Microbes in the Base: A No-Power Winter Salad Bench for Cold Plots

November is the month when UK plots go quiet, but a heap of fresh manure and straw can keep a small corner working. A compost hotbed — sometimes called a hotbed frame — uses the heat of decomposition to warm a growing layer from below. No cable, no thermostat, no electricity bill. Just microbes doing what they do, and a gardener paying close attention.

This is an old idea. Victorian kitchen gardens ran hotbeds on horse manure, and the basic physics has not changed. What has changed is our reason for reaching for it. With glasshouse heat costing what it does and winter light already rationed, a hotbed offers a low-tech way to raise salad, herbs and early cuttings on a cold plot. It is not a miracle. It is a managed compost heap with a job.

The biology: a slow fire that does not burn

A hotbed works because aerobic microorganisms break down carbon-rich material and release heat as they respire. Get the mix right — fresh manure for nitrogen, straw for carbon, enough air, enough moisture — and the heap passes through a thermophilic phase. Temperatures of 55–65°C are common in the core. Actinomycetes and heat-loving bacteria take over, and the heap becomes a slow, damp furnace.

The key word is aerobic. A soggy, compacted heap goes anaerobic, smells sour and cools down. A dry heap stalls. The craft is in building a structure that holds moisture without drowning the air pockets. Aim for a carbon-to-nitrogen ratio in the rough region of 25:1 to 30:1. In practice, that means mixing fresh horse manure with straw at something like one part manure to two or three parts straw by volume, then watering as you build until the material feels like a wrung-out sponge.

This is composting, not sterilising. The heat will knock back many weed seeds and pathogens, but it will not make raw manure safe to eat from directly. That is why the growing layer sits on top, and why the manure never touches the salad.

Siting and safety before you fork anything

Put the hotbed on level ground, away from house walls, sheds, fences and hedges. A hot heap is heavy, damp and biologically active; you do not want it leaning against timber or blocking ventilation. Keep it out of boggy corners and away from ditches, ponds and watercourses, so any runoff is filtered through soil and grass rather than heading straight for the local brook.

A pallet base or a few old paving slabs can keep the heap off standing water, but the ground should still drain. Wear gloves and boots, and do not let children or pets use it as a climbing frame. If you are pregnant, immunocompromised or prone to lung problems, get someone else to turn it or use proper protection — composting dust is not a wellness trend.

One more check: ask your manure supplier about herbicide residues. Aminopyralid and similar persistent herbicides can pass through manure and damage broad-leaved crops. A simple bean test in a pot of the manure mix is a sensible precaution before you build a whole bench on it.

Build a bench in an afternoon

You need four pallets or reclaimed boards, fresh manure, straw, a long-stemmed compost thermometer, topsoil or seed compost, and something breathable for cover — fleece, old nets or a spare cold-frame light. Build a bay roughly 1 metre by 1.5 metres and 60–75 centimetres high. Start with a coarse layer of straw or woody stems for drainage, then add the manure-and-straw mix in thin layers, watering as you go. Do not stamp it down. You want a heap that holds together when squeezed but still has air in it.

Leave the heap to heat for a week or so. Once the core has peaked and begun to settle, add a 10–15 centimetre layer of topsoil or seed compost. That is your growing bench. If you add the growing layer too soon, the heat can cook the roots; if you wait too long, you miss the useful warmth.

Temperature is the only honest metric

Push a compost thermometer into the centre, about 30–45 centimetres down. Expect a rapid rise within a few days. If the core is still below 40°C after a week, suspect dryness, compaction or too little nitrogen. If it is over 65°C, it is working hard — leave it alone and keep an eye on it.

Turn only when you need to. If the heap is cooling unevenly or seems to be stalling, fork the outer material into the middle and the middle out to the edges. One good turn is usually enough. Do not turn a hot heap every day; you are trying to keep the microbial population settled, not audition for a gym class.

Once the core settles to around 30–40°C, the bench is ready for sowing. Keep monitoring at root level. If the growing layer gets too warm, raise the frame or add another thin layer of compost. Salad roots want gentle bottom heat, not a sauna.

What to sow, and what to expect

From late November into January, think hardy leaves rather than summer vegetables. Winter lettuce, lamb's lettuce, land cress, mizuna, mustard greens and spinach are all worth a punt. Sow into trays or direct into the compost layer, keep the frame ventilated on mild days, and cover with fleece at night. Water sparingly with tepid water — cold tap water on a warm bench is a shock nobody needs.

Harvest outer leaves and let the plants keep going. Expect modest yields, not supermarket abundance. A well-run 1.5-metre bench might give you four to six good pickings of cut-and-come-again leaves across the coldest weeks, plus a few early spring thinnings. The heat typically lasts six to ten weeks, depending on the mix and the weather. After that, the heap becomes compost for the plot.

The cold-frame comparison

A cold frame is cheaper and simpler: no manure, no turning, no temperature probe. But it relies on passive solar gain, and in a UK December the soil inside can sit stubbornly cold. Germination is slow and growth even slower. A hotbed gives you bottom heat when the sun is not cooperating, which can mean earlier and more reliable pickings.

Costs are not always lower. Pallets and manure may be free, but straw, seed compost, seeds and a thermometer add up. A realistic scrounged build might cost £30–£50; a bought cold frame can be £50–£150 or more. The hotbed also asks for labour and attention. It is a fair trade if you want an active winter bench and a supply of compost afterwards. It is not a fair trade if you want to build something in October and ignore it until March.

The endgame is still compost

When the heat fades, do not treat the spent heap as waste. Let it finish composting, then use it as mulch or soil conditioner on hungry beds. Keep raw manure away from root crops and wash your hands after handling it. The bench has done its winter job; now it feeds the plot.

A hotbed is not glamorous. It smells, it steams and it needs checking. But on a cold November plot, a bench with a pulse is a quietly radical thing: food grown on waste, warmed by microbes, powered by nothing but biology and a bit of careful observation.