When to plant onions in Boston, Massachusetts

March 7 – March 21

Transplant outdoors · typical Boston year

Climate basis: NOAA station Boston Logan INTL AP, MA (USW00014739), 3 mi (5 km) from downtown Boston (ZCTA 02108 centroid), elevation 12 ft, on the 1991–2020 climate normals. Plant onion sets or transplants from approximately the dates above, anchored to the median last spring 32°F freeze (April 4). One year in ten still freezes after April 17.

Last spring freezeApril 41-in-10 after Apr 17

First fall freezeNovember 91-in-10 before Oct 29

Frost-free season≈ 218 daysmedian at 32°F

Hard freeze, 28°Mar 28 – Nov 21kills even hardy greens

USDA zone7a0 to 5°F winter low — not frost timing

The Boston year

Start seeds indoorsDec 27 – Jan 10

Transplant outdoorsMar 7 – Mar 21

indoors under lights
in the garden
frost-free season
1-in-10 years still freeze to Apr 17

Onions in Boston · freeze context from Boston Logan INTL AP, MA, 1991–2020 normals.

Every method, with its sources

Start seeds indoors

Dec 27 – Jan 10

Sow 8–10 weeks (UNH) to 10–12 weeks (Minnesota) before planting out; derived from the −4-week set-out date using the UNH lead time. Anchored to the median last spring freeze (Apr 4). University of New Hampshire Extension · University of Minnesota Extension

Transplant outdoors

Mar 7 – Mar 21

UNH: set out transplants (or sets) 4 weeks before the last frost; Purdue's hardy class allows 4–6 weeks before. The later UNH point is used, with a 14-day span. Anchored to the median last spring freeze (Apr 4). University of New Hampshire Extension · Purdue University Extension

Variety assumption: Assumes storage-type bulb onions (about 100–115 days from sets; Penn State lists 105).

These windows describe typical years (1991–2020 climate normals), not this year's forecast. Freezes happen outside them — especially in frost pockets, valleys, and urban edges. Watch the forecast before committing tender plants.

This week, not a typical year:

Crop rule sources (3)

University of New Hampshire Extension: Growing Vegetables: When to Plant Your Vegetable Garden [fact sheet] (accessed 2026-07-24)
Per-crop 'time from seed to transplant' and 'safe to set out in garden' relative to last frost.
University of Minnesota Extension: Growing onions (accessed 2026-07-24)
"You can also raise your own transplants by starting seed indoors 10 to 12 weeks before planting outside."
Purdue University Extension: Indiana Vegetable Planting Calendar (HO-186-W) (accessed 2026-07-24)
Hardiness classes: hardy = plant 4-6 wk before last spring frost; semi-hardy = 2-4 wk before; tender = after average last frost; very tender = at least 2 wk after. Table 2: fall planting at least 4-8 wk before first fall frost.

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