GardenCalc
Every sow-by date on a seed packet secretly depends on one number: your last frost date. This planner does the working out for you. It anchors each crop's windows to your own postcode district's safe last-frost date (the date frost had passed by in nine years out of ten), from Met Office HadUK-Grid 5 km gridded observations, 1991–2020.
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Enter your postcode prefix (e.g. B33, TR1, IV2), tick the crops you grow, and get your personal windows.
Your Sowing Plan
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Kit for the Gap Between Sowing and Safe Planting
Links in this section and in the text below are affiliate links to Gardening Naturally, Suttons and Thompson & Morgan, UK suppliers we use ourselves. If you buy through them GardenCalc earns a small commission at no extra cost to you. Prices checked August 2026 and may change.
Steady bottom heat for the sow-indoors window: tomatoes, chillies and aubergines germinate faster and more evenly.
Windowsill propagator (Gardening Naturally, about £15)The cheap route. Self-watering, fits a sill, enough for a first batch of tomatoes or a tray of salad.
Root trainers (Gardening Naturally, about £9)Deep cells for beans, sweet peas and sweetcorn, so the roots go down and the plants lift out without a check.
Peat-free seed compost (Suttons, RHS SylvaGrow)One bag covers the sow-indoors window for most plots. Peat-free and fine enough for seed trays.
Tomato 'Moneymaker' organic seed (Suttons)The reliable outdoor and greenhouse tomato. Sow in the indoor window the planner gives you, plant out after the safe date.
Chilli 'Cayennetta' F1 seed (Thompson & Morgan)Chillies want the earliest sow-indoors start of any crop here. Thompson & Morgan describe this one as British-bred, compact enough for a pot and tolerant of a cold spell.
Fleece, 30 gsm (Gardening Naturally)Insurance for a late cold snap after planting out. Buy the wide roll, not the small pack.
Victorian bell cloche (Gardening Naturally)Warm the soil before direct sowing and shelter young plants for their first weeks outside.
Cold frame (Gardening Naturally)The halfway house for hardening off between the sow-indoors window and the safe date.
How the Planner Works
Each crop in the planner carries a set of windows expressed relative to the frost calendar: how many weeks before the safe date to sow indoors, when it can go outside, and what to do before the first autumn frost. Your postcode gives the anchor dates; the crop data does the rest. The windows follow mainstream UK guidance and each crop's entry carries its sources, listed in the crop data itself.
The safe date is deliberately cautious. It is the date the last spring frost had already passed in nine years out of ten between 1991 and 2020, which is a different and later date than the typical (median) last frost. Tender plants killed by one cold night do not care about medians, so the planner anchors to the safe date.
When to Plant Out Tomatoes in the UK
Tomatoes are the classic example of why postcode matters. Sow them indoors (a reliable variety such as Moneymaker, in peat-free seed compost) during the sow-indoors window the planner shows for your district, then plant out only after your safe last-frost date, hardening off first. In a mild south-western district that can be weeks earlier than in the Scottish Highlands. The planner shows both windows for your own postcode rather than a national average.
Hardening Off Before the Safe Date
Plants raised indoors need a gradual introduction to outdoor conditions before they are planted out. The RHS guidance on hardening off covers the technique. The gap between your sow-indoors window and your safe date is where hardening off happens, and a cold frame or fleece makes it easier.
The Autumn Side: First-Frost Actions
The planner also shows what each crop needs before your district's typical first autumn frost, from picking the last tomatoes to lifting dahlias. If autumn planning is your main question, the first-frost article explains why the smartest gardeners look this date up in high summer, and the Frost Date Calculator gives the full frost picture for your postcode.
Frequently Asked Questions
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About the data. Sow windows are calculated from your postcode district's frost dates, derived from Met Office HadUK-Grid 5 km gridded observations, 1991–2020, used under the Open Government Licence. This is not a forecast: always check the weather before planting out. Crop guidance sources are cited per crop in the planner's data file.