Vegetable · Amaryllidaceae
When to plant onions
Choose a variety suited to your latitude and season. Day length helps trigger bulbing; a frost-based sowing date cannot compensate for an unsuitable short-day or long-day variety.
Start seed indoors
10 weeks before
Counted from your last spring frost.
Plant out
4 weeks before
Counted from your last spring frost.
Days to maturity (estimate)
100-120
From planting out to first harvest; cultivar and conditions matter.
Choose a place for estimated dates and local climate checks.
Timing, in frost-relative terms
These offsets are the starting assumptions used by our calendar for onions. They do not establish suitability in every climate. Your seed packet, cultivar and local growing advice take priority; in frost-free places the calendar uses monthly temperature patterns instead.
- Start seed indoors 10 weeks before the last frost.
- Plant out 4 weeks before the last frost.
- Expect 100–120 days to maturity, counted from planting out.
- Can also be sown in late summer for an autumn crop or to overwinter.
Getting the timing right
Seeds, sets and nursery transplants require different lead times. Check which starting material the packet or supplier's instructions describe before counting backwards from harvest.
Frost tolerance
Cold tolerance varies with cultivar, plant age, acclimatisation and the duration of a freeze. A plant surviving frost does not mean its leaves, heads, flowers or developing crop will remain undamaged. Check the forecast and the variety's local advice.
Classified as hardy, with a catalog cold-damage reference of -8°C (17.6°F)17.6°F (-8°C). Exposure duration, growth stage and acclimatisation change cold tolerance.
Before you plant onions
Use the calendar to plan ahead, then check the conditions in your own garden.
- Measure the soil at sowing depth. Aim for at least 7°C (44.6°F); the calendar estimates soil warmth from air temperatures and cannot measure your bed.
- Frost tolerance does not make saturated or frozen soil suitable for planting. Wait until the bed can be worked without smearing or compacting it.
- Harden off plants raised indoors: gradually expose them to outdoor light, wind and temperature before their final move into the bed.
- Use the maturity range on your seed packet when choosing a variety. Cold, short days and transplant stress can make harvest later than the calendar estimate.
Match bulb onions to day length and intended storage
Seeds, sets and nursery transplants require different lead times. Check which starting material the packet or supplier's instructions describe before counting backwards from harvest.
Use a sunny bed with free drainage and control weeds while the plants are small. Avoid piling wet soil around developing bulbs or treating extra water as a cure for slow growth.
For storage, let the variety reach its proper maturity and cure sound bulbs in a dry, airy place. Use damaged or thick-necked bulbs first rather than mixing them with long-term storage onions.
Growing in a container
A wide container can grow a small bulb crop, but each onion needs its final spacing. Crowding is more appropriate when intentionally harvesting young green onions.
What to record this season
Which cultivar and starting material produced sound bulbs that stored well? Add the cultivar, sowing date and conditions to your growing record so you can compare next season.
Further reading: RHS onions growing guide · Utah State University: Onions in the Garden . General crop guidance; local dates on this site are calculated separately. Source checked 19 September 2026 by Safe to Plant; this is not an expert review.
The RHS guide is written for UK growing conditions. Use its cultivation advice with local cultivar and seasonal guidance; its calendar months are not worldwide recommendations.
How the dates are calculated and their limits · Keep a growing record · Report a local discrepancy