description
Allium cepa is a biennial plant but it is usually grown as an annual. It is part of the Amaryllidaceae family. Modern varieties typically grow to a height of 15 to 45 cm (6 to 18 in) with blueish-green leaves that grow alternately in a flattened, fan-shaped swathe. The leaves fleshy, hollow and cylindrical, with one flattened side. They are at their broadest about a quarter of the way up the stalk beyond which they taper towards a blunt tip. The base of each leaf is a flattened (usually white) sheath that grows out of a basal disc. From the underside of the disc, a bundle of fibrous roots extends for a short way into the soil. As the onion plant matures, food reserves begin to accumulate in the leaf bases and the bulb of the onion will swell.

In the autumn the leaves die back and the outer scales of the bulb become dry and brittle, and this is the time at which the crop is normally harvested. If left in the soil over winter, the growing point in the middle of the bulb begins to develop in the spring. New leaves appear and a long, stout, hollow stem expands, topped by a bract protecting a developing inflorescence. The flower-head takes the form of a globular umbel of white flowers with parts in sixes. The seeds are glossy black and triangular in cross section.

common names & nomenclature
Onion is from Anglo-French union; Old French oignon meaning "onion" (formerly also oingnon), and directly from Latin unionem (nominative unio), a colloquial rustic Roman for "a kind of onion”.

Also known as:
onion, bulb onion, common onion

Allium cepa is known exclusively from cultivation, but related wild species occur in Central Asia.

climate
Onions are a cool-weather crop and are grown in cultivation. Hot temperatures or other stressful conditions cause them to "bolt", meaning that a flower stem begins to grow.

soil
Onions are best cultivated in fertile soils that are well-drained. Sandy loams are good as they are low in sulphur, while clayey soils usually have high sulphur content and produce pungent bulbs. Onions will require a high level of nutrients in the soil.

growing
Onions may be grown from seed or from sets. Onion seeds are short lived and fresh seed germinates better. In suitable climates, certain cultivars can be sown in late summer and autumn to overwinter in the ground and produce early crops the following year.

harvesting
Harvest onions in autumn when the leaves have died back, dry for later use as cut pieces or ground powder.

preserving
Store dried onion in an airtight container in a cool, dry place.