How to Grow Onions — Any Climate
How to grow onions anywhere in the world, without a calendar: wait for a soil temperature, not a date. Onions are a cool-season crop. Start indoors 10-12 weeks before your last frost date, or plant sets/transplants 2-4 weeks before it. Give them full sun, space them 8-10 cm (3-4 in) within row, 30-40 cm (12-16 in) between rows, and allow 100-120 days to maturity. These thresholds hold in any climate and either hemisphere.
When to plant onions — the conditions to wait for
There is no universal date for onions, because the same calendar month means completely different things at different latitudes and in opposite hemispheres. What does not change is the set of conditions the plant responds to.
| Condition | What this crop needs |
|---|---|
| Temperature class | Cool-season |
| When conditions are right | Start indoors 10-12 weeks before your last frost date, or plant sets/transplants 2-4 weeks before it |
| Light | Full sun |
| Spacing | 8-10 cm (3-4 in) within row, 30-40 cm (12-16 in) between rows |
| Germination time | 7-14 days |
| Days to maturity | 100-120 days to maturity |
Observe, measure, decide
- Observe. Watch what your own garden does, not what a calendar says. When does standing water disappear in spring? Where does frost linger longest? Which corner warms first? A season of notes beats any regional average.
- Measure. Two instruments settle almost every timing question: a soil thermometer pushed 5-10cm (2-4in) into the bed at the same time each morning, and your own records of first and last frost. Soil temperature is the number that actually governs germination.
- Decide. Match the measurement to the crop's temperature class, then count backwards from your frost date using days-to-maturity. If the arithmetic does not fit your season, change the cultivar or the crop — not the date.
💡 The one instrument worth owning
A soil thermometer pushed 5-10 cm (2-4 in) into the bed, read at the same time each morning, answers more timing questions than any calendar. Soil temperature — not air temperature — governs whether seed germinates or rots.
Soil and site
Well-drained soil, pH 6.0-7.0. Light feeder.
Watering
Even, consistent moisture through bulb formation; taper off as tops begin to fall over.
Spacing and layout
Space onions 8-10 cm (3-4 in) within row, 30-40 cm (12-16 in) between rows. Crowding is the most common cause of disappointing yields — plants compete for light and water, and poor airflow between them raises disease pressure sharply.
Companion planting
Carrots, beets, lettuce, tomatoes.
Common problems
Onion maggot/fly. Day-length is variety-specific — long-day varieties suit higher latitudes, short-day varieties suit latitudes closer to the equator; the wrong choice for your latitude will not bulb properly.
Turn growing onions into a written plan. The Global Vegetable Garden Planner (printable PDF) has the soil-temperature reference, crop rotation planner, sowing log and harvest tracker — 73 pages, no calendar required.
Frequently Asked Questions
When should I plant onions?
Wait for conditions rather than a date. Start indoors 10-12 weeks before your last frost date, or plant sets/transplants 2-4 weeks before it. This works identically in either hemisphere.
What temperature do onions need?
They are a cool-season crop. Germination takes 7-14 days once conditions are right.
How far apart should onions be planted?
Space them 8-10 cm (3-4 in) within row, 30-40 cm (12-16 in) between rows.
What usually goes wrong with onions?
Onion maggot/fly. Day-length is variety-specific — long-day varieties suit higher latitudes, short-day varieties suit latitudes closer to the equator; the wrong choice for your latitude will not bulb properly.
Last updated 25 August 2026
Tools That Help With This Crop
Use these links when the guide moves from reading into sowing, planting, watering, and keeping the crop healthy.
Plan the next step
Use the printable SoilCommander planner for your region to turn this guide into sowing dates, bed layouts and weekly garden tasks.