When to plant tomato in Portsmouth

Where are you growing?

Comfortable

Comfortable in greenhouse

Comfortable in a cold frame

Tomato in Portsmouth is a good fit outdoors. The suitable growing window of 114 days covers the 55–90 days this plant usually needs.

Start indoors: March 25 – May 16
Transplant outside: May 24 – July 15
Harvest: July 18 – September 14
Season vs crop: 114 grow days vs 55–90 needed
Typical frost (last spring – first fall): March 9 – November 27

Tomato in Portsmouth is a good fit outdoors. The suitable growing window of 114 days covers the 55–90 days this plant usually needs.

Planting conditions

Start indoors
Harden off
Transplant outside
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 row114

Days to harvest55–90 days from transplant

Mar 25 – May 16

Start indoors

May 17 – May 23

Harden off

May 24

Transplant outside

May 25 – Jul 17

Grow outdoors

Jul 18 – Sep 14

Harvest

Activity	Window	Notes
Start indoors	Mar 25 – May 16	Sow indoors before the outdoor window.
Harden off	May 17 – May 23	Acclimate seedlings outdoors before planting.
Transplant outside	May 24	Move plants outside on this day (start of the plant-out window).
Grow outdoors	May 25 – Jul 17	Plants grow outdoors until harvest.
Harvest	Jul 18 – Sep 14	Expected harvest from typical days to harvest.

Blue fill is usually too cold for this plant. Dashed lines are typical last and first frost. Adjust a week or two for your conditions.

Planting and growing across the year

Planting is the sow / plant-out window. Growing is daytime air versus this plant’s ideal (21 – 29 °C) and tolerable (13 – 35 °C) ranges.

Planting uses sow / plant-out rules (frost, soil germination, night, and last-sow): green when plantable, blue diagonal when nights or soil are too cold, amber cross-hatch when too hot, faint for other calendar limits. Growing shows daytime air against this plant’s ideal and tolerable ranges across the year.

Key dates

Typical last spring frostMarch 9

Safer last spring frostApril 8

Typical first fall frostNovember 27

Safer first fall frostNovember 20

Frost-free days262

Earliest indoor seed startMarch 25

Latest indoor seed startMay 16

Transplant outdoorsMay 24 – July 15

Direct sow windownot recommended here

Suitable growing windowMay 24 – September 14 (114 days)

Expected harvestJuly 18 – September 14

How these dates are judged

Ideal conditions: Daytime air from 21 – 29 °C.
Suitable conditions: Daytime air from 13 – 35 °C.
Last frost and first frost: Typical spring and fall frost dates for Portsmouth, shown in the key dates.
Days to harvest: 55–90 days from transplant, 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. The year strip and weekly chart use the same window: air ratings only colour weeks inside it.

Growing window for Tomato: May 24 – Sep 14 (114 days)

Planting window for Tomato: May 24 – Jul 15 (53 days)

Usually too cold: Sep 21 – Dec 31

Outdoors: weekly temperatures and windows

Temperatures across the year for Portsmouth. Shaded bands are this plant’s ideal and tolerable daytime air ranges. Growing is daytime air suitability; Planting is sow / plant-out conditions (same colour language). Window brackets mark each stretch.

Ideal range
Tolerable range
Possible min to max
Average min to max
Suitable
Tolerable
Too cold
Too hot

Selected week · tap a week or use the arrows

Week 21 · May 18 – May 24

Average 10 to 17 °C · Possible 7 to 23 °C

Growing: too cold

Planting: too cold

A week is in range when its average low and high sit inside the tolerance band 13 to 35 °C; it is suitable when that average range also overlaps the ideal 21 to 29 °C band and possible readings stay within 0 °C to 35 °C. Crossing a possible limit, or missing the ideal band, makes it tolerable; any chance of frost (possible low below 0 °C) makes it too cold. When a sowing window is calculated, weeks outside that window are uncoloured even if air is in range.

Based on average frost dates.

Show all 53 weeks

Cover method weekly charts

Outdoor weekly chart is above. Open a cover chart only if you grow under fleece, a cold frame, or an unheated greenhouse.

Other plants for Portsmouth

Calculated by Plan & Plant · Method · Data 2019–2026

Frost and suitability dates are typical figures from climate statistics. Microclimates vary; they are not a guarantee.

How we calculated this

Data: ERA5-Land via Open-Meteo, cell 50.80, -1.09, years 2019–2026 (not a 30-year climate normal yet).
Typical and safer dates: Typical last frost is the date after which half of winters in this sample had no more frost; safer is later in the sample so fewer winters still frosted.
Crop rule: Local windows for Tomato from frost dates, temperature ratings, and crop timing rules.
Confidence: Provisional — short or thin sample for this place.
Full methodology (algorithm 2026-10-season-5).

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