When to plant sweet potato 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. Sweet potato in Portsmouth is not realistic outdoors: 0 suitable days are fewer than the 85 days this plant usually needs.

Sweet potato

How to grow Sweet potato Portsmouth climate Weekly temperatures and windows

Where are you growing?

Not realistic outdoors

Not realistic in greenhouse

Tight fit in a cold frame

Sweet potato in Portsmouth is doable with planning under fleece. 100 suitable days are enough for fast varieties (85 days) but not slow ones (140 days).

When to plantMay 8 – July 2

Harvest: August 1 – November 19
Season vs crop: 100 grow days vs 85–140 needed
Typical frost (last spring – first fall): February 14 – December 29

Sweet potato in Portsmouth is doable with planning under fleece. 100 suitable days are enough for fast varieties (85 days) but not slow ones (140 days).

Start slips March 27 – May 21, then transplant May 8 – July 2 so the crop can finish under fleece.

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

Fleece adds about 1–4 °C on frosty nights, depending on its weight. Remove it on warm days.

Fleece also cuts light: about 5–10 % for light fleece, up to 50–70 % for heavy grades. Take it off by day once frosts are over.

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 February 21 – January 17, depending on the cover and the night.

Planting conditions

Suitable growing days in a row100

Days to harvest85–140 days from planting

No planting window is recorded for this place.

Too cold
Frost line

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 – 32 °C70 – 90 °F21 – 32 °C (70 – 90 °F)) and tolerable (16 – 35 °C61 – 95 °F16 – 35 °C (61 – 95 °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 14

Low-risk last spring frostMarch 5

Typical first fall frostDecember 29

Low-risk first fall frostNovember 30

Frost-free days317

Earliest indoor seed startMarch 27

Latest indoor seed startMay 21

Transplant outdoorsMay 8 – July 2

Direct sow windownot recommended here

Suitable growing windowMay 8 – August 15 (100 days)

Expected harvestAugust 1 – November 19

How these dates are judged

Ideal conditions: Daytime air from 21 – 32 °C70 – 90 °F21 – 32 °C (70 – 90 °F).
Suitable conditions: Daytime air from 16 – 35 °C61 – 95 °F16 – 35 °C (61 – 95 °F).
Last frost and first frost: Typical spring and fall frost dates for Portsmouth, shown in the key dates.
Days to harvest: 85–140 days from planting, 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.

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