Sowing by zone, step by step

1. Three zones and your area

The same date doesn't work in Penzance and in the Highlands: everything is counted from the last spring frost.

1. Three zones — The mild coast has its last frost around 1 April, the lowland zone around 1 May, and the upland zone above ~250 m and the north around 25 May. In the US and Canada, use the frost dates for your town.
2. Average isn't certainty — In roughly half of years there is no more frost after the average date. That's why tender crops go out 2–3 weeks after it, not on the day itself.
3. Your garden isn't the average — A valley bottom is a frost pocket, while a south-facing slope above it can be up to 2 weeks earlier. A town garden by a wall is warmer than an open field.
4. Ask, measure, record — Ask older neighbours or the allotment society when they plant out tomatoes. Keep a max–min thermometer in the garden and record frosts. Old gardeners in Britain wait for the late-May bank holiday; in the northern US it's Memorial Day.

Why: Seed-packet calendars give one date for a whole country, so half of people sow too early and half too late. French beans sown too early rot in cold soil. Carrots sown too late don't make it before the frost.

• Zone dates are averages. In any year frost can come 2–3 weeks earlier or later.
• National weather services publish averages for the nearest station: the Met Office, Met Éireann, NOAA in the US, Environment and Climate Change Canada.

2. Frost and soil temperature

Deadlines are in weeks from the last frost, and the soil decides whether seed is ready to go in.

1. Count in weeks — Take your last frost date and subtract or add weeks. Example: 1 May − 8 weeks = 6 March, the date to sow tomatoes indoors.
2. Measure soil in the morning — With a probe thermometer at sowing depth, 5–10 cm (2–4 in), several days in a row. The date is only the first check.
3. Threshold by crop — Peas, broad beans, spinach and lettuce germinate from 5 °C (41 °F). French beans and sweetcorn need 10–12 °C (50–54 °F). Beans rot in cold, wet soil.
4. Warm the soil earlier — Black or clear plastic over a moist bed for 7–10 days, a low tunnel or a raised bed warm the soil 1–2 weeks earlier.

Why: “− 6” means six weeks before the last frost, “+ 2” two weeks after. The same number holds in every zone. In a cold spring the soil can lag two weeks behind.

• An old test without a thermometer: sit on the bed with a bare hand on the soil. If it feels unpleasantly cold after a minute, it's too cold for beans too.
• Cucumbers, courgettes, peppers and aubergines need 15 °C (59 °F), melons 18 °C (64 °F).

3. Cool-season and warm-season crops · Locked

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4. Direct sowing · Locked

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5. Thinning and successional sowing · Locked

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6. Upland and mild coast · Locked

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