Key takeaways
- A hotbed is a cold frame on top of a deep layer of fresh, strawy horse manure
- Build it in late winter, at least 1.2m square and 60-90cm deep
- Wait until the compost on top drops below about 25C, usually after a week or two, before sowing
- Radish, lettuce, salad leaves, carrots and turnips crop weeks earlier than outdoors
- Open the lid on mild days, as steam and sun can overheat seedlings fast
- Once the heat fades, use the frame for hardening off and the old manure on your beds
A hotbed is a cold frame on top of a deep pile of fresh horse manure. As the manure rots, it gives off heat. The frame traps that warmth, so you can sow salads, radish and early carrots in late winter, weeks before the ground outside is ready. It costs nothing to run and the old manure feeds your beds afterwards.
Victorian kitchen gardens relied on hotbeds for early salads, and even pineapples on grand estates. All you need is a supply of fresh stable manure, a frame and a little patience.
How does a hotbed make heat?
Fresh manure mixed with straw is full of microbes. As they break it down, they give off a lot of heat. It’s the same thing that happens in a hot compost heap.
A hotbed just puts that heat to work. A layer of compost goes on top of the manure, and a glazed frame goes over it. The heat rises through the compost on its way out and warms the roots of your seedlings.
Three things stop it working: manure that’s too old, manure that’s soggy and packed tight, and a pile that’s too small. The guide to hot and cold composting explains the same process in a compost bin.
Fresh stable manure with plenty of straw is what you want. The steam shows it’s already warming up.
Choosing manure for a hotbed
Use fresh horse manure with plenty of straw in it, a few weeks old at most. It should still smell of the stable. Local livery yards and stables are often glad to give it away.
Do a quick check before you collect it:
- Freshness: it should be warm or steaming when you dig into the heap. Dark, crumbly, well-rotted manure has already spent its heat.
- Straw: you want plenty of straw mixed through. Straw lets air in, and the microbes need air to work.
- Moisture: squeeze a handful. It should feel like a damp, wrung-out cloth. Add water if dry, more straw if dripping.
Ask the stable whether their hay or grazing has been treated with weedkiller. Some long-lasting weedkillers pass through horses and can damage crops such as beans, potatoes and tomatoes. The guide to aminopyralid in manure explains what to look out for.
Fresh, strawy manure (left) heats up. Dark, well-rotted manure (right) is great for beds but won’t warm a hotbed.
How to build a hotbed step by step
Build your hotbed in late January to early March, in a sunny, sheltered spot. Size matters most: a small pile loses heat too fast.
- Mark out at least 1.2m square. Bigger holds heat better. A low wall of boards, pallets or straw bales helps keep it in place.
- Add the manure in layers. Fork it in loosely, about 20cm at a time, and tread each layer gently. Aim for 60-90cm of manure in total.
- Leave it a couple of days. It will start to heat and settle down a little.
- Add the compost. Spread 15-20cm of compost or good soil on top. This is where your seeds and roots will grow.
- Put the frame on top. Use a cold frame or a simple timber box with a glazed or polycarbonate lid. Slope the lid towards the sun if you can.
A frame from a skip or an old window works fine. The guide to cold frame gardening has more ideas for frames.
Deep manure at the bottom, a layer of compost on top, and a glazed lid to trap the warmth.
When is it safe to sow?
Wait until the compost on top has cooled below about 25C. For the first few days the manure can top 50C, which would cook seeds. It usually takes a week or two to settle.
Push a soil thermometer about 10cm into the compost, not into the manure below. Once it reads below about 25C for a couple of days in a row, you can sow. The guide to seed germination temperatures shows which seeds like it warmer or cooler.
Check the compost where the roots will grow, not the hot manure underneath. A cheap soil thermometer takes the guesswork out.
What to grow in a hotbed
Quick, low-growing crops do best. Sow them thinly in rows.
- Radish: the fastest crop and very forgiving.
- Lettuce and salad leaves: pick as baby leaves for an early treat.
