When to plant onion in Boise
Onion is cold-hardy, so every date keys off Boise's frost dates of May 2.
Step 1 · Sow
Start seeds indoors
Feb 21 – Mar 14
Before your last frost, on a warm windowsill or heat mat.
Step 2 · Plant
Transplant outdoors
Apr 22 – May 13
Once frost risk has passed and nights stay mild.
Step 3 · Harvest
Harvest window
Jul 21 – Aug 10
Picking runs until conditions turn against the crop.
Window
Days to maturity
90 – 110 d
Boise's 168-day season leaves this much room.
Onion on Boise'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 2
OCT 17
Common questions
When will onion be ready to harvest in Boise?
Expect to harvest from around late July (Jul 21–Aug 10) in Boise, based on its local frost dates. Confidence 80%.
When should I start onion seeds indoors in Boise?
Start seeds indoors around late February (Feb 21–Mar 14) in Boise, based on its local frost dates. Confidence 85%.
When can I transplant onion outdoors in Boise?
Transplant outdoors around late April (Apr 22–May 13) in Boise, based on its local frost dates. Confidence 86%.
How long is the growing season in Boise?
Boise has roughly 168 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 Boise's frost dates.
Confidence 89/100 · Curated · based on NOAA GHCN-Daily 1991–2020 normal · frost dates are probabilities, not guarantees.