How to grow onion
Allium cepa · Amaryllidaceae · Vegetable
When to plant onions: the short answer
Start seeds indoors about 10 weeks before your last spring frost; plant out about 3 weeks before it; sow outdoors about 4 weeks before the last frost. Seeds germinate in soil from 40 to 90°F, fastest near 70°F.
Zone 5: sow Mar 28 – Apr 16. Zone 7: sow Mar 3 – Mar 22. Zone 9: sow Jan 18 – Feb 6. See every zone.
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
Growth stages: how long onions take to grow
From seed to first harvest takes roughly 20-24 weeks: about 7 weeks as a seedling indoors, then 90-120 days in the garden.
When to plant onion
When to plant onions by USDA zone
Dates worked out from each zone’s typical frost dates for 2026. Find your zone on the zone map, or pick your state below for dates from its own frost dates. Zones 11 and above are frost-free, so timing there depends on heat and rain instead.
| Zone | Last / first frost | Start indoors | Sow or plant out |
|---|---|---|---|
| Zone 3 | May 15 / Sep 15 | Feb 27 – Mar 13 | Apr 12 – May 1 |
| Zone 4 | May 10 / Sep 25 | Feb 22 – Mar 8 | Apr 7 – Apr 26 |
| Zone 5 | Apr 30 / Oct 5 | Feb 12 – Feb 26 | Mar 28 – Apr 16 |
| Zone 6 | Apr 15 / Oct 15 | Jan 28 – Feb 11 | Mar 13 – Apr 1 |
| Zone 7 | Apr 5 / Oct 25 | Jan 18 – Feb 1 | Mar 3 – Mar 22 |
| Zone 8 | Mar 20 / Nov 5 | Jan 2 – Jan 16 | Feb 15 – Mar 6 |
| Zone 9 | Feb 20 / Nov 30 | Dec 5 – Dec 19 | Jan 18 – Feb 6 |
| Zone 10 | Jan 30 / Dec 15 | Nov 14 – Nov 28 | Dec 28 – Jan 16 |
Or browse onion planting dates for every state as static pages.
Companions
Carrot, Beet, Lettuce, Cabbage, Tomato
Bush Bean, Pole Bean, Pea
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.
Questions gardeners ask
When should I plant onions?
Start seeds indoors about 10 weeks before your last spring frost; plant out about 3 weeks before it; sow outdoors about 4 weeks before the last frost. Seeds germinate in soil from 40 to 90°F. The table on this page gives dates for each USDA zone, and the planting calendar gives dates for your own state or ZIP code.
How long do onions take to grow?
About 90-120 days from transplanting to the first harvest, or roughly 20-24 weeks from sowing the seed indoors. Seeds germinate in 7-14 days.
How much sun do onions need?
They need at least six to eight hours of direct sun a day.
What soil pH do onions need?
A soil pH between 6.0 and 7.0. Test before adding lime or sulfur - see the soil pH and testing guide.
How far apart should I plant onions?
Space plants about 4 inches apart in rows 12 inches apart, or 9 per square foot in a square-foot bed. Sow seeds about ½ inch deep.
Can I grow onions in a container?
Yes. Use a container at least 8 inches deep with drainage holes and a good potting mix, and water more often than you would in the ground.
Figures are typical home-garden ranges from Cooperative Extension planting guides, cross-checked against multiple state sources. Your variety and conditions will vary.