When to plant onion in Philadelphia
Onion is cold-hardy, so every date keys off Philadelphia's frost dates of Mar 28.
Step 1 · Sow
Start seeds indoors
Jan 17 – Feb 7
Before your last frost, on a warm windowsill or heat mat.
Step 2 · Plant
Transplant outdoors
Apr 8 – Apr 29
Once frost risk has passed and nights stay mild.
Step 3 · Harvest
Harvest window
Jul 7 – Jul 27
Picking runs until conditions turn against the crop.
Window
Days to maturity
90 – 110 d
Philadelphia's 234-day season leaves this much room.
Onion on Philadelphia'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 28
NOV 22
Common questions
When will onion be ready to harvest in Philadelphia?
Expect to harvest from around early July (Jul 7–Jul 27) in Philadelphia, based on its local frost dates. Confidence 78%.
When should I start onion seeds indoors in Philadelphia?
Start seeds indoors around mid January (Jan 17–Feb 7) in Philadelphia, based on its local frost dates. Confidence 83%.
When can I transplant onion outdoors in Philadelphia?
Transplant outdoors around early April (Apr 8–Apr 29) in Philadelphia, based on its local frost dates. Confidence 84%.
How long is the growing season in Philadelphia?
Philadelphia 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 Philadelphia's frost dates.
Confidence 87/100 · Curated · based on NOAA GHCN-Daily 1991–2020 normal · frost dates are probabilities, not guarantees.