When to plant onion in London
Onion is cold-hardy, so every date keys off London's frost dates of Apr 6.
Step 1 · Sow
Start seeds indoors
Jan 26 – Feb 16
Before your last frost, on a warm windowsill or heat mat.
Step 2 · Plant
Transplant outdoors
May 1 – May 22
Once frost risk has passed and nights stay mild.
Step 3 · Harvest
Harvest window
Jul 30 – Aug 19
Picking runs until conditions turn against the crop.
Window
Days to maturity
90 – 110 d
London's 234-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 6
NOV 16
Common questions
When will onion be ready to harvest in London?
Expect to harvest from around late July (Jul 30–Aug 19) in London, based on its local frost dates. Confidence 74%.
When should I start onion seeds indoors in London?
Start seeds indoors around late January (Jan 26–Feb 16) in London, based on its local frost dates. Confidence 78%.
When can I transplant onion outdoors in London?
Transplant outdoors around early May (May 1–May 22) in London, based on its local frost dates. Confidence 80%.
How long is the growing season in London?
London has roughly 234 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 82/100 · Curated · based on NOAA GHCN-Daily 1991–2020 normal · frost dates are probabilities, not guarantees.