When to plant watermelon in Portsmouth

In Portsmouth, typical last spring frost is March 23 and typical first fall frost is December 1. Typical last spring frost: March 23. Watermelon in Portsmouth is not realistic outdoors: 0 suitable days are fewer than the 70 days this plant usually needs.

Watermelon

How to grow Watermelon Portsmouth climate Weekly temperatures and windows

Where are you growing?

Not realistic outdoors

Not realistic in greenhouse

Tight fit in a cold frame

Watermelon in Portsmouth is doable with planning in a cold frame. 85 suitable days are enough for fast varieties (70 days) but not slow ones (100 days).

When to plantJune 8 – August 2

Harvest: August 17 – November 10
Season vs crop: 85 grow days vs 70–100 needed
Typical frost (last spring – first fall): February 8 – January 4

Watermelon in Portsmouth is doable with planning in a cold frame. 85 suitable days are enough for fast varieties (70 days) but not slow ones (100 days).

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.

Typical last frost estimate range March 23 – January 15, depending on the cover and the night.

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 row85

Days to harvest70–100 days from seed

Jun 8 – Aug 2

Direct sow

Aug 3 – Aug 16

Grow outdoors

Aug 17 – Nov 10

Harvest

Activity	Window	Notes
Direct sow	Jun 8 – Aug 2	Plant outdoors in this window.
Grow outdoors	Aug 3 – Aug 16	Plants grow outdoors until harvest.
Harvest	Aug 17 – Nov 10	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

Typical last spring frostFebruary 8

Low-risk last spring frostMarch 4

Typical first fall frostJanuary 4

Low-risk first fall frostDecember 8

Frost-free days328.5

Outdoor sow dateJune 8

Direct sow windowJune 8 – August 2

Suitable growing windowJune 8 – August 31 (85 days)

Expected harvestAugust 17 – November 10

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 Portsmouth, sow watermelon every 21 days from June 8 to about July 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

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

Succession sowing dates for Watermelon in Portsmouth
Sowing	Start indoors	Transplant	Harvest
1	May 11	June 8	August 17 – September 7
2	June 1	June 29	September 7 – September 28
3	June 22	July 20	September 28 – October 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 Portsmouth, 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 Portsmouth

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 Portsmouth use ERA5-Land cell 50.80, -1.09. How we calculated this · Algorithm 2026-10-season-10.

Report a problem