Editing Onion (section)

== Varieties ==

{{further|List of onion cultivars}}

[[File:Cipollatropea.jpg|thumb|upright|''Rossa di Tropea'' onions for sale in Italy]]

=== 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, [[Allium fistulosum#Ambiguous names|salad onions]], and pickling onions belong to this group.{{sfn|Fritsch|2002|pp=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> 

===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.{{sfn|Fritsch|2002|pp=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====

[[File:Sint Jansui (Allium fistulosum var. bulbifera).jpg|thumb|The tree or Egyptian onion is a hybrid of ''A. cepa'' and ''A. fistulosum''.]]

Some hybrids are cultivated that have ''A. cepa'' parentage, such as the [[Diploid#Polyploid types|diploid]] [[tree onion]] or Egyptian onion (''A.'' ×''proliferum''), and the [[Triploid#Polyploid types|triploid]] onion (''A.'' ×''cornutum'').<ref>{{cite journal |author=Friesen, N. & M. Klaas |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 |s2cid=26205471 }}</ref><!--covers diploid and triploid hybrids-->

The tree onion or Egyptian onion produces bulblets in the umbel instead of flowers, and is now known to be a [[hybrid (biology)|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>{{sfn|Fritsch|2002|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>{{sfn|Fritsch|2002|pp=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.{{sfn|Fritsch|2002|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 [[monophyletic]] origin for this species.<ref>{{cite journal |last1=Friesen |first1=N. |last2=Klaas |first2=M. |s2cid=26205471 |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>

[[Spring onion]]s 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>