How to grow onions

Allium cepa is a biennial grown as an annual cool-season vegetable, spending most of its garden time as narrow, hollow green leaves before directing energy into a papery-skinned bulb underground. Shallow-rooted and slow to establish, it occupies a bed from very early spring through midsummer and needs consistent soil moisture throughout that stretch. Gardeners grow it for home-cured storage onions, which can keep under cool, dry conditions for up to six to seven months, and for green onions harvested by progressive thinning as the planting fills in.

The hardest part is matching the plant's photoperiod type to your latitude. Long-day types form bulbs when daylight reaches 14 or more hours, making them the right choice for Northern gardens; short-day types need only about 12 hours and are suited to the deep South. A second-year gardener who plants short-day sets in a Northern bed will grow impressive tops and then harvest small, poorly formed bulbs, because the extended summer days never trigger proper bulbing in a type built for a shorter photoperiod. Planting any type too late in spring carries the same consequence: the plant cannot build enough leaf mass before the bulbing signal arrives, and bulbs stay undersized.