A hotbed is a cold frame built over a pit of fresh horse manure: 30 cm of manure with its bedding straw, wetted and left 3-4 days to heat to 50-60 °C, then covered with 15-20 cm of soil. The decomposing manure warms the soil from below for 6-8 weeks, so seedlings can start 8-12 weeks before the last frost.
A hotbed is a cold frame with a built-in heater: a buried layer of decomposing manure that releases heat for weeks beneath the planting soil.
The Manure Is A Slow Fire, And The Peak Has To Pass Before You Sow
Fresh manure with straw bedding is a compost heap in the making. Microbes eat it and release heat. A layer 30 cm deep, wetted and packed under a lid, climbs to 50-60 °C within 3-4 days and then declines over 6-8 weeks. The planting soil sits above it as a buffer, but the buffer does not make the root zone safe by itself, so the temperature at root depth has to be checked rather than assumed.
That warmth lets seedlings start 8-12 weeks before the last frost. The skill in a hotbed is timing. Seeds sown while the manure is at its peak meet soil that may be hotter than seedlings tolerate, and the gases released in those first days can scorch them. Wait, measure, and sow when the root zone has dropped to a temperature you can hold your hand in.
What You Need
- A cold frame with walls tall enough to hold the layers (see Basic Box Construction)
- Fresh horse or livestock manure with bedding straw — enough for a 30 cm layer over the frame footprint
- Garden soil or compost for a 15-20 cm top layer
- A thermometer, or a long metal rod to feel the temperature at root depth
- Water, for wetting the manure
Method: Build And Time The Hotbed
Step 1: Excavate. Dig 45-60 cm below the footprint of the frame. This depth holds the manure and soil layers and keeps the lid within reach.
Step 2: Fill with manure. Add fresh manure with its bedding straw to 30 cm depth. Pack it lightly rather than tightly, so air still reaches the microbes.
Step 3: Water and wait. Water it thoroughly and close the lid. Over 3-4 days the internal temperature should reach 50-60 °C. A thermometer pushed into the layer, or a metal rod left in for ten minutes, tells you.
Step 4: Cover with soil. Spread 15-20 cm of good garden soil or compost over the manure. The total of manure plus soil, 45-50 cm, fits the 45-60 cm pit.
Step 5: Check the root zone before sowing. Push your thermometer or rod into the soil layer. Sow when the soil is warm but not hot to the hand. If it still burns, wait a few more days and ventilate.
Step 6: Vent daily. Open the lid every day, even in cold weather. Venting lets out the ammonia and surplus heat that would otherwise scorch the seedlings and cook the leaves.
Worked Example
Volume of manure for a standard 120 × 180 cm frame at 30 cm depth: 1.2 m × 1.8 m × 0.30 m = 0.648 m³. Soil layer at 15-20 cm: 1.2 × 1.8 × 0.15 = 0.324 m³ up to 1.2 × 1.8 × 0.20 = 0.432 m³. Total depth 30 + 15 = 45 cm up to 30 + 20 = 50 cm, which fits the 45-60 cm pit with 0-10 cm of rim above it.
Common Mistakes
- Sowing on the peak — the soil is too hot and the air is full of ammonia. Wait for the temperature to fall and vent.
- Dry manure — it will not heat. Water it until it is damp throughout but not dripping.
- Too little manure — a shallow layer peaks lower and fades sooner than the 6-8 weeks you are counting on.
- Compacting it flat — air cannot get in and the pile cools.
Safety
Gas, spores and heat from fresh manure
Fresh manure in a closed box releases ammonia and carbon dioxide, which accumulate under the lid; ammonia scorches seedlings and irritates eyes and lungs. Never build a hotbed inside a closed room, cellar or shed where people breathe, and never lean your face into a freshly closed frame — lift the lid and let it air first.
Manure also holds tetanus spores. Wear gloves, cover cuts, and wash hands before eating.
What’s Next
- Learn next: Hot Composting — the same microbial heat, managed deliberately
- Combine with: Succession Planting Schedules — what to sow into the warm bed and when