When to plant onion in Chicago
Onion is cold-hardy, so every date keys off Chicago'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
Apr 21 – May 12
Once frost risk has passed and nights stay mild.
Step 3 · Harvest
Harvest window
Jul 20 – Aug 9
Picking runs until conditions turn against the crop.
Window
Days to maturity
90 – 110 d
Chicago's 206-day season leaves this much room.
Onion on Chicago'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
OCT 31
Common questions
When will onion be ready to harvest in Chicago?
Expect to harvest from around mid July (Jul 20–Aug 9) in Chicago, based on its local frost dates. Confidence 76%.
When should I start onion seeds indoors in Chicago?
Start seeds indoors around late January (Jan 30–Feb 20) in Chicago, based on its local frost dates. Confidence 80%.
When can I transplant onion outdoors in Chicago?
Transplant outdoors around late April (Apr 21–May 12) in Chicago, based on its local frost dates. Confidence 81%.
How long is the growing season in Chicago?
Chicago has roughly 206 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 Chicago's frost dates.
Confidence 84/100 · Curated · based on NOAA GHCN-Daily 1991–2020 normal · frost dates are probabilities, not guarantees.