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Allium is a large genus of flowering plants with around 1000 accepted species, making Allium the largest genus in the family Amaryllidaceae and amongst the largest plant genera in the world. Many of the species are edible, and some have a long history of cultivation and human consumption as a vegetable including the onion, garlic, scallion, shallot, leek, and chives, with onions being the second most grown vegetable globally after tomatoes as of 2023. (2025). 9781789249972, CAB International. ISBN 9781789249972
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), and tropical Africa ( A. spathaceum). They vary in height between . The form an umbel at the top of a leafless stalk. The vary in size between species, from small (around 2–3 mm in diameter) to rather large (8–10 cm). Some species (such as Welsh onion A. fistulosum and leeks ( A. ampeloprasum)) develop thickened leaf-bases rather than forming bulbs as such.
Carl Linnaeus first described the genus Allium in 1753. The generic name Allium is the Latin word for garlic,Gledhill, David (2008). "The Names of Plants". Cambridge University Press. (hardback), (paperback). pp 43 and the type species for the genus is Allium sativum which means "cultivated garlic". Allium In: Index Nominum Genericorum. In: Regnum Vegetabile (see below). The decision to include a species in the genus Allium is Plant taxonomy difficult, and species boundaries are unclear. Estimates of the number of species are as low as 260,Knud Rahn. 1998. "Alliaceae" pages 70-78. In: Klaus Kubitzki (editor). The Families and Genera of Vascular Plants volume III. Springer-Verlag: Berlin;Heidelberg, Germany. and as high as 979. The Plant List, for genus Allium In the APG III classification system, Allium is placed in the family Amaryllidaceae, subfamily Allioideae (formerly the family Alliaceae). In some of the older Systematics, Allium was placed in Liliaceae.James L. Brewster, "Onions and Other Alliums" (Wallingford: CABI Publishing, 2008)Haim D. Rabinowitch, Leslie Currah, "Allium Crop Sciences: Recent Advances" (Wallingford: CABI Publishing, 2002)Penny Woodward, "Garlic and Friends: The History, Growth and Use of Edible Alliums" (South Melbourne: Hyland House, 1996) Molecular phylogenetic studies have shown this circumscription of Liliaceae is not monophyletic.
Various Allium species have been cultivated from the earliest times. About a dozen species are economically important as crops, or garden vegetables, and an increasing number of species are important as ornamental plants. Plants of the genus produce chemical compounds, mostly derived from cysteine , that give them a characteristic 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. The characteristic Allium flavor depends on the sulfate content of the soil the plant grows in. In the rare occurrence of sulfur-free growth conditions, all Allium species completely lose their usual pungency.
The bulbs are solitary or clustered and tunicate and the plants are perennialized by the bulbs reforming annually from the base of the old bulbs, 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 Allium tricoccum. The leaves are sessile, and very rarely narrowed into a petiole.
The flowers, which are produced on scapes are erect or in some species pendent, having six petal-like produced in two whorls. The flowers have one Gynoecium and six epipetalous ; the and pollen can vary in color depending on the species. The ovaries are superior, and three-lobed with three .
The fruits are capsules that open longitudinally along the capsule wall between the partitions of the locule. The seeds are black, and have a rounded shape.
The or flattened flowering scapes are normally persistent. The inflorescences are , in which the outside flowers bloom first and flowering progresses to the inside. Some species produce bulbils within the umbels, and in some species, such as Allium paradoxum, 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.
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 pickling.
Many of the species of Allium have been used as food items throughout their ranges. There are several unrelated species that are somewhat similar in appearance to Alliums but are poisonous (e.g. in North America, death camas, Toxicoscordion venenosum), but none of these has the distinctive scent of onions or garlic.Peterson, R.P. 1982. A Field Guide to Edible Wild Plants: Eastern and central North America. Houghton Mifflin, Boston.Gibbons, E. 1962. Stalking the wild asparagus. David McKay, New York.
Allium includes a number of taxonomic groupings previously considered separate genera ( Caloscordum Herb., Milula Prain and Nectaroscordum Lindl.) Allium spicatum had been treated by many authors as Milula spicata, the only species in the Monotypic taxon genus Milula. In 2000, it was shown to be Paraphyly in Allium.
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. Meanwhile, the number of new species continued to increase, reaching 800 by 2009, and the pace of discovery has not decreased. Detailed studies have focused on a number of subgenera, including Amerallium. Amerallium is strongly supported as monophyletic. Subgenus Melanocrommyum has also been the subject of considerable study (see below), while work on subgenus Allium has focussed on section Allium, including Allium ampeloprasum, although sampling was not sufficient to test the monophyly of the section.
The major evolutionary lineages or lines correspond to the three major clades. Line one (the oldest) with three subgenera is predominantly bulbous, the second, with five subgenera and the third with seven subgenera contain both bulbous and rhizomatous taxa.
Various Allium species are used as food plants by the of the leek moth and onion fly as well as other 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, a moth that feeds exclusively on Allium species.
The following cultivars, of uncertain or mixed parentage, have gained the Royal Horticultural Society's Award of Garden Merit:
This genus also includes species that are abundantly gathered from the wild such as wild garlic ( Allium ursinum) in Europe and ramps ( Allium tricoccum) in North America.
Genetics
The genus Allium has very large variation between species in their genome size that is not accompanied by changes in ploidy level. This remarkable variation was noted in the discussion of the evolution of junk DNA and resulted in the Onion Test, a "reality check for anyone who thinks they have come up with a universal function for junk DNA". vary between 7.5 Gb in A. schoenoprasum and 30.9 Gb in Allium ursinum, both of which are diploid.
Telomere
The unusual telomeric sequence of 'Allium cepa' was discovered and cytologically validated to be CTCGGTTATGGG
A bioinformatics method for detecting this unique telomere sequence was demonstrated using SERF de novo Genome Analysis
Cultivation
Live show]]Many Allium'' species have been harvested through human history, but only about a dozen are still economically important today as crops or garden .
Ornamental
Many Allium species and hybrids are Horticulture as Ornamental plant.Anthony Huxley, Mark Griffiths, and Margot Levy (1992). The New Royal Horticultural Society Dictionary of Gardening. The Macmillan Press, Limited: London. The Stockton Press: New York. (set). These include A. cristophii and Allium giganteum, which are used as border plants for their ornamental flowers, and their "architectural" qualities.Brickell, Christopher (Editor-in-chief), The Royal Horticultural Society A–Z Encyclopedia of Garden Plants, p. 95, Dorling Kindersley, London, 1996, 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). RHS Plant Finder 2009–2010, p. 68, Dorling Kindersley, London, 2009, 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 ( Allium caeruleum), to yellow ( Allium flavum) and purple ( A. giganteum). By contrast, other species (such as invasive A. triquetrum and Allium ursinum) can become troublesome garden .Lloyd, Christopher & Rice, Graham, (1991). Garden Flowers From Seed, p. 45, Viking,
Toxicity
Dogs and cats are very susceptible to poisoning after the consumption of certain species. Cattle have occasionally suffered onion toxicosis as well. Vegetables of the Allium genus can cause digestive disorders for human beings.
Uses
The genus includes many economically important species. These include ( A. cepa), ( A. oschaninii), leeks ( A. ampeloprasum), garlic ( A. sativum), and such as scallions (various Allium species) and chives ( A. schoenoprasum). Some have been used as traditional medicines.
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