Hotbeds — Using Fermenting Manure for Early Crops

What Is a Hotbed?

A hotbed is a growing frame whose base is filled with fresh, fermenting horse manure. As the manure breaks down, it generates heat — enough to maintain temperatures of 20–25°C inside the frame through cold spring nights, weeks before the soil outdoors has warmed. It is one of the oldest techniques in kitchen gardening, widely used before glass was cheap enough for heated glasshouses, and it requires no electricity, no fuel, and no cost beyond the manure itself.

How It Works

Fresh horse manure — particularly when mixed with straw bedding — contains enormous populations of bacteria and fungi. As these micro-organisms break down the organic matter, the process is exothermic: it generates heat. A well-built hotbed can reach 60–70°C in its first week before settling to the 20–25°C growing range. The art is to allow the initial heat to pass before placing tender plants.

Building a Hotbed

1. Collect fresh horse manure with a good proportion of straw — the fresher the better. Avoid poultry manure for this purpose; it heats unevenly and burns more quickly.
2. Build a mound or pit 45–60cm deep, slightly larger than your cold frame. Firm it gently as you build.
3. Place the cold frame on top and close the lid.
4. Wait 5–7 days for the initial violent heat to pass. Check with a thermometer — you want 25°C or below before sowing.
5. Add a layer of good compost or topsoil (10–15cm) over the manure, then sow or plant directly.

What to Grow

Hotbeds are ideal for early lettuce, radish, spring onions, and mustard greens from January or February. They are also excellent for starting off tomato, pepper, aubergine, and cucumber seedlings in March — all crops that need warmth to germinate and establish.

Manuring the Garden

When the hotbed is spent — typically after 8–10 weeks — the manure has lost its heat but has partially composted into an excellent soil amendment. Spread it on beds where brassicas or hungry feeders will grow.

Historical Note

Hotbeds were standard practice in walled kitchen gardens serving large houses throughout the 17th, 18th, and 19th centuries. Head gardeners would time their manure deliveries carefully to have successions of hotbeds ready through late winter and spring. The ability to produce cucumbers and melons in February was considered a mark of exceptional skill.