How To Make a Hot Bed in Your Polytunnel

Installing a polytunnel heater is not the only way to provide extra heat for under-cover plants. Polytunnel heating can often be energy intensive and is not always a sustainable or eco-friendly way to grow food or other plants over the winter months. The good news is that polytunnel gardens can also utilise a range of other methods to protect polytunnel plants over winter. One such method is harnessing natural heat given off my decomposing organic material and making a hot bed in your polytunnel.

What is a Hot Bed?

A hot bed was an extremely popular method for greenhouse horticulture in the Victorian period. A hot bed is a raised bed that takes advantage of the fact that organic material gives off heat as it decomposes. The heat from beneath can help to protect tender plants from frost over the winter months.

A hot bed consists of a layer of a decomposing mix of straw and manure, topped by a thinner layer of growing medium into which seeds can be sown, or trays of seedlings can be placed.

What Materials Do I Need To Make A Hot Bed?

To make a heated bed in your polytunnel you will need:

• Edges to contain your raised bed. (Wooden pallets are a good choice. You could also increase the thermal mass (heat storing capacity) in your polytunnel by using stone/ brick/ clay or other such materials.)
• Enough straw/ manure or other organic, compostable materials to create a layer within your raised bed that is around 60-90cm deep (once compressed by treading down firmly, which will increase the heat release).
• A 1:1 mix of topsoil and compost, to create a topping of 20-30cm depth. (Ratio of heat-producing material to growing medium should be 3:1, as this will help to achieve an ideal temperature of around 24 degrees C.

How To Make a Hot Bed

To make your raised composting bed:

• Create a raised bed frame using your edging material. (The size and shape can vary – as long as you stick to a 3:1 ratio as described above.)
• Shovel your mix of manure and straw into the raised bed area that you have prepared.
• Tread on the mix to compress it and increase its heat producing capacity.
• Top this mix with your growing medium and rake this to a fine tilth (if direct sowing).
• Direct sow lettuce/ salad greens and other crops earlier in the year than it would be possible to sow elsewhere in your polytunnel or garden.
• You can also harness the heat even more effectively by covering your hot bed with a mini polytunnel, row cover or cloche, to create a much warmer micro-climate than the rest of your undercover growing area.

Have you created a hot bed in your polytunnel? Let us know how you have fared in the comments below. Looking for more tips for winter polytunnel gardening? Check out our other articles and guides.