When to plant onion in Missoula
Onion is cold-hardy, so every date keys off Missoula's frost dates of May 15.
Step 1 · Sow
Start seeds indoors
Mar 6 – Mar 27
Before your last frost, on a warm windowsill or heat mat.
Step 2 · Plant
Transplant outdoors
May 16 – Jun 6
Once frost risk has passed and nights stay mild.
Step 3 · Harvest
Harvest window
Aug 14 – Sep 3
Picking runs until conditions turn against the crop.
Window
Days to maturity
90 – 110 d
Missoula's 138-day season leaves this much room.
Onion on Missoula'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
MAY 15
OCT 4
Common questions
When will onion be ready to harvest in Missoula?
Expect to harvest from around mid August (Aug 14–Sep 3) in Missoula, based on its local frost dates. Confidence 75%.
When should I start onion seeds indoors in Missoula?
Start seeds indoors around early March (Mar 6–Mar 27) in Missoula, based on its local frost dates. Confidence 79%.
When can I transplant onion outdoors in Missoula?
Transplant outdoors around mid May (May 16–Jun 6) in Missoula, based on its local frost dates. Confidence 81%.
How long is the growing season in Missoula?
Missoula has roughly 138 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 Missoula's frost dates.
Confidence 83/100 · Curated · based on NOAA GHCN-Daily 1991–2020 normal · frost dates are probabilities, not guarantees.