 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 [[Biology:Monophyletic|monophyletic]] origin for this species.<ref>{{cite journal |author1=Friesen, N. |author2=M. Klaas |s2cid=26205471 |name-list-style=amp |year=1998 |title=Origin of some vegetatively propagated ''Allium'' crops studied with RAPD and GISH |journal=Genetic Resources and Crop Evolution |volume=45 |issue=6 |pages=511–523 |doi=10.1023/A:1008647700251}}</ref>

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.<ref>{{cite book |last=Brewster |first=James L. |title=Onions and other vegetable alliums |edition=1st |year=1994 |publisher=CAB International |location=Wallingford, UK |isbn=978-0-85198-753-8 |page=3}}</ref>

==See also==
*[[Biology:List of Allium species|List of ''Allium'' species]]
*[[Biology:List of onion cultivars|List of onion cultivars]]
*Pyruvate scale

==References==
{{Reflist}}

==Further reading==
{{Refbegin}}
*{{cite book | last = Block | first= E. |title=Garlic and Other Alliums: The Lore and the Science |publisher=Royal Society of Chemistry (UK) |year=2010 |isbn= 978-0-85404-190-9}}
*{{cite journal | last1 = Gripshover | first1 =  Margaret M. | first2= Thomas L.  | last2 = Bell|title=Patently Good Ideas: Innovations and Inventions in U.S. Onion Farming, 1883–1939| journal = Material Culture | date= Spring 2012 | volume= 44| issue = 1 | pages= 1–30 | jstor=23145818}}
*{{cite book |author=Sen, Colleen T. | year= 2004|title=Food culture in India| publisher = Greenwood Publishing |isbn= 0-313-32487-5}}
{{Refend}}

==External links==
{{Sister project links
<!-- Configuration parameters. Do not leave empty; populate, or remove -->
 |1= |auto= |collapsible= |display= |position= |style=

<!-- Projects that default to "yes" when auto=no -->
 |b= |c= |n= no|q=no |s=1911 Encyclopædia Britannica/Onion |v=no |wikt= onion

<!-- Projects that default to "no" when auto=no -->
 |d= |iw= |m= |mw= |species=yes |species_author= |voy=

 |author= <!-- force search for author name in wikisource -->
 |commonscat= Allium cepa
 |cookbook= Onion
}}
*{{Cookbook-inline|Onion powder}}
*[https://web.archive.org/web/20140301113518/http://database.prota.org/dbtw-wpd/exec/dbtwpub.dll?AC=QBE_QUERY&BU=http%3A%2F%2Fdatabase.prota.org%2Fsearch.htm&TN=PROTAB~1&QB0=AND&QF0=Species+Code&QI0=Allium+cepa&RF=Webdisplay PROTAbase on ''Allium cepa''] (archived 1 March 2014)

{{Allium}}
{{Transient receptor potential channel modulators}}
{{Taxonbar|from=Q23485}}

[[Category:Allium]]
[[Category:Onions]]
[[Category:Herbs]]

{{Sourceattribution|Onion}}