How to grow onion

Allium cepa · Amaryllidaceae · Vegetable

Onion is a cool-season vegetable in the Amaryllidaceae family, usually started indoors and set out as transplants. It wants full sun (six or more hours of direct light) and steady, even moisture. From sowing to first pick is about 90–120 days. It handles frost well and a light freeze often improves the flavour.

Grower’s note: Match the day-length type to your latitude: long-day onions for the north, short-day for the south, day-neutral anywhere. Bulbing is triggered by day length, not calendar date.

Sowing

Sow about 0.5″ deep, into soil between 40–90°F — germination is quickest near 70°F. Germination takes a week or two (7–14 days). For an early crop, start seed indoors about 10 weeks before your last spring frost and grow it under bright light.

Spacing & layout

Give each plant about 4″ along the row with roughly 12″ between rows. On a square-foot grid that is 9 per square foot. Allow at least 12″ of loose, workable soil for the roots. It grows well in containers at least 8″ deep.

Fitting it to your season

As a cool-season crop, onion does its best growing in the mild weather of spring and fall and tends to bolt or turn bitter in summer heat.

Quick facts

When to plant onion

Companions

Common problems

Pests: onion maggot, onion thrips.

Diseases & disorders: downy mildew, white rot, neck rot, purple blotch.

Same family (Amaryllidaceae)

Rotate these so they don’t follow each other in the same bed year to year.

Figures are typical home-garden ranges from Cooperative Extension planting guides, cross-checked against multiple state sources. Your variety and conditions will vary.