When to plant onions in Cincinnati, Ohio

March 19 – April 2

Transplant outdoors · typical Cincinnati year

Climate basis: NOAA station Cincinnati Lunken AP, OH (USW00093812), 5 mi (7 km) from downtown Cincinnati (ZCTA 45202 centroid), elevation 490 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 16). One year in ten still freezes after May 3.

Last spring freezeApril 161-in-10 after May 3

First fall freezeOctober 251-in-10 before Oct 12

Frost-free season≈ 190 daysmedian at 32°F

Hard freeze, 28°Apr 2 – Nov 4kills even hardy greens

USDA zone6b-5 to 0°F winter low — not frost timing

The Cincinnati year

Start seeds indoorsJan 8 – Jan 22

Transplant outdoorsMar 19 – Apr 2

indoors under lights
in the garden
frost-free season
1-in-10 years still freeze to May 3

Onions in Cincinnati · freeze context from Cincinnati Lunken AP, OH, 1991–2020 normals.

Every method, with its sources

Start seeds indoors

Jan 8 – Jan 22

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 16). University of New Hampshire Extension · University of Minnesota Extension

Transplant outdoors

Mar 19 – Apr 2

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 16). 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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