UK seed sowing calendar heat mat: month-by-month tender crops, set-points and off-heat timing
If you use a seedling heat mat through the UK cold season, timing matters as much as temperature. This month-by-month schedule covers tender crops from January through April: when to sow, which compost set-point to use, when to turn heat off after emergence, and when to start hardening off before planting out after the risk of frost.
January–April schedule for tender crops
| Month | Crops on heat | Compost target | Mat off-heat | Notes |
|---|---|---|---|---|
| January | Chinense chilli | 28–30°C | At first cotyledons | Needs grow lights—do not sow without supplemental light |
| February | Chinense, early tomato | 25–28°C | At emergence | Vent lid by day once sprouted |
| March | Tomato, pepper, aubergine | 22–25°C | 3–5 days after emergence | Greenhouse bench may still need night fleece |
| April | Courgette, cucumber (late) | 20–22°C | Quickly after sprout | Fast crops stretch if heat stays on |
Crops that should not use bottom heat
Lettuce, peas, beans, root vegetables and most hardy herbs germinate better without extra heat. Cool-season crops on a mat may emerge too fast and become leggy under low winter light.
UK frost and hardening-off
Last frost dates vary by region—use "after risk of frost" language rather than a single national date. Plan one week of hardening off in a cold frame or sheltered spot before outdoor planting. Move seedlings to brighter, cooler conditions as soon as heat comes off.
Checklist column for each sowing month
- Soil thermometer probe in compost, not tray surface.
- Clear propagator lid for humidity; vent once sprouted.
- Grow lights from February for windowsill crops.
- Prepare off-heat location before sowing so transition is immediate.
Who should skip the January chilli row
Without supplemental light, January-sown seedlings will stretch on a windowsill regardless of perfect compost temperature. Add lights or delay sowing to February.
Product reference
The Plants Seedling Heating Mat with Thermostat covers 20–30°C set-points—match your month column to the controller range. Listed at £28.99 inc. VAT on the live page.
Can I reuse the mat for cuttings in summer?
Many growers use bottom heat for softwood cuttings—clean the mat surface and verify the controller still holds set-point before reuse.