When to plant onion in San Jose
Onion is cold-hardy, so every date keys off San Jose's frost dates of Feb 12.
Step 1 · Sow
Start seeds indoors
Dec 4 – Dec 25
Before your last frost, on a warm windowsill or heat mat.
Step 2 · Plant
Transplant outdoors
Feb 4 – Feb 25
Once frost risk has passed and nights stay mild.
Step 3 · Harvest
Harvest window
May 5 – May 25
Picking runs until conditions turn against the crop.
Window
Days to maturity
90 – 110 d
San Jose's 294-day season leaves this much room.
Onion on San Jose'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 12
DEC 3
Common questions
When will onion be ready to harvest in San Jose?
Expect to harvest from around early May (May 5–May 25) in San Jose, based on its local frost dates. Confidence 74%.
When should I start onion seeds indoors in San Jose?
Start seeds indoors around early December (Dec 4–Dec 25) in San Jose, based on its local frost dates. Confidence 78%.
When can I transplant onion outdoors in San Jose?
Transplant outdoors around early February (Feb 4–Feb 25) in San Jose, based on its local frost dates. Confidence 80%.
How long is the growing season in San Jose?
San Jose has roughly 294 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 Jose's frost dates.
Confidence 82/100 · Curated · based on NOAA GHCN-Daily 1991–2020 normal · frost dates are probabilities, not guarantees.