When to plant winter squash in Seattle, Washington

March 27 – April 10

Transplant outdoors · typical Seattle year

Climate basis: NOAA station Seattle Boeing Fld, WA (USW00024234), 5 mi (8 km) from downtown Seattle (ZCTA 98104 centroid), elevation 20 ft, on the 1991–2020 climate normals. Transplant winter squash outdoors from approximately the dates above, anchored to the median last spring 32°F freeze (March 13). One year in ten still freezes after April 1.

Last spring freezeMarch 131-in-10 after Apr 1

First fall freezeNovember 171-in-10 before Nov 1

Frost-free season≈ 247 daysmedian at 32°F

Hard freeze, 28°Feb 18 – Dec 2kills even hardy greens

USDA zone9a20 to 25°F winter low — not frost timing

The Seattle year

Start seeds indoorsFeb 27 – Mar 6

Transplant outdoorsMar 27 – Apr 10

Direct-sow outdoorsMar 27 – Apr 10

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

Winter squash in Seattle · freeze context from Seattle Boeing Fld, WA, 1991–2020 normals.

Every method, with its sources

Start seeds indoors

Feb 27 – Mar 6

Sow in pots 3–4 weeks before the outdoor window opens (UNH 3–4 weeks; Missouri G6570 3–5 weeks). Anchored to the median last spring freeze (Mar 13). University of New Hampshire Extension · University of Missouri Extension

Transplant outdoors

Mar 27 – Apr 10

UNH: set out 2 weeks after last frost; Purdue very-tender class agrees. Anchored to the median last spring freeze (Mar 13). University of New Hampshire Extension · Purdue University Extension

Direct-sow outdoors

Mar 27 – Apr 10

Purdue: very tender — at least two weeks after the average last frost; Penn State earliest sowing May 15 (+14 days) agrees. Anchored to the median last spring freeze (Mar 13). Purdue University Extension · Penn State Extension (York County Master Gardeners)

Variety assumption: Assumes an average butternut/acorn-type variety (about 90–110 days; Penn State lists 90). Fruit must mature before fall frost.

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 (4)

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 Missouri Extension: Starting Plants Indoors from Seeds (G6570) (accessed 2026-07-24)
Weeks needed to grow vegetable transplants indoors (Table 2).
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.
Penn State Extension (York County Master Gardeners): Vegetable Seed Planting Guide (accessed 2026-07-24)
Earliest/latest sowing dates for a May 1 last-frost / Oct 1 first-frost baseline; offsets derived by date difference from May 1. Also days-to-maturity figures.

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