Soil temperature — not air temperature — determines whether your seeds will germinate. Enter your current soil temperature or select a month to see exactly which crops are ready to sow right now.
Average UK / temperate climate soil temps
| Crop | Min °C | Optimal °C | Max °C |
|---|---|---|---|
| Broad Bean | 2°C | 15°C | 25°C |
| Pea | 4°C | 15°C | 25°C |
| Spinach | 4°C | 16°C | 24°C |
| Lettuce | 2°C | 18°C | 26°C |
| Onion | 5°C | 20°C | 35°C |
| Leek | 5°C | 18°C | 28°C |
| Carrot | 7°C | 22°C | 30°C |
| Beetroot | 7°C | 20°C | 30°C |
| Parsnip | 6°C | 18°C | 25°C |
| Cabbage / Kale | 5°C | 25°C | 38°C |
| Broccoli | 5°C | 25°C | 38°C |
| Celery | 10°C | 22°C | 30°C |
| Tomato | 10°C | 24°C | 32°C |
| Pepper | 15°C | 27°C | 35°C |
| Aubergine | 15°C | 27°C | 35°C |
| Courgette | 12°C | 30°C | 38°C |
| Cucumber | 15°C | 30°C | 38°C |
| Pumpkin / Squash | 13°C | 30°C | 40°C |
| French Bean | 10°C | 25°C | 35°C |
| Runner Bean | 12°C | 25°C | 35°C |
| Sweetcorn | 10°C | 27°C | 38°C |
| Basil | 15°C | 24°C | 35°C |
Many gardeners sow seeds based on the calendar or air temperature, then wonder why germination is patchy or slow. The real signal your seeds respond to is the temperature of the soil around them — not the temperature of the air above.
Air temperature can fluctuate wildly between day and night. Soil temperature is far more stable and accurately reflects the warmth your seeds will actually experience. A spell of warm March sunshine might push air temperatures to 16°C, but the soil may still be sitting at 6°C — far too cold for warm-season crops.
The most accurate tool is a soil thermometer — a probe thermometer you insert 5–10 cm (2–4 inches) into the ground. Take readings in the morning, when soil is coolest. For seed sowing, the morning reading is the most conservative and reliable guide.
No soil thermometer? As a rough guide, soil temperature at 10 cm depth typically lags about 4–6 weeks behind air temperature trends. A simple kitchen probe thermometer inserted into the soil also works perfectly well.
Black mulch, cloches, and polythene covers can raise soil temperature by 3–5°C, letting you sow 2–4 weeks earlier than uncovered ground. This is especially useful for courgettes, squash, and sweetcorn.
Crops broadly divide into two categories based on their germination temperature requirements.
Cold-season crops — peas, broad beans, lettuce, spinach, carrots, brassicas — germinate happily in soil as cool as 4–7°C. They also grow better in cooler conditions and tend to bolt or turn bitter in high summer heat.
Warm-season crops — tomatoes, peppers, courgettes, cucumbers, beans, sweetcorn — need soil of at least 10–15°C before germination begins reliably. Below this threshold, seeds sit dormant and often rot before they can sprout.
What depth should I measure soil temperature at?
For seed sowing, measure at 5 cm (2 inches) depth — the level where small seeds typically germinate. For transplanting larger seedlings, 10 cm (4 inches) is more relevant as it reflects the temperature experienced by the root zone.
What time of day should I measure soil temperature?
Morning is best, before the sun has warmed the surface. Early morning readings represent the minimum overnight temperature your seeds will experience. For the warmest reading of the day, measure in mid-afternoon — but for sowing decisions, use the morning figure.
Can I sow if my soil is at the minimum temperature?
Yes, but germination will be slow and irregular. Seeds germinate fastest at the optimal temperature, not the minimum. If you sow at minimum temperatures, expect germination to take two to three times longer than packets suggest.
How do I warm up my soil quickly?
Cover the soil with black polythene or a cloche 2–4 weeks before planting. Black polythene absorbs more heat than clear, but clear polythene also traps warmth effectively. You can raise soil temperature by 3–5°C this way.
Why do my courgettes keep rotting instead of sprouting?
Cold, wet soil is almost always the cause. Courgette seeds need a minimum of 12°C to germinate. Below this, they sit in damp soil for weeks and rot. Either wait until the soil is warm enough or start seeds indoors in warm conditions (21–25°C) before transplanting.
Connect your garden to live weather data. The app tracks your local conditions and nudges you the moment soil temperatures hit the right threshold for each crop.
Weather integration
Live local weather and temperature data connected to your garden (Pro)
Smart sowing reminders
Get email alerts when conditions are right to sow each crop (Pro)
Smart seasonal suggestions
App auto-recommends what to start based on your current season (Pro)
Planting calendar
Full zone-based sow, transplant and harvest calendar for every crop
Task tracker
Log when you sowed and get reminders for watering, feeding and harvest
Free to start
2 beds included · task reminders · plant database
Start Free → Already have an account? Sign in