Onion · London, England, United Kingdom
When to plant onions in London
Main outdoor window
17 Mar - 7 Apr
The window this page is built around.
Timed from last spring frost
14 April
In London, on your chosen risk.
Ready to pick
25 Jun - 15 Jul
If you plant at the start of the window.
The average last frost. Half of all years are later than this.
Step by step in London
Each window is a range, not a single day. Earlier in the range buys you a longer season; later in it buys you a safer one.
Start indoors
3-24 Feb
Ten weeks before your last frost, under cover or on a windowsill.
Sow for overwintering
17 Jun - 8 Jul
Counting back 134 days from your first frost so the crop matures in time.
Plant out
17 Mar - 7 Apr
Four weeks before your last frost.
Harvest
25 Jun - 15 Jul
Onions through the year
Against the frost risk in London
Why these dates
Start seed ten weeks before last frost. Choose long-day cultivars above roughly 38 degrees latitude and short-day types below it - the wrong choice produces no bulb regardless of timing.
Young plants take -8C. Autumn-planted sets overwinter reliably in zone 6 and warmer.
Seed packets and extension guides express timing as weeks either side of the last spring frost, because that is the only reference point that travels. Onions are timed from 10 weeks before that frost for an indoor start, with planting out 4 weeks before it, and this page simply resolves those offsets against the frost date computed for London.
The median frost-free season here is 219 days, against 100 to 120 days to maturity for this crop counted from planting out. That margin is what decides whether a late sowing is worth attempting.
Also in London
Other vegetables to time
Growing onions in London
When should I plant onions in London?
Aim for 17 Mar - 7 Apr in an average year. If you would rather not gamble on the weather, wait until 7-28 Apr - by then frost has finished in nine years out of ten.
When will onions be ready to harvest in London?
Expect to start picking around 25 Jun - 15 Jul, based on 100 to 120 days to maturity from the planting date calculated for this location.
How much frost will onions tolerate?
Young plants take -8C. Autumn-planted sets overwinter reliably in zone 6 and warmer.