When to plant onion in San Bernardino
Onion is cold-hardy, so every date keys off San Bernardino's frost dates of Feb 23.
Step 1 · Sow
Start seeds indoors
Dec 15 – Jan 5
Before your last frost, on a warm windowsill or heat mat.
Step 2 · Plant
Transplant outdoors
Dec 26 – Jan 16
Once frost risk has passed and nights stay mild.
Step 3 · Harvest
Harvest window
Mar 26 – Apr 15
Picking runs until conditions turn against the crop.
Window
Days to maturity
90 – 110 d
San Bernardino's 284-day season leaves this much room.
Onion on San Bernardino'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
FEB 23
DEC 9
Common questions
When will onion be ready to harvest in San Bernardino?
Expect to harvest from around late March (Mar 26–Apr 15) in San Bernardino, based on its local frost dates. Confidence 70%.
When should I start onion seeds indoors in San Bernardino?
Start seeds indoors around mid December (Dec 15–Jan 5) in San Bernardino, based on its local frost dates. Confidence 74%.
When can I transplant onion outdoors in San Bernardino?
Transplant outdoors around late December (Dec 26–Jan 16) in San Bernardino, based on its local frost dates. Confidence 76%.
How long is the growing season in San Bernardino?
San Bernardino has roughly 284 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 San Bernardino's frost dates.
Confidence 78/100 · Curated · based on NOAA GHCN-Daily 1991–2020 normal · frost dates are probabilities, not guarantees.