- Spinach: a useful early leafy crop.
- Early carrots: choose a quick, stumpy or early variety such as Amsterdam Forcing or Early Nantes.
- Turnips and salad onions: both grow well in the warmth.
You can also start trays of seeds on the warm compost, such as tomatoes or brassicas, then move them on. Once the heat fades in spring, the hotbed becomes a handy spot for hardening off seedlings.
Radish and early carrots enjoy the gentle warmth. You’ll be pulling them weeks before outdoor sowings.
Opening the lid: watering and airflow
A hotbed is more likely to overheat than freeze. Rotting manure gives off steam, and a shut lid on a sunny morning can turn the frame into an oven. Damp, still air also invites damping off, where seedlings collapse at soil level.
- Open the lid a little on mild days, more on warm sunny days.
- Close it by mid-afternoon in cold weather to keep the warmth in overnight.
- Wipe off heavy condensation from the underside of the lid.
- Water the compost lightly, in the morning, only when it’s dry.
- Sow thinly so air can move between seedlings.
The guide to damping off explains how to spot it early.
Gardener’s tip: Hang a max-min thermometer inside the frame at seedling height. It tells you if the frame got too hot while you were out.
Prop the lid on a block to let out steam and stop the frame overheating on sunny days.
Month-by-month hotbed calendar
| Month | What to do |
|---|---|
| December | Find a stable and arrange to collect fresh manure |
| January | Build the hotbed in late January in milder areas |
| February | Build now if not done. Sow radish, lettuce and salad once the compost cools |
| March | Sow early carrots, spinach and turnips. Open the lid on mild days |
| April | Pick radish and salad. Heat fading, so use the frame for hardening off |
| May | Fork the old manure and compost onto your beds |
| June to November | Use the frame as an ordinary cold frame |
Common hotbed mistakes
Sowing too soon. In the first few days the compost can be too hot for seeds. Wait until it is below about 25C.
Building too small. A small pile loses its heat quickly. Go for at least 1.2m square.
Using well-rotted manure. It’s lovely on beds but has no heat left. Use fresh, strawy manure.
Keeping the lid shut. Steam and sunshine can cook seedlings. Open up on mild days.
Soggy manure. Wet, packed manure smells but doesn’t heat. Mix in more straw.
Is a hotbed worth the effort?
Yes, if you can get fresh manure. A hotbed gives you a warm growing space for free, weeks of early salads, and a load of rotted manure for your beds in late spring. For other ways to get a head start, see the guide to forcing crops for earlier harvests.
Frequently asked questions
What is hotbed gardening?
A hotbed is a cold frame heated from below by rotting fresh manure. As the manure breaks down it gives off heat, which warms a layer of compost on top. That lets you sow early crops in late winter with no electricity.
What manure is best for a hotbed?
Fresh horse manure mixed with plenty of straw works best. It should be a few weeks old at most and still smell of the stable. Well-rotted manure has already used up its heat, so it won’t warm the bed.
How hot does a hotbed get?
The manure can get very hot, often above 50C, in the first week. That’s far too hot for seeds. The compost on top is cooler and settles to a safe temperature after a week or two.
When can you sow into a hotbed?
Sow once the compost has dropped below about 25C. Check with a soil thermometer pushed into the compost, not the manure. This usually takes a week or two after building.
What can you grow in a hotbed?
Radish, lettuce, salad leaves, spinach, early carrots, turnips and salad onions grow well. Radish is the quickest. Once the heat fades, use the frame for hardening off young plants.
How long does a hotbed stay warm?
A well-built hotbed usually gives about six to eight weeks of useful warmth. Bigger beds of fresher, strawy manure stay warm for longer than small ones.
Once the heat has gone, read the guide to growing winter salad leaves for crops to keep the frame busy next autumn, or browse more growing guides.
Our team produces practical UK gardening guidance from source research, contributor records and established horticultural practice. Editorial standards are overseen by Matt W.
