When to plant onion in Boston
Onion is cold-hardy, so every date keys off Boston's frost dates of Apr 4.
Step 1 · Sow
Start seeds indoors
Jan 24 – Feb 14
Before your last frost, on a warm windowsill or heat mat.
Step 2 · Plant
Transplant outdoors
Apr 23 – May 14
Once frost risk has passed and nights stay mild.
Step 3 · Harvest
Harvest window
Jul 22 – Aug 11
Picking runs until conditions turn against the crop.
Window
Days to maturity
90 – 110 d
Boston's 220-day season leaves this much room.
Onion on Boston'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
APR 4
NOV 9
Common questions
When will onion be ready to harvest in Boston?
Expect to harvest from around late July (Jul 22–Aug 11) in Boston, based on its local frost dates. Confidence 80%.
When should I start onion seeds indoors in Boston?
Start seeds indoors around late January (Jan 24–Feb 14) in Boston, based on its local frost dates. Confidence 85%.
When can I transplant onion outdoors in Boston?
Transplant outdoors around late April (Apr 23–May 14) in Boston, based on its local frost dates. Confidence 86%.
How long is the growing season in Boston?
Boston has roughly 220 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 Boston's frost dates.
Confidence 89/100 · Curated · based on NOAA GHCN-Daily 1991–2020 normal · frost dates are probabilities, not guarantees.