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ZIP 96813 planting calendar

When to plant watermelon in Honolulu, HI - Zone 12b

In ZIP 96813, plant watermelon around Jan 15; this uses the Honolulu frost station, USDA zone 12b, and watermelon timing rules.

Your Honolulu calendar has a 364-day estimated frost-free season. This estimate uses the representative Honolulu frost station, USDA zone 12b, and watermelon timing rules.

Source: honolulu-intl frost station, USDA zone 12b, 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 honolulu-intl frost station and USDA zone 12b, last updated 2026-06-28.

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

USDA zone 12b Honolulu frost station University of Hawaii Cooperative Extension Updated 2026-06-28
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Answer first

When to start watermelon seeds indoors in Honolulu

In ZIP 96813, start watermelon seeds indoors around Dec 11, about 21 days before the Jan 1 last-frost estimate.

Start watermelon seeds indoors in Honolulu around Dec 11, about 21 days before the Jan 1 last-frost estimate.

Seed starting schedule

Watermelon dates for Honolulu, HI

StepDateHow to use it
Start IndoorsDecember 11, 2025Start seeds under lights or in a bright indoor seed-starting space.
Transplant OutdoorsJanuary 15, 2026Move hardened-off seedlings outside after the frost-risk window.
Direct SowJanuary 15, 2026Sow seed outside when the crop and soil can handle the window.

Last frost

When is the last frost in Honolulu?

Honolulu (ZIP 96813) has an average last spring frost around Jan 1.

First frost

When is the first frost in Honolulu?

The average first fall frost is around Dec 31, giving about 364 frost-free days.

Season map

Watermelon windows by month

JanFebMarAprMayJunJulAugSepOctNovDec
Start indoors
Dec 11
Direct sow
Jan 15
Transplant
Jan 15
Harvest
Apr 15
Frost watch
Oct 2

Reminder workflow

Every date has a next action

Start watermelon indoorsDec 11
Last frost windowJan 1
Direct sow watermelonJan 15
Transplant watermelonJan 15
First watermelon harvest windowApr 15
Last watermelon sowing targetOct 2
SignalValueWhy it matters
Last frost estimateJanuary 1, 2026Warm-season transplant risk starts falling after this window.
First frost estimateDecember 31, 2026Fall planting counts backward from this date.
Season length364 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

Honolulu frost history and USDA zone 12b shift the safe planting window. GrowZone uses a local representative station instead of one national rule.

University of Hawaii Cooperative 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 Dec 31. A conservative last sowing target is around Oct 2.