Allium cepa Seeds | Onion

Onion (Allium cepa) — Onion (*Allium cepa*): native to Central Asia, from the Iran to Pakistan border through the western Himalayas, domesticated across the Near East and long cultivated worldwide.

LATIN NAME: Allium cepa

COMMON NAME: Onion

Plant Profile

A bulb recovered from a Bronze Age settlement in the Near East proves the onion was already a stored food before the first written recipes, and its dry, transportable form made it a rare provision rich in keeping quality that travelled without spoiling. Few crops can claim such a continuous place at the human table, which is why the onion appears in kitchen records from Egypt to China. What looks like a simple vegetable is in fact one of the most ancient cultivated plants still grown in an unchanged form.

The onion is a tunicate bulb: a short underground stem, the basal plate, carries concentric fleshy scale leaves that are really swollen storage leaves, wrapped in dry, membranous tunics. The visible part above ground is a fan of hollow, cylindrical, blue-green leaves, each one a modified blade. A scape may rise in the second season carrying a spherical umbel of small, six-petalled, white to greenish flowers, after which the plant sets small, black, angular seeds.

Wild Allium species across central Asia show the ancestry of the cultivated bulb, but Allium cepa itself lost the ability to set fertile seed freely in the wild and now depends on people. Its close relatives include the bunching onion Allium fistulosum and the perennial tree onion, yet the common onion is unusual among alliums in forming a single large bulb rather than a cluster. Cultivated forms divide into storage types with tight necks and sweet types with looser flesh.

Science & Biotechnology

The onion’s bite and its famous ability to bring tears are both products of a two-part chemical system kept in separate cells until damage joins them. The flavour begins as odourless sulphur precursors and ends as a cloud of volatile compounds above the cutting board.

The principal precursor in the common onion is isoalliin, an S-alk(en)yl cysteine sulphoxide. When cells are crushed, the enzyme alliinase acts on it and produces a range of thiosulphinates plus a separate pathway led by the enzyme lachrymatory-factor synthase, which makes the volatile propanethial S-oxide that stings the eyes. Different onion groups carry different balances of these compounds, which is why a sweet onion barely makes you weep while a sharp storage onion clouds the kitchen.

Among the breakdown products is ajoene, a sulphur-rich compound formed from allicin that has been studied for its effects on blood and on microbes. Alongside the sulphur pool, the scale tissue stores fructans that the plant draws on while the bulb rests, and these soluble sugars are what slowly soften the raw bite during long storage. The chemistry is the reason the onion works as both a flavour base and a traditional remedy.

Sliced thin and cooked slowly in a little fat, the fructose and the sulphur compounds transform into a deep, sweet, savoury mass that anchors French soups and Indian curries alike. The long, gentle heat drives off the sharp volatiles and concentrates the sugars held in the scale tissue.

Uses & Benefits

Almost every savoury cuisine builds its base from the onion, and the same bulb that flavours a stew once served as a household medicine for coughs and wounds in European folk practice.

Dehydrated and granulated onion keeps the sulphur aroma in a shelf-stable form for the off-season cook, and the dried product is a staple of camp and emergency provisioning because the bulb’s low moisture is easy to remove and hard to regain. The fresh bulb, though, remains the reference point for taste.

Growing from Seed

Onions grow best from a fine seed sown early into a prepared bed, then moved as young plants once the weather steadies. A medium of equal parts loam, compost, and sharp sand drains well and warms quickly in spring.

Press the small black seed onto the surface of firm, moist soil and cover it with only a thin veil of fine soil or vermiculite, because light helps it sprout evenly. Keep the bed damp but not soggy until the green thread leaves appear.

Transplant when the seedling shows three true leaves and a pencil-thick stem, setting each plant so the basal plate sits just at the soil surface, about ten centimetres apart in the row. Thin by removing the weakest once six true leaves stand, and avoid burying the neck, which invites rot. For a different bulb habit the Southport White Globe is the keeper to try, while the Walla Walla gives a sweet alternative and the Dutch shallot offers the clustered form.

Our Take

I sow a short row of the common onion every spring and count on it for the base note of almost every meal. The Egyptian Onion china Allium x proliferum is a cheerful perennial neighbour, and the Welsh Onion Allium fistulosum Evergreen keeps the kitchen supplied with green tops long after the bulbs are spent; the Garlic Chives seed Allium tuberosum and the Chives Allium schoenoprasum round out the allium bed.

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