When to plant onion in Phoenix
Onion is cold-hardy, so every date keys off Phoenix's frost dates of Jan 27.
Step 1 · Sow
Start seeds indoors
Nov 18 – Dec 9
Before your last frost, on a warm windowsill or heat mat.
Step 2 · Plant
Transplant outdoors
Dec 27 – Jan 17
Once frost risk has passed and nights stay mild.
Step 3 · Harvest
Harvest window
Mar 27 – Apr 16
Picking runs until conditions turn against the crop.
Window
Days to maturity
90 – 110 d
Phoenix's 314-day season leaves this much room.
Onion on Phoenix'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
JAN 27
DEC 7
Common questions
When will onion be ready to harvest in Phoenix?
Expect to harvest from around late March (Mar 27–Apr 16) in Phoenix, based on its local frost dates. Confidence 74%.
When should I start onion seeds indoors in Phoenix?
Start seeds indoors around mid November (Nov 18–Dec 9) in Phoenix, based on its local frost dates. Confidence 78%.
When can I transplant onion outdoors in Phoenix?
Transplant outdoors around late December (Dec 27–Jan 17) in Phoenix, based on its local frost dates. Confidence 80%.
How long is the growing season in Phoenix?
Phoenix has roughly 314 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 Phoenix's frost dates.
Confidence 82/100 · Curated · based on NOAA GHCN-Daily 1991–2020 normal · frost dates are probabilities, not guarantees.