When to plant onion in Washington
Onion is cold-hardy, so every date keys off Washington's frost dates of Mar 24.
Step 1 · Sow
Start seeds indoors
Jan 13 – Feb 3
Before your last frost, on a warm windowsill or heat mat.
Step 2 · Plant
Transplant outdoors
Apr 6 – Apr 27
Once frost risk has passed and nights stay mild.
Step 3 · Harvest
Harvest window
Jul 5 – Jul 25
Picking runs until conditions turn against the crop.
Window
Days to maturity
90 – 110 d
Washington's 234-day season leaves this much room.
Onion on Washington'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 24
NOV 17
Common questions
When will onion be ready to harvest in Washington?
Expect to harvest from around early July (Jul 5–Jul 25) in Washington, based on its local frost dates. Confidence 80%.
When should I start onion seeds indoors in Washington?
Start seeds indoors around mid January (Jan 13–Feb 3) in Washington, based on its local frost dates. Confidence 85%.
When can I transplant onion outdoors in Washington?
Transplant outdoors around early April (Apr 6–Apr 27) in Washington, based on its local frost dates. Confidence 86%.
How long is the growing season in Washington?
Washington has roughly 234 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 Washington's frost dates.
Confidence 89/100 · Curated · based on NOAA GHCN-Daily 1991–2020 normal · frost dates are probabilities, not guarantees.