General · EdgepediaSeptember 17, 20268 min read

Onion

The onion (Allium cepa) is a cultivated bulb plant of the amaryllis family (Amaryllidaceae), grown worldwide for the layered, pungent bulb formed at ground level by thickened leaf bases.¹ It is one of the earliest vegetables cultivated by humans, with archaeological evidence of cultivation dating back more than 4,700 years, and it ranks second only to tomato in global vegetable production.²

Key fact	Detail
Scientific name and family	Allium cepa, amaryllis family (Amaryllidaceae); the species epithet cepa is Latin for onion¹
Origin	Thought to originate from central Asia and central Persia; the exact native origin is unknown, and genomic work supports Allium vavilovii as the wild progenitor¹^•²
Cultivation history	Archaeological evidence more than 4,700 years old; genome analysis estimates the onion lineage diverged from A. vavilovii about 6,100 to 8,800 years ago²
Genome size	A near-complete assembly (DH066619-V2) spans 15.86 Gb; the genome is over 95% repetitive DNA with an estimated 61,619 to 65,730 gene models²^•³
Bulbing trigger	Bulb formation is initiated by day length: short-day cultivars bulb at about 10 to 12 hours, intermediate-day at 12 to 14 hours, and long-day at 14 to 16 hours⁴
Growing conditions	Cool-season crop needing full sun and moist, well-drained fertile soil; optimal soil pH 6.0 to 6.5¹^•⁴
Storage	Cured onions store best at 65 to 70% relative humidity with about 1 m³/min per m³ of onion of air circulation; fungal growth resumes above 15 °C (59 °F)⁵
Toxicity	Contains N-propyl disulfide and is poisonous to cats, dogs, and horses, with low severity for humans¹

Plant description

When growing, onions look somewhat like grass but have hollow, round stems and long green leaves with a glaucous surface. The bulb sits at ground level and consists of fleshy, concentric leaf bases (scales) attached to a short, flattened underground stem called a disc; dry, papery outer leaves protect the bulb. Plants reach roughly 1 to 1.5 feet in height and 6 inches to 1 foot across, and produce small white or pink star- or bell-shaped flowers in umbels in summer.¹^•⁶

The bulb is a storage organ that lets the plant survive adverse conditions. During the growing season the bulb produces several offsets, and the crop is ready to harvest when at least two thirds of the tops have died down.¹

Types and relatives

The FAO school-garden factsheet distinguishes three main cultivated types. The common onion, from Central Asia, forms a single bulb that can grow as large as the palm of a hand. Garlic and shallots, from cooler climates, form a clump of small bulbs that fit together tightly. Bunching onions, from Eastern Asia, are also called spring onions, scallions, or chives in common usage; they are grown for their leaves rather than large bulbs, and can keep producing a few leaves weekly for six months after a single planting.⁶ In the Cepa section, A. cepa and the related wild species A. vavilovii, A. oschaninii, and A. galanthum are found chiefly in the Irano-Turanian region.³

Origin and genome

Despite millennia of cultivation, the origin and domestication of the onion long remained debated because of discordance among morphological, cytogenetic, and molecular evidence within the large genus Allium. A 2025 near-complete genome assembly, DH066619-V2, spanning 15.86 Gb, provided the contiguity needed to resolve this. Population analysis of 122 accessions traced a photoperiod-driven dispersal history and gave strong genomic support for Allium vavilovii as the wild progenitor.²

The same analysis dated the split of the onion lineage from A. vavilovii to roughly 6,100 to 8,800 years ago, which predates the earliest archaeological evidence for cultivation (more than 4,700 years ago); the authors note this is consistent with domestication occurring after lineage divergence, possibly under continued gene flow. Two genetic groups diverged about 2,300 years ago, likely reflecting human trade that spread onions from the center of domestication to Europe and the Mediterranean region, and a further divergence around 1,000 years ago matches expansion into higher latitudes, supported by Viking Age archaeological finds of onions in southern Scandinavia.²

The onion genome is enormous for a crop plant, about 16 Gb, and over 95% of it is repetitive DNA dominated by Gypsy- and Copia-type LTR retrotransposons that expanded in bursts. Despite this, the genome carries an estimated 61,619 to 65,730 gene models, with expansions in gene families linked to abiotic stress, carbohydrate metabolism, and flavor traits.³ Breeding progress has been slowed by the crop's biennial life cycle, cross-pollination, and severe inbreeding depression.²

Cultivation

Onions are cool-season crops planted in moist, well-drained, fertile loose soils in full sun, from seeds, sets, or transplants. Bulbs are set about half an inch to 1 inch deep and 6 inches apart, and the soil pH should be adjusted to 6.0 to 6.5. Because the root system is shallow, with most water taken from the upper 6 to 8 inches of soil, the crop is highly sensitive to poor drainage, compaction, and water stress, and drip irrigation is recommended over overhead watering.¹^•⁴

