How to Grow Onion

Growing guide

Overview

Onion is a cool-season bulb vegetable grown for its layered bulbs and green tops. Growers value it for long storage and kitchen versatility. It thrives in full sun, well-drained loamy or sandy soil, and moderate moisture.

Soil & planting prep

Onions do best in loose, well-drained loamy or sandy soil with plenty of compost worked in. Aim for a soil pH of 6.0-6.8 if you test. Space sets or transplants 4-6 inches apart in rows 12-18 inches apart, and plant them about 1 inch deep.

When to plant

Start onion seeds indoors 8 weeks before your last frost date. Transplant seedlings or plant sets outdoors 2-4 weeks before the last frost, as soon as the soil can be worked; onions are frost tolerant. Choose a day-length type suited to your latitude before planting.

How to plant

1. Choose the right day-length type for your region: short-day in the South, long-day in the North, or day-neutral in many areas.
2. Start seeds indoors 8 weeks before your last frost date, or buy sets or transplants.
3. Work compost into loose, well-drained soil and rake the bed smooth.
4. Plant sets or transplants 1 inch deep and 4-6 inches apart in rows 12-18 inches apart.
5. Water the bed moderately after planting, and keep the soil evenly moist while plants establish.

5 common mistakes to avoid

• Choosing the wrong day-length type for your latitude causes either no bulbing or prematurely small bulbs; choose short-day, long-day, or day-neutral varieties based on your region.
• Planting onion sets or transplants too deep leads to thick necks and poorly formed bulbs; set them only about 1 inch deep so the bulb can develop at the soil surface.
• Letting weeds crowd the bed weakens onions because their shallow roots compete poorly; remove weeds regularly and shallowly to avoid disturbing the bulbs.
• Applying heavy nitrogen after bulbs begin to form encourages soft, thick-necked onions that store poorly; switch to a lower-nitrogen or balanced feed once bulbing starts.
• Growing onions in compacted or soggy soil causes rot and misshapen bulbs; loosen the soil and add compost before planting so water drains freely.

Harvesting & storage

Onions are ready when about half to three-quarters of the tops have fallen over and begun to yellow. Pull or dig bulbs on a dry day, then cure them in a warm, dry, airy spot for 7-14 days. Trim tops and store cured bulbs in a cool, dry, dark place with good air circulation.

Companion planting

Good companions include carrots, beets, lettuce, and cabbage family crops. Avoid planting onions near beans and peas, which alliums can suppress.

Growing in containers

Onions grow well in containers; use a container at least 5 gallons and about 8-10 inches deep. Space plants 4-6 inches apart and use loose, well-drained potting mix.

Pro tips

Growing from sets is easiest for beginners. Plant in early spring — onions are frost-tolerant. Day-length sensitive — choose short-day long-day or day-neutral varieties based on your latitude.