Making a Hotbed in your Polytunnel

Deano Martin
Friday, 28th February 2014

Hotbeds are a mixture of manure, straw, poultry bedding... anything that will heat up as it decomposes. You can then grow tender vegetables like sweet peppers and French beans much earlier in the season without a heater.

I have been intrigued by the idea of a hotbed for years, but the trigger for this project was reading Hot Beds by Jack First. This book is a great little resource, packed full of information on how to make a hotbed, and what to grow in one once made. Having put up a decent sized polytunnel (hoop house) last Summer, I've been waiting for the right time to build a hotbed. According to Jack, the ideal time for a full depth bed is the end of January, so that's what I have done (but you can build them later and still get the benefits).

This post contains lots of pictures of the hotbed, along with some text. However the construction for an outdoor bed is a little different, and if that's what you're looking for, you probably need the book.

Hotbed Basics

A hotbed uses the heat from decomposing organic matter to raise the temperature of soil in an enclosed space, in order to extend the growing season. One reason for that is that by the time the last frost has passed, so have a lot of days with long day length. For example our last safe frost date is normally the first week in June. That is only two weeks before the Summer solstice, after which daylength starts to shorten again. By providing a warm and sheltered environment for growing crops, a hotbed will let me make use of that additional sunlight.


Hotbed Construction

Jack recommends a hotbed size of about 6ft x 6ft (180cm x 180cm), to provide a growing space of about 4ft x 4ft. The growing space has it's own frame partly buried in the hotbed, with the extra space all round helping to prevent heat loss. The depth of the hotbed determines how early to start building it. Two feet to 2 feet 6 inches (60-75cm) of horse manure, and wet stable bedding (straw) is supposed to maintain warmth through until the weather is warm enough.

My hotbed

The central bed in my polytunnel is 5 feet wide, made 18 inches tall with timber planks. The bed runs North to South, so I decided to make the hotbed at the Southern (entrance) end, to get the most from the low, early season sun. That section of the bed had settled since the Summer, and was about 6 inches below the level of the planks, so I decided to remove another 6 inches of soil, and then build a two foot deep hotbed, by adding another two planks. This would give me an overall bed size of 6 ft x 5 ft, 2 ft deep. The two pictures below show the bed just before all of the soil had been removed.

Preparing the hotbed

The soil that I removed filled eight dustbins, I knew that I would need about two thirds of the soil to put on top of the manure and straw mix. This gives a good growing medium for seeds. The remainder will be used as the base for my seed and potting mix.

Hotbed construction

After the soil had been removed, I built up the hotbed in layers. My horse manure is given to me without bedding, so I alternated a layer of manure with a layer of poultry bedding. The bedding that I use is shredded Miscanthus. Shredding exposes more surface area to microbes which could lead to the bed heating up, and cooling down too quickly. The decomposition in a normal bed is slowed by compacting the materials. This restricts oxygen, which reduces microbial activity, and therefore slows everything down. To try and slow the process down further, I added a little more bedding than I would have done with undamaged straw, and made sure that the layers were well compacted. I'll monitor the temperature this year to see how successful that is.?

Building up the hotbed in layers

The picture above shows the first layer in place. As the bedding was dry, I watered it, and added urine, seaweed, comfrey concentrate, and the liquid from my wormeries. These additions were to improve the overall nutrient availability, and to add some extra nitrogen. This isn't something from the hotbed book, but seemed like a good idea. However the extra nitrogen may speed the decomposition up, and make it too quick. I'll have to monitor that.

The picture below shows the hotbed after an extra 12 inches of planking had been added. The lower plank is full length, and the top one is only the length of the hotbed. I did it this way as I always have a surplus of compost, and so I have decided to raise the whole of this bed ( 40ft x 5ft) by another 6 inches. I'll stagger this, building a new hotbed each year, moving back along the bed.?

Adding another layer to the hotbed

The picture above shows the hotbed before the additional planks were added, whereas the one below shows it after all of the timber was in place. If you look carefully at the the picture below, you can see the three levels of timber. The short plank for the top layer, which steps down to a long plank added to the existing bed. Beyond that the bed sides drop again to the original level.

(The black corrugated bitumen is there to prevent a passionflower from being buried by the manure/straw mix. The material should shrink by around a half, so the bitumen can be removed when the hotbed is taken apart.)

More layers in the hotbed

The change in levels can be seen better in the next picture, taken from the opposite direction.

Three levels in a hotbed

The plank to the left of the plants has been added to raise the height of the whole bed, and there is a short plank raising the height of the hotbed still further.

Last layer in place

The picture above shows the last but one layer of the straw and manure mix in place. Originally I had intended to build the hotbed the full 2ft deep, and then add a 4 inch layer of growing medium. The depth of the hotbed determines how long it will remain warm for. Two feet roughly equates to three months of warmth outdoors. However I started to run short of horse manure. As I'm building the hotbed inside a polytunnel, I'm happy for the bed to cool down slightly quicker, as the protection given by the polytunnel should compensate for that.

Adding soil to the hotbed

The picture above shows the compacted top of the manure and straw mix on the left, and the 4 inch layer of soil that I added on the right. The soil is from the stuff that I removed at the beginning, but sieved. I put the larger lumps at the bottom, and the finer soil on top. Both can be seen in the picture. The sieved soil should give me a better medium for seed sowing.


The picture above shows the finished bed. The round disc is the top of a compost thermometer, which is measuring the temperature in the bottom half of the hotbed (18 inches below the surface). I have two plastic cold frames that were too flimsy for outdoor use. These will sit in the center of the hotbed, and will help to create a warm and protected environment for my early vegetables. The mild Winter (so far), and generous growing of salads late last year, has meant that I already have a surplus of early salad. So I'm going to try growing some tender plants, French beans, and sweet peppers, as well as some extra spinach and rocket (aragula).

Monitoring Hotbed Temperature

Compost Thermometer

When first built, less than a week ago, the temperature was 11oC, roughly 6 degrees above the ambient temperature at the time. The temperature has since climbed to 18oC. I now need to monitor the temperature change. Planting or sowing starts once the temperature steadies, or starts to decline. In order to get a head start, I have sown some of my first batch of plants in modules, in a propagator.

Seeds in electric propagator

Once the hotbed is ready for planting I'll post an update.

Deano Martin runs a smallholding and is a Trustee of the Permaculture Association (Britain). He writes a regular blog The Sustainable Smallholding and article and reviews for Permaculture.

Further Resources

Hot Beds: How to Grow Early Crops Using an Age-Old Technique by Jack First for just £8.99 from our Green Shopping site

Heating a greenhouse with compost & manure

How to make hot compost

Watch: 500 showers from a pile of compost

Colin Bennett |
Wed, 24/12/2014 - 22:29

On a compositing course, we measured peak temperatures over 60C in a standard home compost bin, done with layers of kitchen veg waste and cardboard/paper torn up. Just right 'greens' and 'browns', stacked up all at once.

Your system here reached 18C. What can you do if it risks getting too hot? Water pipes that loop outside the polytunnel could maybe help lose heat, or vents somehow..

At Tatton, their pineapple house wasn't powered by a manure system, but by fermenting oak leaves. They had water pipes under and I assumed four extra heat, but could also have cooled. Pineapples were sat in pots, rather than rooting into the hotbed.

Hoping to grow a pineapple top back into a full plant this year! Hope to try hotbeds, black-painted water tanks with beds on top, and a rocketstove mass heater system. May the best 1 win!