Photoperiod controls bulbing. Cultivars are classified by the day length required to initiate bulbing, not by planting date: short-day types bulb at about 10 to 12 hours and suit southern regions, intermediate-day types at 12 to 14 hours, and long-day types at 14 to 16 hours, which grow best in northern areas such as Delaware, Minnesota, and Canada.⁴^•⁷ Bulb development also requires a combination of long days and high temperatures; in the tropics, where day lengths never exceed 13 hours, temperate varieties usually fail to form bulbs, so matching the cultivar to the environment is the key to successful production.⁸ Planting too early in fall raises the risk of bolting, the premature formation of a seedstalk, which produces small, thick-necked bulbs that cure poorly and are generally unmarketable.⁴

Weed control is most critical during the first 6 to 8 weeks after emergence or transplanting, when competition can cause permanent yield loss. Onion thrips (Thrips tabaci) are the most economically important insect pest in North Carolina production, and major diseases include downy mildew, purple blotch, Botrytis leaf blight, and soilborne white rot; a 3 to 4 year rotation away from Allium crops is recommended.⁴

Harvest, curing, and storage

Onions are harvested when 50 to 80% of plants have undergone neck fall, signaling that bulb enlargement is complete; garden guidance is to dig when at least two thirds of the tops have died down, then cure the bulbs for 2 to 3 weeks before use. Curing dries the neck tissue and outer scales, forming a protective barrier against moisture loss and disease, and proper curing essentially prevents Botrytis neck rot, the watery decay that otherwise moves down through the bulb in storage.¹^•⁴^•⁵

Storage conditions matter as much as curing. Dry onions keep at 65 to 70% relative humidity with adequate air circulation, about 1 m³/min per m³ of onion; low temperature delays fungal growth, but growth resumes above 15 °C (59 °F). Recognized storage disorders include freezing injury, translucent scales (worsened when cold storage is delayed 3 to 4 weeks after curing), greening of outer scales in light, black mold from Aspergillus niger often associated with bruising, and brown-black blotches from ammonia gas leaks.⁵

Food use and nutrition

Onion and its relatives are considered essential in many countries for the taste, smell, and perceived health benefits they add to meals. Fresh slices or chopped leaves go into salads and garnishes, and chopped onion is cooked with rice, beans, and meat, and in relishes, soups, sauces, curries, and stews; consumer choice often follows variety, with white onion preferred in salads and red onion in stews.⁶^•⁸ Mature bulb onions are not a substantial source of calories or nutrients, but bunching onion leaves contain valuable amounts of vitamins A and C needed daily by children, and onion is pungent enough that it is not recommended for young babies.⁶

Clinical evidence. A 2023 systematic review and meta-analysis of 14 randomized controlled trials found that onion supplementation significantly improved body fat percentage, LDL cholesterol, total cholesterol, HDL cholesterol, systolic blood pressure, adiponectin, and the liver enzyme AST, with larger effects when interventions lasted more than 12 weeks or used doses above 300 mg/day.⁹ A separate meta-analysis of five clinical trials, all conducted in Korea, found significant reductions in body weight, BMI, waist circumference, and triglycerides, with the effect most pronounced for onion peel, which is rich in the flavonoid quercetin.¹⁰ A 2019 umbrella review of allium vegetable consumption and health, however, rated 88% of the 50 pooled outcomes as very low or low quality under the GRADE system, noted statistically significant publication bias for gastric cancer with onion, and found that for prostate cancer a protective effect appeared only for garlic, with no significant effect for onion.¹¹ A broader systematic review of traditional and modern uses cataloged antimicrobial, antioxidant, anti-inflammatory, anti-diabetic, hypolipidemic, anti-hypertensive, and immunoprotective pharmacological properties of A. cepa.¹²

Toxicity and safety

The plant contains N-propyl disulfide and is listed as poisonous to cats, dogs, and horses, with symptoms including vomiting, breakdown of red blood cells (hemolytic anemia), blood in urine, weakness, and high heart rate; severity is rated low for humans, and the bulbs and tops are edible raw or cooked for people.¹

References

Topic: Encyclopedia › Life and health › Plants and algae › Seed plants › Monocots › Lilioid monocots and Asparagales › Allium (allium genus) › Edible Allium species (non-crop-vegetable)

Initially written Sep 17, 2026 · Reviewed: Oct 21, 2026 · Edited: Oct 15, 2026; Oct 21, 2026 · Last review: Oct 21, 2026

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