When to plant watermelon in San Francisco

In San Francisco, typical last spring frost is December 24 and typical first fall frost is December 22. Typical last spring frost: December 24. Watermelon in San Francisco is not realistic outdoors: 0 suitable days are fewer than the 70 days this plant usually needs.

Watermelon

How to grow Watermelon San Francisco climate Weekly temperatures and windows

Where are you growing?

Not realistic outdoors

Not realistic in greenhouse

Tight fit in a cold frame

Watermelon in San Francisco is a good fit in a cold frame. The suitable growing window of 158 days covers the 70–100 days this plant usually needs.

When to plantSeptember 1 – October 26

Harvest: November 10 – February 3
Season vs crop: 158 grow days vs 70–100 needed
Typical frost (last spring – first fall): December 24 – December 22

Watermelon in San Francisco is a good fit in a cold frame. The suitable growing window of 158 days covers the 70–100 days this plant usually needs.

Estimate. Protection varies with the cover, the weather and how you vent it.

A closed cold frame keeps nights up to about 3–6 °C warmer than outside. Open it on sunny days.

Cover dates are modelled from weather-model data using published greenhouse studies. They are not measured in your greenhouse. A max–min thermometer inside will tell you more.

Planting conditions

Direct sow
Grow outdoors
Harvest

Local activity windows for this place and growing method. Blue fill is usually too cold for this plant. Dashed lines are typical last and first frost.

Suitable growing days in a row158

Days to harvest70–100 days from seed

Sep 1 – Oct 26

Direct sow

Oct 27 – Nov 9

Grow outdoors

Nov 10 – Feb 3

Harvest

Activity	Window	Notes
Direct sow	Sep 1 – Oct 26	Plant outdoors in this window.
Grow outdoors	Oct 27 – Nov 9	Plants grow outdoors until harvest.
Harvest	Nov 10 – Feb 3	Expected harvest from typical days to harvest.

Direct sow
Grow outdoors
Harvest

Blue fill is usually too cold for this plant. Dashed lines are typical last and first frost (35 seasons (1991–2025)). Adjust a week or two for your conditions.

Protected frost and sowing dates are modelled estimates from weather-model data and published greenhouse studies, not measured in your greenhouse. A max–min thermometer inside will tell you more.

Planting and growing across the year

Planting is the sow / plant-out window. Growing is daytime air versus this plant’s ideal (21 – 35 °C70 – 95 °F21 – 35 °C (70 – 95 °F)) and tolerable (16 – 38 °C61 – 100 °F16 – 38 °C (61 – 100 °F)) ranges, from modelled cover temperatures for the growing method you selected.

Ideal / plantable
Tolerable
Too cold
Too hot
Outside window

Planting uses sow / plant-out rules; growing shows daytime air against this plant’s ideal and tolerable ranges.

Key dates

Frost-free days365

Outdoor sow dateSeptember 1

Direct sow windowSeptember 1 – October 26

Suitable growing windowApril 15 – September 19 (158 days)

Expected harvestNovember 10 – February 3

Sow again for a steady harvest

Sow quick crops like radish and lettuce every 1–2 weeks, and slower ones like beans and carrots every 2–3 weeks. Stop when there is not enough time left before your first autumn frost for the crop to mature.

In San Francisco, sow watermelon every 21 days from May 5 to about June 16. That is about 3 sowings in a typical year — a short row or a few square feet each time. Shorter autumn days slow growth, so we allow two extra weeks before the season end.

Start indoors
Grow indoors
Harden off
Transplant
Grow outdoors
Harvest

Each row is one succession sowing. Dashed lines are typical last and first frost.

Succession sowing dates for Watermelon in San Francisco
Sowing	Start indoors	Transplant	Harvest
1	April 7	May 5	July 14 – August 4
2	April 28	May 26	August 4 – August 25
3	May 19	June 16	August 25 – September 15

These dates are deliberately cautious. They come out earlier than common rules like '8 weeks before first frost', because we use the low-risk frost date, leave time for picking and add two weeks for shorter autumn days.

How these dates are judged

Ideal conditions: Daytime air from 21 – 35 °C70 – 95 °F21 – 35 °C (70 – 95 °F).
Suitable conditions: Daytime air from 16 – 38 °C61 – 100 °F16 – 38 °C (61 – 100 °F).
Last frost and first frost: Typical spring and fall frost dates for San Francisco, shown in the key dates.
Days to harvest: 70–100 days from seed, from typical days to harvest.
Sowing window: Dated sow and harvest bars use frost dates, soil minima, night limits, and last-sow count-back from typical crop timing. Air ratings on the weekly chart colour only weeks inside that same window — see weekly temperatures and windows.

Other plants for San Francisco

Calculated by Plan & Plant. Method. Data 35 seasons (1991–2025).

Frost and suitability dates are typical figures from climate statistics.

Based on 35 years (1991–2025) of ERA5-Land data from Open-Meteo, for an area of about 10 km. Typical dates, not a guarantee.

Dates for San Francisco use ERA5-Land cell 37.77, -122.42. How we calculated this · Algorithm 2026-10-season-10.

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