A greenhouse hotbed uses heat from decomposing organic material to warm a growing area, helping seeds germinate and young plants grow when the greenhouse is cool. To make one, build a contained bed, prepare fresh manure mixed with dry plant material, and cover it with a layer of growing medium; careful temperature checks are essential because the heat can be uneven or too intense.
Hotbeds are most useful when you want to start seeds earlier than unheated greenhouse conditions allow. They do not provide precise, thermostatically controlled warmth, so treat them as a natural heat source rather than a set-and-forget substitute for a heated propagator. Before beginning, check that you have enough space, a way to ventilate the greenhouse, and access to suitable fresh manure. If those conditions are not practical, an electric propagator or another controlled method may be easier to manage.
What You’ll Need
- A sturdy frame or contained bed, such as a timber-sided box, with room for a planting surface above the heating material.
- Fresh, straw-rich manure, commonly horse manure, and dry straw or similar coarse plant material for mixing and aeration. Avoid material with unknown contamination or chemical treatment.
- A fork or shovel, gloves, and a watering can or hose with a gentle flow.
- A soil thermometer to monitor the growing medium, plus a separate compost thermometer if available.
- Good-quality seed-starting mix or clean, prepared soil for the top layer, along with seed trays or pots.
- A transparent cover, such as a greenhouse lid or suitable glazing, and a way to prop it open for ventilation.
- Seeds, labels, and a notebook for recording temperatures and sowing dates.
Step-by-Step
- Choose a safe, workable position. Set the hotbed inside the greenhouse where it receives useful daylight and is easy to reach for watering and temperature checks. The floor must support the weight of the frame and its contents. Keep the bed clear of paths and away from plants that could be damaged by heat or leachate. If you are building directly on the ground, choose a spot with drainage; standing water can make the heating layer cold, sour, and difficult to manage.
- Build and prepare the frame. Make a robust box or use an existing bed with solid sides. The frame needs enough depth for a substantial heating layer beneath the growing surface, while still leaving room for seed trays and headroom under the cover. A cold frame placed inside a greenhouse can also serve as the top enclosure. Make sure the lid opens easily and can be secured safely in a ventilated position.
- Prepare the heating material. Break apart compacted manure and mix it with dry straw or other coarse, untreated plant material. The aim is a loose, springy mixture rather than a dense, wet mass. A mix that is too compact can become oxygen-starved and heat poorly; one that is too dry may not begin decomposing evenly. If the material is already very wet, do not add more water until you have assessed its condition.
- Fill the lower part of the bed. Add the mixture in manageable layers, distributing it evenly and pressing it only lightly. Do not stamp it down: airflow supports decomposition and helps the heat spread. Leave room above for the growing layer and for the cover. Avoid making the heating layer excessively deep, which can produce more heat than the bed can shed and make temperatures harder to control.
- Moisten and let the bed warm. Check the mixture by feel. It should be moist, not dripping wet; add water gradually if it is dry. Close the cover, then allow the material time to heat as it begins to decompose. The warm-up time varies with the material, moisture, packing, and greenhouse conditions, so do not rely on a fixed number of days. Measure the temperature within the heating layer and the growing medium rather than guessing from the air temperature.
- Add the growing layer. Once the heating material is warming steadily, cover it with a generous, even layer of clean seed-starting mix or prepared soil. The growing medium separates roots from raw manure and moderates the heat. Do not sow directly into the manure. Let the top layer settle and check its temperature at the depth where seeds or pots will sit.
- Wait for suitable growing conditions. Hotbed heat changes as the material warms and later cools. Sow only when the growing medium is within the suitable range for the seeds you plan to start. Check each seed packet or reliable horticultural guidance for that crop’s germination needs. If the bed is too hot, ventilate it and wait; if it is not warming adequately, troubleshoot moisture, compaction, and material quality before sowing.
- Sow seeds or set out trays. Sow at the depth recommended for each crop, label clearly, and water gently. For delicate or valuable seeds, use trays or pots rather than sowing directly into the bed. This makes it easier to move plants if the heat shifts. Keep the growing medium evenly moist, not waterlogged, and avoid splashing seedlings with dirty water from the heating layer.
- Manage temperature and ventilation daily. Check both the growing-medium temperature and the greenhouse conditions. Open the cover or greenhouse vents when heat builds up, then close them again as conditions cool. Do not leave a sealed hotbed in bright sun without checking it: the air under glass can become much warmer than the composting layer suggests. Keep a record so you can spot rising or falling trends.
