How to grow Onion

Where to plant

Onions need full sun and fertile, well-drained soil rich in organic matter. They have shallow roots and compete poorly with weeds, especially in the first two months, so start with a clean bed.

Onions form bulbs in response to day length. Varieties are sold as short-day, intermediate (mid) or long-day, depending on how many hours of daylight they need to bulb. Short-day types suit the South, long-day types the North, and intermediate types the middle of the country. A northern variety grown in the South never bulbs properly, and the reverse gives small bulbs too early.

• Florida, the Gulf South, Texas, the Desert Southwest and Southern California: plant short-day onions such as 'Granex', the Vidalia type, from September through December. UF/IFAS notes that bulbing onions in Florida must be planted in fall and be short-day types.
• The Mid-South, Mid-Atlantic, Southern Plains, Northern California and Pacific Northwest: plant intermediate-day onions in late winter or early spring, or short-day types in fall where winters are mild.
• The Northeast, Midwest, Northern Plains, Mountain West and Alaska: plant long-day or regionally recommended onions in early spring as soon as the soil can be worked; transplants need about 8 weeks of growth before planting out.

Choosing a variety

For Florida, UF/IFAS suggests 'Granex' for bulbs and 'Evergreen Bunching' or 'White Lisbon Bunching' for green onions. In Utah and similar northern areas, Utah State recommends 'Utah Sweet Spanish', 'Candy', 'Walla Walla', 'Cabernet' (red) and 'Sweet Sandwich', which store 2 to 4 months. Check the day-length type on any seed or set package.

How to plant

1. Sow seed 1/4 to 1/2 inch deep, or plant sets about 1 inch deep, upright, or set out transplants.
2. Thin or space plants 3 to 4 inches apart in the North, or 4 to 6 inches apart in Florida, in rows 8 to 16 inches apart. Eat the thinnings as green onions.
3. For green onions, plant closer and thin as you harvest.

Watering and feeding

Keep the soil moist; onions have shallow roots, and drought reduces bulb size and affects flavor. Soak the soil about a foot deep when you water, ideally by drip. Stop watering once the tops begin to fall over, as wet soil at maturity delays curing and causes storage rots.

Work a complete fertilizer into the bed before planting, then side-dress with nitrogen during active growth: monthly in Florida, or twice in late spring and early summer in the North. Do not feed late, as extra nitrogen near maturity reduces storage life.

Pruning and care through the year

• Keep weeds down with shallow cultivation or mulch, especially in the first two months.
• Rotate onions with crops outside the onion family to limit soil diseases such as pink root and white rot.

Harvesting

Pull green onions any time once they are big enough. Bulb onions are ready when the tops start to fall over; UF/IFAS suggests harvesting in Florida when about a quarter of the tops have fallen, usually 4 to 5 months after planting.

Lift the bulbs and cure them in a dry, airy, shaded place for one to two weeks until the necks are dry and the skins papery. Trim the tops an inch or two above the bulb and store in mesh bags in a cool, dry place. Use any that soften or sprout first.