When to plant leeks in Chicago, Illinois

March 13 – March 27

Transplant outdoors · typical Chicago year

Climate basis: NOAA station Chicago Northerly Is, IL (USC00111550), 2 mi (3 km) from downtown Chicago (ZCTA 60602 centroid), elevation 583 ft, on the 1991–2020 climate normals. Transplant leeks outdoors from approximately the dates above, anchored to the median last spring 32°F freeze (April 10). One year in ten still freezes after April 25.

Last spring freezeApril 101-in-10 after Apr 25

First fall freezeNovember 51-in-10 before Oct 24

Frost-free season≈ 208 daysmedian at 32°F

Hard freeze, 28°Apr 1 – Nov 15kills even hardy greens

USDA zone6b-5 to 0°F winter low — not frost timing

The Chicago year

Start seeds indoorsJan 2 – Jan 16

Transplant outdoorsMar 13 – Mar 27

indoors under lights
in the garden
frost-free season
1-in-10 years still freeze to Apr 25

Leeks in Chicago · freeze context from Chicago Northerly Is, IL, 1991–2020 normals.

Every method, with its sources

Start seeds indoors

Jan 2 – Jan 16

Sow 8–10 weeks before the set-out window opens (UNH: time from seed to transplant 8–10 weeks). Anchored to the median last spring freeze (Apr 10). University of New Hampshire Extension

Transplant outdoors

Mar 13 – Mar 27

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 (Apr 10). University of New Hampshire Extension · Purdue University Extension

Direct-sow outdoors

Not calculated

Leeks are typically transplanted (Purdue marks leek 'typically transplanted'); the cited guides do not time direct sowing.

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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