When to plant watermelon in Philadelphia
In Philadelphia, typical last spring frost is April 3 and typical first fall frost is November 12. Typical last spring frost: April 3. Watermelon in Philadelphia is a good fit outdoors. The suitable growing window of 131 days covers the 70–100 days this plant usually needs.
Watermelon
Comfortable
Comfortable in greenhouse
Comfortable in a cold frame
Watermelon in Philadelphia is a good fit outdoors. The suitable growing window of 131 days covers the 70–100 days this plant usually needs.
When to plantMay 9 – July 3
- Harvest
- July 18 – October 11
- Season vs crop
- 131 grow days vs 70–100 needed
- Typical frost (last spring – first fall)
- April 3 – November 12
Watermelon in Philadelphia is a good fit outdoors. The suitable growing window of 131 days covers the 70–100 days this plant usually needs.
Planting conditions
- Direct sow
- Grow outdoors
- Harvest
| Activity | Window | Notes |
|---|---|---|
| Direct sow | May 9 – Jul 3 | Plant outdoors in this window. |
| Grow outdoors | Jul 4 – Jul 17 | Plants grow outdoors until harvest. |
| Harvest | Jul 18 – Oct 11 | Expected harvest from typical days to harvest. |
- Direct sow
- Grow outdoors
- Harvest
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.
- Ideal / plantable
- Tolerable
- Too cold
- Too hot
- Outside window
Key dates
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 Philadelphia, sow watermelon every 21 days from May 9 to about June 20. 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
| Sowing | Start indoors | Transplant | Harvest |
|---|---|---|---|
| 1 | April 11 | May 9 | July 18 – August 8 |
| 2 | May 2 | May 30 | August 8 – August 29 |
| 3 | May 23 | June 20 | August 29 – September 19 |
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 Philadelphia, 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 Philadelphia
- Radish Planting dates
- Arugula Planting dates
- Spinach Planting dates
- Lettuce Planting dates
- Kale Planting dates
- Beet Planting dates
- Zucchini Planting dates
- Cucumber Planting dates
- Green bean Planting dates
- Tomato Planting dates
- Bell pepper Planting dates
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 Philadelphia use ERA5-Land cell 39.95, -75.16. How we calculated this · Algorithm 2026-10-season-10.