When to plant onion in London
Onion is cold-hardy, so every date keys off London's frost dates of Apr 10.
Step 1 · Sow
Start seeds indoors
Jan 30 – Feb 20
Before your last frost, on a warm windowsill or heat mat.
Step 2 · Plant
Transplant outdoors
Mar 20 – Apr 10
Once frost risk has passed and nights stay mild.
Step 3 · Harvest
Harvest window
Jun 18 – Jul 8
Picking runs until conditions turn against the crop.
Window
Days to maturity
90 – 110 d
London's 209-day season leaves this much room.
Onion on London's growing year
Frost-bounded season with crops mapped onto the 12-month axis.
Frost risk Growing Harvest
JAN
FEB
MAR
APR
MAY
JUN
JUL
AUG
SEP
OCT
NOV
DEC
FROST
FROST
APR 10
NOV 5
Common questions
When will onion be ready to harvest in London?
Expect to harvest from around mid June (Jun 18–Jul 8) in London, based on its local frost dates. Confidence 79%.
When should I start onion seeds indoors in London?
Start seeds indoors around late January (Jan 30–Feb 20) in London, based on its local frost dates. Confidence 84%.
When can I transplant onion outdoors in London?
Transplant outdoors around mid March (Mar 20–Apr 10) in London, based on its local frost dates. Confidence 88%.
How long is the growing season in London?
London has roughly 209 frost-free days, which is enough time to grow onion to maturity outdoors.
The rule of thumb
Sow onion as soon as the soil is workable — it tolerates cool weather and light frost. Every date on this page is derived from that rule and London's frost dates.
Confidence 88/100 · NOAA GHCN-Daily · 1991–2020 normals · frost dates are probabilities, not guarantees.