When Should You Plant Onion Sets?
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GardenSays
Direct-sow onions outside Apr 14 – Apr 21 — they shrug off frost.
- Spring sowing
- Apr 14 – Apr 21
- Last frost (typ.)
- May 5
In Minneapolis, MN, the typical last frost lands around May 5, and onions are cold-hardy — so unlike tomatoes, you don't wait for frost to pass. Sow 14-21 days before the last frost: Apr 14 – Apr 21, as soon as soil is workable and around 40°F.
- This year's spring sowing window (Apr 14 – Apr 21) has passed — dates shown are for planning ahead.
Do this this week
- 1.Passed for this season — plan next spring: sow Apr 14 – Apr 21
Calculated result · 3 verified sources · Checked 2026-07-18 · How we decide
Why
Sow a short row every 2 weeks inside the window for a steady harvest instead of one glut.
- Onions germinate in cold soil (~40°F) and tolerate frost that would kill warm-season crops — waiting until after the last frost wastes their best growing weather.
- The 50% frost date is a planning midpoint — for hardy crops an early start risks little.
When this doesn't apply
- UMN states mid-April planting for Minnesota and 'as soon as soil is workable' for sets; converted to a Minneapolis-relative estimate (last frost ~May 5). Most Minnesota onion sets are short-day type — check your region's day-length type before buying sets.
- Cold, waterlogged soil rots seed — 'workable' means it crumbles in your hand, not mud.
How this was calculated
- 1. Last spring frostMay 5Source guidance
Minneapolis, MN: typical (50% probability) last 32°F frost
- 2. Direct-sow windowApr 14 – Apr 21Calculated result
Onion is cold-hardy: sow outside 14-21 days before the last frost, once soil reaches ~40°F
Data sources
- University of Minnesota Extension — Growing onions in home gardens — Plant onion sets as soon as the soil is workable in spring; soil temperatures of 40-50F, usually late April to early May, or mid-April per the general planting guide. Onions grown from sets or transplants tolerate cold soil and light frosts. Thin seedlings to 3-4 inches apart. Shallow-rooted, need consistent moisture — soak weekly if rainfall is under an inch. (checked 2026-07-18)
- NOAA NCEI — 1991-2020 U.S. Climate Normals (freeze date probabilities) (checked 2026-07-15)
- National Gardening Association — frost date lookup (checked 2026-07-15)
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