When to plant onion in London

Where are you growing?

Comfortable

Comfortable in greenhouse

Comfortable in a cold frame

Onion in London is a good fit outdoors. The suitable growing window of 297 days covers the 90–120 days this plant usually needs.

When to plantFebruary 18 – April 14

Harvest: May 19 – August 12
Season vs crop: 297 grow days vs 90–120 needed
Typical frost (last spring – first fall): April 1 – December 13

Onion in London is a good fit outdoors. The suitable growing window of 297 days covers the 90–120 days this plant usually needs.

Planting conditions

Direct sow

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 row297

Days to harvest90–120 days from seed

Feb 18 – Apr 14

Direct sow

Activity	Window	Notes
Direct sow	Feb 18 – Apr 14	Plant outdoors in this window.

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 (15 – 24 °C) and tolerable (4 – 30 °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 frostApril 1

Safer last spring frostApril 1

Typical first fall frostDecember 13

Safer first fall frostDecember 13

Frost-free days255

Outdoor sow dateFebruary 18

Direct sow windowFebruary 18 – April 14

Suitable growing windowFebruary 18 – December 11 (297 days)

How these dates are judged

Ideal conditions: Daytime air from 15 – 24 °C.
Suitable conditions: Daytime air from 4 – 30 °C.
Last frost and first frost: Typical spring and fall frost dates for London, shown in the key dates.
Days to harvest: 90–120 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. The year strip and weekly chart use the same window: air ratings only colour weeks inside it.

Growing window for Onion: Feb 18 – Dec 11 (297 days)

Planting window for Onion: Feb 18 – Apr 14 (56 days)

Usually too cold: Jan 1 – Feb 15

Outdoors: weekly temperatures and windows

Temperatures across the year for London. 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 8 · Feb 16 – Feb 22

Average 6 to 11 °C · Possible −2 to 15 °C

Growing: tolerable

Planting: suitable

A week is in range when its average low and high sit inside the tolerance band 4 to 30 °C; it is suitable when that average range also overlaps the ideal 15 to 24 °C band and possible readings stay within 4 °C to 30 °C. Crossing a possible limit, or missing the ideal band, makes it tolerable; a possible low below 4 °C makes it tolerable. 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 London

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 51.50, -0.10, 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 Onion from frost dates, temperature ratings, and crop timing rules.
Confidence: Provisional — short or thin sample for this place.
Full methodology (algorithm 2026-10-season-6).

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