Onion

The onion's wild ancestor is believed to be Allium cepa var. celebicum or related wild species native to Central Asia, specifically the region spanning modern-day Afghanistan, Iran, and the Central Asian republics.

• Domesticated at least 5,000 years ago — among the oldest cultivated plants
• Ancient Sumerians grew onions extensively; they are documented in cuneiform tablets from 2400 BCE
• Ancient Egyptians revered onions, placing them in tombs and swearing oaths on them — the concentric rings symbolized eternal life
• Roman gladiators rubbed onions on their muscles to firm them before combat
• Christopher Columbus brought onions to the Americas on his second voyage in 1493
• The Pilgrims brought onions to North America in 1620
• Modern breeding has produced thousands of cultivars adapted to virtually every climate zone
• The genus Allium contains approximately 750 to 900 species, including garlic, leeks, chives, and shallots

Allium cepa is a biennial plant producing a swollen, subterranean storage bulb.

Roots:
• Fibrous, shallow root system extending only 30 to 45 cm deep
• White, unbranched, and relatively weak compared to many vegetables

Bulb:
• The edible organ — a compressed, subterranean stem surrounded by fleshy, modified leaf bases (scales)
• Variable in size, shape, and color depending on cultivar: flattened globe to elongated, 5 to 20 cm in diameter
• Outer dry scales (tunic) may be golden-yellow, red, purple, brown, or white
• Inner flesh white, cream, pale green, or tinged with purple
• Pungency ranges from mild and sweet to intensely sharp and burning

Leaves:
• Hollow, tubular, blue-green, waxy, 30 to 100 cm tall
• Produced in a fan from the top of the bulb

Flowers:
• Spherical umbels 5 to 10 cm across on a tall, hollow scape (seed stalk)
• Numerous small white to greenish-white flowers with 6 tepals
• Insect-pollinated, primarily by bees and flies

Seeds:
• Small, black, angular, 2 to 3 mm long
• Short viability — typically 1 to 2 years under normal storage

Onions are moderately nutritious and provide important dietary phytochemicals.

Per 100 g raw onion:
• Energy: approximately 40 kcal
• Carbohydrates: 9.3 g (including 4.2 g sugars and 1.7 g fiber)
• Protein: 1.1 g
• Fat: 0.1 g
• Vitamin C: 7.4 mg (12% DV)
• Vitamin B6: 0.12 mg (9% DV)
• Folate: 19 mcg (5% DV)
• Potassium: 146 mg
• Manganese: 0.129 mg

Phytochemicals:
• Rich in quercetin — a powerful flavonoid antioxidant with anti-inflammatory properties
• Contains allicin and other organosulfur compounds with demonstrated cardiovascular benefits
• Fructo-oligosaccharides (prebiotic fibers) support gut health
• Chromium content helps regulate blood sugar
• Regular consumption associated with reduced risk of certain cancers, heart disease, and stroke

Generally safe for human consumption but with notable cautions.

• The tear-inducing compound syn-propanethial-S-oxide is formed when cells are cut — it converts to sulfuric acid upon contact with eye moisture, causing burning and tearing
• Can cause gastrointestinal distress in some individuals, particularly those with IBS or onion intolerance
• Toxic to dogs, cats, and livestock — causes hemolytic anemia by damaging red blood cells
• The toxic principle is N-propyl disulfide and related organosulfur compounds
• Affected animals develop weakness, pale gums, elevated heart rate, and reddish-brown urine
• All parts of the plant are toxic to susceptible animals, whether raw, cooked, or dried

Onions are adaptable to a wide range of growing conditions.

Soil and site:
• Prefer loose, well-drained, fertile, sandy loam soil with pH 6.0 to 7.0
• Heavy clay soils can restrict bulb expansion
• Full sun is essential for good bulb development

Planting:
• Can be started from seed, sets (small dormant bulbs), or transplants
• Direct-seed in early spring as soon as soil can be worked (cold-hardy)
• Sets provide the quickest harvest — push into soil with the pointed end up, 2 to 5 cm deep
• Space 10 to 15 cm apart in rows 30 to 45 cm apart
• Long-day varieties for northern latitudes (form bulbs with 14+ hours of daylight)
• Short-day varieties for southern latitudes (form bulbs with 10 to 12 hours of daylight)

Care:
• Keep soil consistently moist but not waterlogged during active growth
• Cease watering when tops begin to fall over — this signals bulb maturity
• Avoid high nitrogen fertilizers after bulbing begins
• Mulch to conserve moisture and suppress weeds

Harvest:
• Harvest when 75% of tops have fallen over and begun to dry
• Cure in a warm, dry, well-ventilated area for 2 to 3 weeks until outer scales are papery
• Store cured onions in a cool (0 to 4°C), dry, dark location — can keep for months

Onions are arguably the most versatile vegetable in global cuisine.

Culinary uses:
• Raw: sliced in salads, burgers, sandwiches, salsas, and garnishes
• Sautéed: the foundation of mirepoix (France), sofrito (Spain/Latin America), and suppengrün (Germany)
• Caramelized: slow-cooked onions develop deep sweetness and complexity
• Fried: onion rings, French fried onions, crispy toppings
• Pickled: red onions pickled in vinegar are a universal condiment
• Roasted: whole or halved onions become meltingly sweet
• Dehydrated: onion powder and flakes are essential pantry spices
• Boiled: base for stocks, soups, stews, and sauces
• Grilled: charred onions as a side dish or topping
• Fermented: used in kimchi and other fermented preparations
• Sweet varieties (Vidalia, Walla Walla, Maui) eaten raw like apples

Medicinal and other uses:
• Traditionally used for coughs, colds, and respiratory complaints
• Onion syrup (sliced onions soaked in honey) used as a folk remedy for sore throats
• Quercetin supplements derived from onion skins
• Onion juice used as a natural insect repellent in gardens
• Historically used as a dye — onion skins produce yellow, orange, and brown dyes