When to plant onion in Riverside
Onion is cold-hardy, so every date keys off Riverside's frost dates of Feb 16.
Step 1 · Sow
Start seeds indoors
Dec 8 – Dec 29
Before your last frost, on a warm windowsill or heat mat.
Step 2 · Plant
Transplant outdoors
Jan 26 – Feb 16
Once frost risk has passed and nights stay mild.
Step 3 · Harvest
Harvest window
Apr 26 – May 16
Picking runs until conditions turn against the crop.
Window
Days to maturity
90 – 110 d
Riverside's 293-day season leaves this much room.
Onion on Riverside'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 16
DEC 6
Common questions
When will onion be ready to harvest in Riverside?
Expect to harvest from around late April (Apr 26–May 16) in Riverside, based on its local frost dates. Confidence 70%.
When should I start onion seeds indoors in Riverside?
Start seeds indoors around early December (Dec 8–Dec 29) in Riverside, based on its local frost dates. Confidence 74%.
When can I transplant onion outdoors in Riverside?
Transplant outdoors around late January (Jan 26–Feb 16) in Riverside, based on its local frost dates. Confidence 78%.
How long is the growing season in Riverside?
Riverside has roughly 293 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 Riverside's frost dates.
Confidence 78/100 · Curated · based on NOAA GHCN-Daily 1991–2020 normal · frost dates are probabilities, not guarantees.