When to plant onions in Cheyenne, Wyoming

April 16 – April 30

Transplant outdoors · typical Cheyenne year

Climate basis: NOAA station Cheyenne Wfo, WY (USC00481676), 7 mi (12 km) from downtown Cheyenne (ZCTA 82001 centroid), elevation 6119 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 (May 14). One year in ten still freezes after May 28.

Last spring freezeMay 141-in-10 after May 28

First fall freezeOctober 11-in-10 before Sep 17

Frost-free season≈ 139 daysmedian at 32°F

Hard freeze, 28°May 3 – Oct 10kills even hardy greens

USDA zone5b-15 to -10°F winter low — not frost timing

The Cheyenne year

Start seeds indoorsFeb 5 – Feb 19

Transplant outdoorsApr 16 – Apr 30

indoors under lights
in the garden
frost-free season
1-in-10 years still freeze to May 28

Onions in Cheyenne · freeze context from Cheyenne Wfo, WY, 1991–2020 normals.

Every method, with its sources

Start seeds indoors

Feb 5 – Feb 19

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 (May 14). University of New Hampshire Extension · University of Minnesota Extension

Transplant outdoors

Apr 16 – Apr 30

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 (May 14). 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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