How to grow onion
Bulbing is triggered by day length: choose long-day types in the north, short-day in the south.
Your onion dates
Companions
BeetBroccoliCabbageCarrotCauliflowerDillKaleKohlrabiLeekLettuceParsnipPepperStrawberrySwiss chardTomatoTurnip
Keep apart from
Rotation and family
Onion belongs to the alliums (Amaryllidaceae). Shallow-rooted; onion white rot survives in soil for many years.
In a four-bed crop rotation it goes in the roots & alliums bed. Avoid growing it where any alliums grew in the last three years.
When to plant onion by zone
| Zone | Start indoors | Sow / plant out |
|---|---|---|
| 3 | Mar 11 – Mar 25 | Apr 22 – May 13 |
| 4 | Mar 3 – Mar 17 | Apr 14 – May 5 |
| 5 | Feb 19 – Mar 5 | Apr 2 – Apr 23 |
| 6 | Feb 9 – Feb 23 | Mar 23 – Apr 13 |
| 7 | Jan 28 – Feb 11 | Mar 11 – Apr 1 |
| 8 | Jan 15 – Jan 29 | Feb 26 – Mar 19 |
| 9 | Dec 14 – Dec 28 | Jan 25 – Feb 15 |
| 10 | Nov 23 – Dec 7 | Jan 4 – Jan 25 |
Based on typical frost dates for each zone; b half-zones run about a week earlier in spring. Zones 11–13 are frost-free in most years.
Onion questions
When should I plant onion?
Plant onion outdoors 4 weeks before your last spring frost. Start seeds indoors about 10 weeks before your last frost.
How far apart should I plant onion?
About 4 inches apart, or 9 per square foot in a square foot garden.
What grows well with onion?
Traditional companions include beet, broccoli, cabbage, carrot, cauliflower, dill, kale, kohlrabi, leek, lettuce, parsnip, pepper, strawberry, swiss chard, tomato, turnip. Keep it away from asparagus, bush bean, pea, pole bean.
How long does onion take to grow?
Typically 90–120 days from transplanting to first harvest, depending on variety and weather.