GrowZone

ZIP 19103 planting calendar

When to plant watermelon in Philadelphia, PA - Zone 7b

In ZIP 19103, plant watermelon around Apr 24; this uses the Philadelphia frost station, USDA zone 7b, and watermelon timing rules.

Your Philadelphia calendar has a 209-day estimated frost-free season. This estimate uses the representative Philadelphia frost station, USDA zone 7b, and watermelon timing rules.

Source: philadelphia-intl frost station, USDA zone 7b, last updated 2026-06-28. Verify locally before planting high-value crops because microclimates, current weather, and yard exposure can move the safe date.

Reviewed by GrowZone Editorial. Calculated from the philadelphia-intl frost station and USDA zone 7b, last updated 2026-06-28.

In the Philadelphia area, watermelon timing follows the Philadelphia frost window, so the local ZIP date can differ from a broad zone-only calendar.

USDA zone 7b Philadelphia frost station Penn State Extension Updated 2026-06-28
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Answer first

When to start watermelon seeds indoors in Philadelphia

In ZIP 19103, start watermelon seeds indoors around Mar 20, about 21 days before the Apr 10 last-frost estimate.

Start watermelon seeds indoors in Philadelphia around Mar 20, about 21 days before the Apr 10 last-frost estimate.

Seed starting schedule

Watermelon dates for Philadelphia, PA

StepDateHow to use it
Start IndoorsMarch 20, 2026Start seeds under lights or in a bright indoor seed-starting space.
Transplant OutdoorsApril 24, 2026Move hardened-off seedlings outside after the frost-risk window.
Direct SowApril 24, 2026Sow seed outside when the crop and soil can handle the window.

Last frost

When is the last frost in Philadelphia?

Philadelphia (ZIP 19103) has an average last spring frost around Apr 10.

First frost

When is the first frost in Philadelphia?

The average first fall frost is around Nov 5, giving about 209 frost-free days.

Season map

Watermelon windows by month

JanFebMarAprMayJunJulAugSepOctNovDec
Start indoors
Mar 20
Direct sow
Apr 24
Transplant
Apr 24
Harvest
Jul 23
Frost watch
Aug 7

Reminder workflow

Every date has a next action

Start watermelon indoorsMar 20
Last frost windowApr 10
Direct sow watermelonApr 24
Transplant watermelonApr 24
First watermelon harvest windowJul 23
Last watermelon sowing targetAug 7
SignalValueWhy it matters
Last frost estimateApril 10, 2026Warm-season transplant risk starts falling after this window.
First frost estimateNovember 5, 2026Fall planting counts backward from this date.
Season length209 daysLonger seasons can support succession crops.
Spacing36-60 inAirflow and root space affect yield.
Soil temperature70 F+Seeds and transplants stall in cold soil.

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Local data

Why this date changes by ZIP

Philadelphia frost history and USDA zone 7b shift the safe planting window. GrowZone uses a local representative station instead of one national rule.

Penn State Extension is the kind of local source to verify against when a crop is expensive, weather is unusual, or your yard has a cold pocket.

Crop note

Watermelon timing notes

Wait for warm soil; watermelon stalls when nights stay cool. Watermelon prefers full sun, 36-60 in spacing, and soil near 70 F+.

For fall, count backward from Nov 5. A conservative last sowing target is around Aug 7.