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ZIP 94102 planting calendar

When to plant watermelon in San Francisco, CA - Zone 10a

In ZIP 94102, plant watermelon around Feb 8; this uses the San Francisco frost station, USDA zone 10a, and watermelon timing rules.

Your San Francisco calendar has a 324-day estimated frost-free season. This estimate uses the representative San Francisco frost station, USDA zone 10a, and watermelon timing rules.

Source: san-francisco-downtown frost station, USDA zone 10a, 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 san-francisco-downtown frost station and USDA zone 10a, last updated 2026-06-28.

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

USDA zone 10a San Francisco frost station University of California Agriculture and Natural Resources Updated 2026-06-28
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Answer first

When to start watermelon seeds indoors in San Francisco

In ZIP 94102, start watermelon seeds indoors around Jan 4, about 21 days before the Jan 25 last-frost estimate.

Start watermelon seeds indoors in San Francisco around Jan 4, about 21 days before the Jan 25 last-frost estimate.

Seed starting schedule

Watermelon dates for San Francisco, CA

StepDateHow to use it
Start IndoorsJanuary 4, 2026Start seeds under lights or in a bright indoor seed-starting space.
Transplant OutdoorsFebruary 8, 2026Move hardened-off seedlings outside after the frost-risk window.
Direct SowFebruary 8, 2026Sow seed outside when the crop and soil can handle the window.

Last frost

When is the last frost in San Francisco?

San Francisco (ZIP 94102) has an average last spring frost around Jan 25.

First frost

When is the first frost in San Francisco?

The average first fall frost is around Dec 15, giving about 324 frost-free days.

Season map

Watermelon windows by month

JanFebMarAprMayJunJulAugSepOctNovDec
Start indoors
Jan 4
Direct sow
Feb 8
Transplant
Feb 8
Harvest
May 9
Frost watch
Sep 16

Reminder workflow

Every date has a next action

Start watermelon indoorsJan 4
Last frost windowJan 25
Direct sow watermelonFeb 8
Transplant watermelonFeb 8
First watermelon harvest windowMay 9
Last watermelon sowing targetSep 16
SignalValueWhy it matters
Last frost estimateJanuary 25, 2026Warm-season transplant risk starts falling after this window.
First frost estimateDecember 15, 2026Fall planting counts backward from this date.
Season length324 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

San Francisco frost history and USDA zone 10a shift the safe planting window. GrowZone uses a local representative station instead of one national rule.

University of California Agriculture and Natural Resources 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 Dec 15. A conservative last sowing target is around Sep 16.