When to plant onions in Kansas City, Missouri
March 7 – March 21
Transplant outdoors · typical Kansas City yearClimate basis: NOAA station Kansas City Downtown AP, MO (USW00013988), 2 mi (3 km) from downtown Kansas City (ZCTA 64106 centroid), elevation 742 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 20.
The Kansas City year
- indoors under lights
- in the garden
- frost-free season
- 1-in-10 years still freeze to Apr 20
Every method, with its sources
Start seeds indoors
Dec 27 – Jan 10Sow 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 21UNH: 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.
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.