Onion

Sowing: Onions can be started from seed, sets, or transplants in early spring as soon as the ground can be worked. Seed is sown about 1/4 to 1/2 inch deep, either indoors four to eight weeks before planting out or direct-sown once soil is workable; sets are planted pointed-end up, roughly 1 to 2 inches deep; transplants are set with about an inch of the lower stem buried. Spacing depends on the intended harvest: plants left close together and thinned are pulled young as green onions, while wider spacing of about 4 to 8 inches, with 12 to 24 inches between rows, allows bulbs to size up.

Siting and feeding: Onions need full sun and loose, well-drained, fertile soil rich in organic matter, with a near-neutral pH; heavy clay should be amended. They have a high nutrient demand and benefit from fertilizer worked in before planting plus side-dressing as bulbs begin to enlarge. Bulb size is directly tied to consistent watering through the season, and because roots are shallow, weeds must be controlled by hand-pulling or shallow cultivation rather than left to compete; soil should not be hilled up over the bulb, since this can promote neck and stem rot. Cultivars are grouped by the day length needed to trigger bulbing, and gardeners are advised to match long-day, short-day, or intermediate types to their latitude.

Pests: Onion crops are attacked by onion maggot, onion thrips, cutworms, wireworms, bulb mites, and allium leafminer, with thrips often hiding in the inner leaf folds near the bulb.

Diseases: Fungal problems include downy mildew, purple blotch and other Alternaria leaf spots, Botrytis leaf blight and neck rot, pink rot, smudge, smut, and white rot, alongside bacterial soft rot, slippery skin, and sour skin; viral diseases such as onion yellow dwarf and stem and bulb nematode infestations can also occur. Bulb rot develops readily in soils that stay too wet.

Disorders: Cold exposure early in growth or overly early planting can trigger premature flowering, or bolting, instead of bulb formation, and drought or waterlogging can cause additional stress and edema.

Picking: Green onions are pulled once their tops reach about 6 inches tall, becoming stronger in flavor as they age; bulb onions are left in the ground until at least two-thirds of the tops have died down and fallen over. Bulbs should be lifted gently, ideally with a garden fork, to avoid bruising or damaging the skin.

Storage: After digging, bulbs are cured by drying for two to three weeks before use, then kept in mesh bags or crates that allow good air circulation to extend storage life.

Culinary: The onion bulb and its green tops are eaten both raw and cooked, and the crop is grown chiefly as a savory kitchen staple used across many cuisines.

Historical: Accounts spanning more than 5,000 years describe onions being used by past peoples as food and medicine, as well as in art and in the mummification process.

Onion is considered a cultigen, a plant with no known wild counterpart, and its precise native origin remains undetermined, though it is traced broadly to Central Asia and central Persia. Its closest wild relatives, alliums with hollow leaves and a similar pungent smell, are scattered across the Middle East and Central Asia. The crop has been grown since the earliest days of agriculture in the Fertile Crescent and is thought to have spread outward from that region over roughly the past five thousand years.