Allium is a monocot genus of flowering plants, informally referred to as the onion genus. The generic name Allium, the Latin word for garlic, is said to come from the Greek αλεω, to avoid, because of its offensive smell. The genus, including the various edible onions, garlics, chives, and leeks, has played a pivotal role in cooking worldwide, as the various parts of the plants, either raw or cooked in many ways, produce a large variety of flavors and textures.
The genus contains hundreds of distinct species; many have been harvested through human history, but only about a dozen are still economically important today as crops or garden vegetables. Many others are cultivated as ornamental plants.
Allium is taxonomically difficult and species boundaries are unclear. Most authorities accept about 750 species. Estimates of the number of species have been as low as 260, and as high as 979. The type species for the genus is Allium sativum.
Allium species occur in temperate climates of the Northern Hemisphere, except for a few species occurring in Chile (such as A. juncifolium), Brazil (A. sellovianum) or tropical Africa (A. spathaceum). They can vary in height between 5 cm and 150 cm. The flowers form an umbel at the top of a leafless stalk. The bulbs vary in size between species, from very small (around 2–3 mm in diameter) to rather large (8–10 cm). Some species (such as Welsh onion, A. fistulosum) develop thickened leaf-bases rather than forming bulbs as such. Allium is a genus of perennial bulbous plants that produce chemical compounds (mostly derived from cysteine sulfoxides) that give them a characteristic (alliaceous) onion or garlic taste and odor. Many are used as food plants, though not all members of the genus are equally flavorful. In most cases, both bulb and leaves are edible. Their taste may be strong or weak, depending on the species and on ground sulfur (usually as sulfate) content (in the rare occurrence of sulfur-free growth conditions, all Allium species will lack their usual pungency altogether).
In the APG III classification system, Allium is placed in the family Amaryllidaceae, subfamily Allioideae (formerly the family Alliaceae). In some of the older classification systems, Allium was placed in Liliaceae. Molecular phylogenetic studies have shown this circumscription of Liliaceae is not monophyletic.
Allium is one of about 57 genera of flowering plants with more than 500 species. It is by far the largest genus in the Amaryllidaceae, and also in the Alliaceae in classification systems in which that family is recognized as separate.
Allium species are herbaceous perennials with flowers produced on scapes. They grow from solitary or clustered tunicate bulbs and many have an onion odor and taste. Plants are perennialized by bulbs that reform annually from the base of the old bulb, or are produced on the ends of rhizomes or, in a few species, at the ends of stolons. A small number of species have tuberous roots. The bulbs' outer coats are commonly brown or grey, with a smooth texture, and are fibrous, or with cellular reticulation. The inner coats of the bulbs are membranous.
Many alliums have basal leaves that commonly wither away from the tips downward before or while the plants flower, but some species have persistent foliage. Plants produce from one to 12 leaves, most species having linear, channeled or flat leaf blades. The leaf blades are straight or variously coiled, but some species have broad leaves, including A. victorialis and A. tricoccum. The leaves are sessile, and very rarely narrowed into a petiole.
The terete or flattened flowering scapes are normally persistent. The inflorescences are umbels, in which the outside flowers bloom first and flowering progresses to the inside. Some species produce bulbils within the umbels, and in some species the bulbils replace some or all the flowers. The umbels are subtended by noticeable spathe bracts, which are commonly fused and normally have around three veins.
The flowers are erect or in some species pendent, having six petal-like tepals produced in two whorls. The flowers have one style and six epipetalous stamens; the anthers and pollen can vary in color depending on the species. The ovaries are superior, and three-lobed with three locules.
Some bulbous alliums increase by forming little bulbs or "offsets" around the old one, as well as by seed. Several species can form many bulbils in the flowerhead; in the so-called "tree onion" or Egyptian onion (A. × proliferum) the bulbils are few, but large enough to be pickled.
Many of the species of Allium have been used as food items throughout their ranges. There are several poisonous species that are somewhat similar in appearance (e.g. in North America, death camas, Toxicoscordion venenosum), but none of these has the distinctive scent of onions or garlic. 
The taxonomy of Allium is poorly understood, with incorrect descriptions being widespread. Allium spicatum has been treated by many authors as Milula spicata, the only species in the monospecific genus Milula. In 2000, it was shown to be embedded in Allium.
In 2006, a phylogeny of Allium was published based on the nuclear ribosomal gene ITS. The authors of this study divided Allium into 15 subgenera and 72 sections. They defined the subgenus Rhizirideum in a much narrower sense than in previous classifications.
Subsequent molecular phylogenetic studies have shown the 2006 classification is a considerable improvement over previous classifications, but some of its subgenera and sections are probably not monophyletic. One of these studies focused on the subgenus Amerallium, which is strongly supported as monophyletic. Another study focused on Allium ampeloprasum and its relatives within the section Allium of subgenus Allium. Sampling in this study was not sufficient to test the monophyly of section Allium.
