History 

Manure-fuelled hotbeds have been around since the Egyptians (who used them to incubate eggs as well as to grow plants) and have never really gone out of fashion. Mad Roman emperor Tiberius grew cucumbers in wheeled hotbeds and the heat generating ability of manure was well understood by the ancient Greeks.  The Moors of Southern Europe raised seedlings in small boxes filled with donkey manure and by the Dark Ages the practice had spread to the monastery gardens of England.  The heyday of hotbeds really began, though, in the 18th century with the need to cosset the precious new seeds and cuttings that came flooding in from the New World, and culminated in the Victorian era, when whole sections of walled kitchen gardens were set aside for hotbeds tended by armies of gardeners dedicated to supplying out of season vegetables for the kitchen of the big house.

 

Traditionally hotbeds were heated by fermenting waste - usually horse manure - but in the 18th century, oak bark, which was left over from the process of tanning leather, was ground up and then piled into frames. The heat generated from this tanners bark lasted for a good 6 months. With the technological leap of the industrial revolution, steam and hot water pipes came into vogue in the grand gardens, then in the early 1900’s electricity took over and greenhouse hotbeds and frames were heated by cables laid snake-like in the soil. A 1930’s edition of Popular Science magazine proclaimed that ‘Gardener Works Miracles with Electric Hotbeds’, allowing him to ‘obtain plants that will make him the envy of all gardeners in the neighbourhood’, if, that is, he had a reliable supply of electricity.  

For anyone with a ready supply of horse manure, though, the dry concentrated heat of electrically heated hotbeds is a poor second to good old muck. Properly made, a manure powered hotbed will supply free, consistent warmth for two months and then provide a rich, moist bed for cucumbers, courgettes, squash or melons for the remainder of the summer.  For starting seedlings off in spring, for striking cuttings and for furnishing a warm bed for tender plants, a hotbed is the perfect, ecological solution.

 

Fundamentals of Design

The basic design of any hotbed is a shallow growing frame placed on top of a fermenting pile of straw and fresh manure.  As anybody who has ever turned a compost heap knows, fermenting waste generates heat. Manure is a storehouse of heat energy; but the trick isn’t just in the generation, but in the maintenance of heat; anybody can build a hotbed, but the knack is knowing how to keep it cooking. Manure on its own will burn out in a couple of weeks but if you mix in an equal amount of straw, by volume, the release of the stored energy will be eked out over two months rather than two weeks, providing a regular, gentle heat that will continue until the warmth of  the spring sun takes over.

 

Hotbeds can be built above or below ground and be sited either in a sheltered spot in the garden or inside your greenhouse or polytunnel. A sunken pit takes advantage of the extra insulation provided by the soil by minimizing radiant heat loss, but the risk of water-logging in the pit make the above ground model a better option in many cases. A sheltered spot against a south wall or in the lee of a fence or hedge will help to regulate the temperature of outdoor frames and provide protection from the worst of the wind and the weather. More satisfactory still - if space will allow - is to site your frame in your greenhouse where you will gain the extra advantage of any lost heat from your hotbed raising the ambient temperature in the house.

 

Hotbeds, like compost heaps, do not come in one particular shape or size, and, like compost heaps, the fundamentals must be understood for the whole thing to work.  The real essential to grasp is that the whole project hinges on how you treat your manure. 

 

Building the frame and the base

Building the hotbed can be broken down into 3 parts: building the frame and base, preparing the manure and finally, assembling the hotbed. 

 

To the cost-conscious home-farmer the overall size of the hotbed will probably be determined by the size of available timber, glazing materials and space. The front of the frame should be several inches lower than the back to shed the rain and to make best use of the sunlight, and the lid should be hinged at the back of the frame to allow easy access. The simplest lid is made from old wooden windows but a more practical, though less picturesque, solution are polycarbonate roofing sheets fit into a light-weight frame.

