Onion
Yellow Onion, Red Onion, White Onion, Sweet Onion, Bunching Onion
Long-day varieties for latitudes above 36°N; short-day for southern regions. Start from transplants or sets for earlier bulbing. Bulbing is triggered by day length — choose correct type for your latitude. Stop watering when tops fall over. Cure 2–4 weeks in warm, dry, well-ventilated place before storage.
- Germination
- 10–14 days
- Soil temperature
- 16–24 °C
- Harvest after planting
- 100–120 days
- Spacing
- 10–15 cm
When to plant onion
Start seeds indoors 10–12 weeks before your last spring frost, and plant out 2–4 weeks before it.
Dashed line: typical last spring frost in zone 7 (Apr 5). Zone dates are averages; most of the UK is zone 8–9, central Europe 6–7 and the Baltics and Scandinavia 4–6. In the app, Tomatograph uses your garden's own frost dates.
How to grow onion
- 1
Pick the right type
Bulbing depends on day length. Long-day varieties suit northern Europe and the northern US; short-day varieties suit the south.
- 2
Sow or plant sets
Sow indoors 10–12 weeks before planting out, or push sets into the soil as soon as it can be worked, tips just showing.
- 3
Plant out
Plant seedlings out when they are pencil-thick, 2–4 weeks before the last frost, 10 cm apart. Keep weeds down; onions hate competition.
- 4
Harvest and cure
Stop watering when the tops fall over, 100–120 days after planting. Lift and cure for 2–4 weeks somewhere warm, dry and airy before storing.
Growing conditions
Light
Full sun
Water
Medium water
Temperature
4–30 °C
Frost tolerance
Frost-hardy — tolerates frost
Days to maturity
100 days
Plant spacing
10 cm
How Tomatograph watches your onion
Tomatograph checks your local forecast every morning and only speaks up when one of these applies to your garden.
- First frost within 14 daysReminder to mulch the roots before winter.
- 3 days at 35 °C or aboveHeat-wave alert while plants are flowering or fruiting: shade and water early.
- Humidity above 80% for 48 hoursCheck for downy mildew, which thrives in those conditions.
- Within a week of maturityHarvest-window reminder based on when you planted.
Onion questions
- When should I plant onion?
- Start seeds indoors 10–12 weeks before your last spring frost, and plant out 2–4 weeks before it.
- How long do onion seeds take to germinate?
- Usually 10–14 days at a soil temperature of 16–24 °C.
- How long until I can harvest onion?
- About 100–120 days after planting out, depending on variety and weather.
- How far apart should I plant onion?
- Allow 10–15 cm between plants.
- What grows well with onion?
- Good neighbours include carrot, beet, lettuce, tomato.
Growing guide written by Tomatograph. Planting figures from openplantdb (CC0 1.0). Zone frost dates are averages from openplantdb (CC0 1.0).