- Adjust as the bed matures. As the organic material decomposes, its heat output declines and the bed may settle. Top up or level the growing medium if needed, while keeping it separate from the raw heating layer. Move established seedlings to a cooler position when they no longer need bottom warmth. When the bed has cooled, the decomposed material can be handled and used according to local composting guidance; do not assume fresh manure is suitable for every garden use.
Common Mistakes
- Sowing before checking the temperature: The bed can be much hotter in one spot than another. Test the growing layer where pots or seeds will sit, and wait if conditions are unsuitable.
- Packing the heating layer tightly: Dense material has less airflow and may heat unevenly or turn anaerobic. Keep it loose and avoid heavy compression.
- Using material that is too wet: Waterlogged manure can lose the air needed for active decomposition. Add water in small amounts only when the mix feels dry.
- Putting seeds in raw manure: Fresh manure can be too hot and is not a seed-starting medium. Keep it below a separate layer of clean growing mix.
- Ignoring ventilation: A closed greenhouse can overheat even when the weather outside seems mild. Check the bed regularly and provide a way to release excess heat.
- Expecting steady, controllable heat: A hotbed’s output naturally rises and falls. It needs monitoring, and sensitive crops may be better started with a more controlled heat source.
Tips
Build the bed before you need to sow, so you can observe how it warms and settles without risking a tray of seedlings. Use a thermometer at seed depth, not just a reading from the greenhouse air or the centre of the manure. Temperatures can vary across the bed, so check more than one location when possible.
Keep a simple log of readings, watering, ventilation, and changes in the material. This helps you learn how your particular combination of manure, straw, frame, and greenhouse behaves. If the heat is uneven, rotate trays only when doing so will not disturb emerging seedlings, and avoid placing pots directly over a very hot patch.
Choose crops suited to the available warmth and light. Warmth can encourage germination, but seedlings still need adequate daylight and sensible watering. If stems become long and weak, the problem may be insufficient light rather than a lack of heat. Harden plants gradually before moving them into colder conditions.
Troubleshooting
The bed does not warm up: Check whether the material is dry, overly compacted, or already well decomposed. Loosen compacted areas and moisten dry material gradually. If it is waterlogged, improve drainage and add dry, coarse plant material. Old, finished compost may not generate useful heat, so replacing it with suitable fresh material may be necessary.
The bed is too hot: Open the cover and ventilate the greenhouse, then measure the growing layer again. Keep seed trays out of the hot zone until the temperature falls into a suitable range. Do not try to cool a very hot bed by flooding it; excess water can create other problems and may not reduce heat evenly.
Some parts are warm and others are cold: Uneven moisture, different degrees of compaction, or irregular filling can cause hot and cool spots. Check several locations, redistribute trays if safe, and correct the underlying issue gradually. Avoid turning the heating layer after sowing unless necessary, because it can disrupt the bed and release heat unpredictably.
There is a sour smell or liquid pooling: These can indicate a poorly aerated, waterlogged heating layer. Stop adding water, improve drainage, and gently loosen the material if the bed is not planted or can be worked without disturbing roots. If the material remains foul or unusable, remove it carefully and rebuild with a more open mix.
Seedlings wilt or fail to emerge: Check the temperature at tray level, then inspect moisture and sowing depth. Overheating, drying out, and waterlogging can all prevent healthy establishment. Follow the seed’s specific guidance, and move affected trays to a more stable environment if you cannot control the hotbed conditions.
Frequently Asked Questions
What is a greenhouse hotbed?
A greenhouse hotbed is a growing bed warmed by heat produced as organic material, usually manure mixed with straw, decomposes beneath a layer of soil or seed-starting mix.
Can I make a hotbed without manure?
Other actively decomposing organic materials can produce heat, but results vary. A hotbed depends on suitable material, moisture, and airflow; if you do not have a reliable heat source, a propagator or another controlled method may be more predictable.
How deep should the heating layer be?
There is no single depth that suits every frame or material. Make it deep enough to provide useful, even warmth, but not so deep that the bed becomes difficult to manage or overheats. Use temperature readings to guide adjustments.
When should I sow into a hotbed?
Sow when the growing medium, at seed depth, is suitable for the crop. Check the seed packet or trusted crop-specific guidance, and measure the bed rather than relying on a calendar or the temperature of the greenhouse air.
Does a hotbed need ventilation?
Yes. The greenhouse and hotbed cover can trap heat, especially in bright conditions. Provide a way to open them, check temperatures regularly, and ventilate before warmth becomes excessive.
Can I use the finished heating material in the garden?
It may be compostable or useful after it has decomposed, but suitability depends on its ingredients and local guidance. Keep fresh manure away from edible crops and follow official horticultural advice on handling and use.