When to plant kohlrabi in Madison, Wisconsin

April 3 – April 17

Transplant outdoors · typical Madison year

Climate basis: NOAA station Uw Arboretum - Madison, WI (USC00470273), 4 mi (6 km) from downtown Madison (ZCTA 53703 centroid), elevation 870 ft, on the 1991–2020 climate normals. Transplant kohlrabi outdoors from approximately the dates above, anchored to the median last spring 32°F freeze (May 1). One year in ten still freezes after May 17.

Last spring freezeMay 11-in-10 after May 17

First fall freezeOctober 91-in-10 before Sep 26

Frost-free season≈ 161 daysmedian at 32°F

Hard freeze, 28°Apr 20 – Oct 20kills even hardy greens

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

The Madison year

Start seeds indoorsFeb 20 – Mar 6

Transplant outdoorsApr 3 – Apr 17

Fall plantingAug 14 – Sep 11

indoors under lights
in the garden
frost-free season
1-in-10 years still freeze to May 17

Kohlrabi in Madison · freeze context from Uw Arboretum - Madison, WI, 1991–2020 normals.

Every method, with its sources

Start seeds indoors

Feb 20 – Mar 6

Sow 4–6 weeks before the set-out window opens (UNH: time from seed to transplant 4–6 weeks). Anchored to the median last spring freeze (May 1). University of New Hampshire Extension

Transplant outdoors

Apr 3 – Apr 17

UNH: safe to set out 4 weeks before the last frost; Purdue's hardy class allows 4–6 weeks before. The later UNH start is used. Anchored to the median last spring freeze (May 1). University of New Hampshire Extension · Purdue University Extension

Fall planting

Aug 14 – Sep 11

Purdue Table 2: plant fall kohlrabi at least 4–8 weeks before the first fall frost. Anchored to the median first fall freeze (Oct 9). Purdue University Extension

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 (2)

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.
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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