When to plant summer squash & zucchini in Philadelphia, Pennsylvania

April 13 – April 27

Transplant outdoors · typical Philadelphia year

Climate basis: NOAA station Phila Franklin Inst, PA (USC00366886), 1 mi (1 km) from downtown Philadelphia (ZCTA 19107 centroid), elevation 60 ft, on the 1991–2020 climate normals. Transplant summer squash & zucchini outdoors from approximately the dates above, anchored to the median last spring 32°F freeze (March 30). One year in ten still freezes after April 12.

Last spring freezeMarch 301-in-10 after Apr 12

First fall freezeNovember 171-in-10 before Nov 4

Frost-free season≈ 232 daysmedian at 32°F

Hard freeze, 28°Mar 17 – Nov 29kills even hardy greens

USDA zone7b5 to 10°F winter low — not frost timing

The Philadelphia year

Start seeds indoorsMar 16 – Mar 23

Transplant outdoorsApr 13 – Apr 27

Direct-sow outdoorsApr 13 – Apr 27

indoors under lights
in the garden
frost-free season
1-in-10 years still freeze to Apr 12

Summer squash & zucchini in Philadelphia · freeze context from Phila Franklin Inst, PA, 1991–2020 normals.

Every method, with its sources

Start seeds indoors

Mar 16 – Mar 23

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 30). University of New Hampshire Extension · University of Missouri Extension

Transplant outdoors

Apr 13 – Apr 27

UNH: set out 2 weeks after last frost; Purdue very-tender class agrees. Anchored to the median last spring freeze (Mar 30). University of New Hampshire Extension · Purdue University Extension

Direct-sow outdoors

Apr 13 – Apr 27

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 30). Purdue University Extension · Penn State Extension (York County Master Gardeners)

Variety assumption: Assumes an average zucchini-type variety (about 50–60 days from seed; Penn State lists 60).

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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