The majority of Allium species are native to the Northern Hemisphere, mainly in Asia. A few species are native to Africa and Central and South America. Species grow in various conditions from dry, well-drained mineral-based soils to moist, organic soils; most grow in sunny locations, but a number also grow in forests (e.g., A. ursinum), or even in swamps or water.
Many Allium species have been harvested through human history, but only about a dozen are still economically important today as crops or garden vegetables. These include onions (A. cepa), French shallots (A. oschaninii), leeks (A. ampeloprasum), scallions (various Allium species), and herbs such as garlic (A. sativum) and chives (A. schoenoprasum). Others are cultivated as ornamentals.
Some Allium species, including A. cristophii and A. giganteum, are used as border plants for their ornamental flowers, and their "architectural" qualities. Several hybrids have been bred, or selected, with rich purple flowers. A. hollandicum 'Purple Sensation' is one of the most popular and has been given an Award of Garden Merit (H4). These ornamental onions produce spherical umbels on single stalks in spring and summer, in a wide variety of sizes and colours, ranging from white (Allium 'Mont Blanc'), blue (A. caeruleum), to yellow (A. flavum) and purple (A. giganteum). By contrast, other species (such as invasive A. triquetrum and A. ursinum) can become troublesome garden weeds.
Various Allium species are used as food plants by the larvae of the leek moth and onion fly as well as some Lepidoptera including cabbage moth, common swift moth (recorded on garlic), garden dart moth, large yellow underwing moth, nutmeg moth, setaceous Hebrew character moth, turnip moth and Schinia rosea, a moth that feeds exclusively on Allium species.
See List of Allium species for a full list. Some important species include:
- Allium acuminatum — tapertip onion, Hooker's onion
- Allium allegheniense — Appalachian onion
- Allium ampeloprasum — (broadleaf) wild leek
- Allium anceps — twinleaf onion
- Allium angulosum — mouse garlic
- Allium aflatunense — flowering onion
- Allium atrorubens — dark red onion
- Allium caeruleum — blue globe onion
- Allium campanulatum — dusky onion
- Allium canadense — Canadian garlic
- Allium cepa — onion, garden onion, bulb onion, common onion
- Allium cernuum — nodding onion
- Allium chinense — Chinese onion, Chinese scallion, Japanese scallion, Oriental onion
- Allium fistulosum — Welsh onion
- Allium galanthum
- Allium giganteum — giant onion
- Allium hollandicum — flowering onion
- Allium libani — Lebanese onion
- Allium neapolitanum — white garlic
- Allium nevii — Nevius' garlic
- Allium nigrum — black garlic
- Allium oleraceum — field garlic
- Allium oschaninii — shallot
- Allium paradoxum — few-flowered garlic
- Allium ramosum — fragrant garlic
- Allium sativum — garlic
- Allium schoenoprasum — chives
- Allium scorodoprasum — sand leek
- Allium siculum — Mediterranean bells, Sicilian honey garlic
- Allium sieberianum — Schult.f.
- Allium stipitatum — Persian shallot
- Allium textile — prairie onion
- Allium tricoccum — wild leek, ramp
- Allium triquetrum — three-cornered leek, triquetous garlic
- Allium tuncelianum — Tunceli garlic, Ovacik garlic
- Allium tuberosum — Chinese chives
- Allium ursinum — buckrams, wild garlic, broad-leaved garlic, wood garlic, bear's garlic
- Allium vineale — crow garlic, wild garlic
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||This article includes a list of references, but its sources remain unclear because it has insufficient inline citations. (May 2012)|
- Pacific Bulb Society: Rhizomatous Alliums
- J. G. Dubouzet, K. Shinoda and N. Murata. Phylogeny of Allium L. subgenus Rhizirideum (G. Don ex Koch) Wendelbo according to dot blot hybridization with randomly amplified DNA probes TAG Theoretical and Applied 'Genetics. Volume 95, Number 8, December, 1997
- Haim D. Rabinowitch, Lesley Currah. Allium crop science: recent advances. CABI Publishing Series, 2002. ISBN 978-0-85199-510-6
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- Block, E. (2010). Garlic and Other Alliums: The Lore and the Science. Royal Society of Chemistry. ISBN 978-0-85404-190-9.
- Brewster, J. L. (2008). Onions and Other Alliums. CABI Publishing. ISBN 978-1-84593-399-9.
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- Rabinowitch, H. D., Currah, L. (2002). Allium Crop Sciences: Recent Advances. CABI Publishing. ISBN 0-85199-510-1.
- Woodward, P. (1996). Garlic and Friends: The History, Growth and Use of Edible Alliums. Hyland House. ISBN 1-86447-009-7.
|Wikimedia Commons has media related to Allium.|
- "WCSP". World Checklist of Selected Plant Families. (enter "Allium" in search box).
- Allium At:Index Nominum Genericorum At:References At:NMNH Department of Botany
- Bloomsta.com Florist Community
- Pacific Bulb Society