When to plant onion in St. Louis
Onion is cold-hardy, so every date keys off St. Louis's frost dates of Mar 30.
Step 1 · Sow
Start seeds indoors
Jan 19 – Feb 9
Before your last frost, on a warm windowsill or heat mat.
Step 2 · Plant
Transplant outdoors
Mar 31 – Apr 21
Once frost risk has passed and nights stay mild.
Step 3 · Harvest
Harvest window
Jun 29 – Jul 19
Picking runs until conditions turn against the crop.
Window
Days to maturity
90 – 110 d
St. Louis's 218-day season leaves this much room.
Onion on St. Louis'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
MAR 30
NOV 5
Common questions
When will onion be ready to harvest in St. Louis?
Expect to harvest from around late June (Jun 29–Jul 19) in St. Louis, based on its local frost dates. Confidence 76%.
When should I start onion seeds indoors in St. Louis?
Start seeds indoors around mid January (Jan 19–Feb 9) in St. Louis, based on its local frost dates. Confidence 80%.
When can I transplant onion outdoors in St. Louis?
Transplant outdoors around late March (Mar 31–Apr 21) in St. Louis, based on its local frost dates. Confidence 81%.
How long is the growing season in St. Louis?
St. Louis has roughly 218 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 St. Louis's frost dates.
Confidence 84/100 · Curated · based on NOAA GHCN-Daily 1991–2020 normal · frost dates are probabilities, not guarantees.