When to plant onions in Knoxville, Tennessee

March 14 – March 28

Transplant outdoors · typical Knoxville year

Climate basis: NOAA station Knoxville Exp Stn, TN (USC00404946), 5 mi (8 km) from downtown Knoxville (ZCTA 37902 centroid), elevation 830 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 11). One year in ten still freezes after April 29.

Last spring freezeApril 111-in-10 after Apr 29

First fall freezeOctober 281-in-10 before Oct 13

Frost-free season≈ 198 daysmedian at 32°F

Hard freeze, 28°Mar 31 – Nov 6kills even hardy greens

USDA zone7b5 to 10°F winter low — not frost timing

The Knoxville year

Start seeds indoorsJan 3 – Jan 17

Transplant outdoorsMar 14 – Mar 28

indoors under lights
in the garden
frost-free season
1-in-10 years still freeze to Apr 29

Onions in Knoxville · freeze context from Knoxville Exp Stn, TN, 1991–2020 normals.

Every method, with its sources

Start seeds indoors

Jan 3 – Jan 17

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

Transplant outdoors

Mar 14 – Mar 28

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 11). 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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