Lesson 3
Soil temperature and sowing
Air temperature is what you feel. Soil temperature is what seeds feel, and they will not germinate until the ground is warm enough. Peas need 5°C. Carrots want 7°C. Courgettes and squash need 10°C minimum, preferably 12–15°C, or they rot in the ground before they sprout. This is why direct-sown courgettes fail in April but romp away in late May.
A soil thermometer costs eight pounds and removes all guesswork. Push the probe 5cm down at 9am, away from direct sun. If you do not own one, use touch and timing: soil warm enough to sit on comfortably for thirty seconds (around 10°C) is warm enough for beans. Soil that feels cold to your palm is below 8°C — wait another week.
Covering soil with black plastic or cloches for two weeks before sowing raises the temperature by 2–4°C, effectively bringing your season forward by a fortnight. In a cold spring, this is the difference between success and watching your neighbour's beans sprint past yours because she warmed her soil first.
Quick reference — minimum soil temperatures
- Broad beans, peas, onion sets — 5°C
- Lettuce, spinach, radish, early carrots — 6°C
- Beetroot, chard, parsnips, maincrop carrots — 7°C
- Dwarf French beans, runner beans — 10°C
- Courgettes, squash, sweetcorn, climbing French beans — 12°C
- Cucumbers (outdoor), basil — 15°C
If you sow into cold soil and nothing happens for three weeks, the seed has rotted — start again and wait for warmth.