 

The manure and straw mixture can be contained within a timber or brick support or left free-standing. If left free-standing the sides should be gently sloped and covered with something to prevent heat loss, erosion in heavy rain or damage by being kicked while you are attending your plants.  Inverted grass turves are the ideal solution, but sacking or old feed bags would do the job at a pinch. The minimum depth for the finished pile should be 2 feet; the optimum is 3. The minimum length and width should be 2 feet by 3 feet in order to maintain an even temperature. Traditionally the manure pile, whether freestanding or encased in a base of brick or timber, was 12 to 18 inches larger all round than the frame which sat on top of it, in order to ensure an even temperature within the frame. However in a small greenhouse or garden this would be a luxury, so build it a scant 6 inches wider and keep an eye on the plants that are growing in the cooler regions around the edges. Just keep in mind that you will need to reach all of your plants so don’t make the frame too large.

 

The important bit: preparing the manure

This part of the process takes nine days and will result in a mixture that is ready to put into the base or make into a freestanding pile. To begin with assemble an adequate quantity of old straw and fresh horse manure, the fresher the better, and with a garden fork mix the two together well, breaking up any clumps and incorporating as much air into the mix as you can. Sprinkle the mix with water and fork it into a neat pile and leave it for three days to begin fermentation.

On the third day, turn the heap, again incorporating as much air as possible and if it is dry, sprinkle it with water. To achieve an even heat it is important that the straw and manure is evenly distributed and any clumps are removed. Again leave it in a pile to cook for three days and then, on the sixth day, turn the heap again.

​

On the ninth day, repeat the process and then fork the fermenting mixture into its final position, adding it in 6 inch layers and settling it firmly into place. As air is a key ingredient in the fermentation process the pile must be firm but shouldn’t be overly compacted. Once the base has been filled (or the freestanding pile has been fashioned) leave it to heat up for 3 or 4 days. Insert a stick into the centre of the pile and leave it there to act as a thermostat. To begin with the temperature will rise sharply, but it will then fall and level out to provide a gentle consistent heat. Don’t be tempted to finish your hotbed before this heating and cooling process is complete as the very high temperature will damage the chemistry of the soil if it is added too early. Check the temperature by extracting the stick and holding it; your bed is ready for use when the stick can be held comfortably in your hand.

 

Now is the time to lift the frame into place and fill it with soil. Keep an eye on the hotbed for the first few days to see if steam is still escaping from the pile. If it is leave the lid open to allow it to escape and wait until all steam has dissipated before planting. For the first year, the soil you use can be any good peat-free cutting or seedling mix laid on top of the fresh manure to a depth of 6 inches. After that the soil can be made up of the nutritious remains of the previous year’s manure, well mixed with the same quantity of garden loam and riddled to a fine tilth.

 

Put your plants to bed by closing the lid and laying an old curtain or sacking across the glazing to stop all the precious heat from radiating out each night; tacked to a piece of lathe, the curtain can be easily rolled up each morning. During the day, regulate the temperature in the frame by opening and closing the lid, just as you would a coldframe.

 

Choosing your plants

Long season vegetables like pumpkins, squash and melons will appreciate the extra growing time if they are planted early and allowed to establish a good root system before being transplanted out into the garden. Warm climate vegetables like sweet potatoes will repay the extra warmth and early start with a healthy, heavy crop. Strawberries, too, are good hotbed crops as they grow quick and sweet in the heat.

 

Hotbeds are terrific for striking cuttings, as the bottom heat is exactly what they need to quickly grow roots and be ready for planting out in a seedbed in late spring to grow on over the summer. Gooseberry, raspberry and other bush fruit and herb cuttings will appreciate the addition of sharp sand mixed into the soil to increase drainage.

 

Many flower and vegetable seeds fancy a bit of warmth to get them going and the gentle heat of a hotbed is ideal. The seeds of all the cucurbits will appreciate a warm bed, as will tomatoes, peppers and impatiens. Beware though as some cold weather plants such as lettuce, carrots, delphiniums and  geraniums will demonstrate very poor germination if planted in soil which is too warm..

 

Summer built outdoor hotbeds can be used to extend the growing season for any quick growing plants that are day-neutral. Baby beets, pak choi, fennel, kohlrabi, dumpy carrots, rocket, turnips, lamb’s lettuce and land cress can all be started in late August, September or even October and will keep on cropping in the heat and protection of a hotbed. Cool weather crops may bolt or grow woody if they get too hot, so be ready to harvest crops like radishes and spinach while they are young.

 

Within the extra protection of a polytunnel or large greenhouse, bumper crops of sweet potatoes and melons can be grown in pit hotbeds glazed with polythene sheeting stretched across a frame, rather than with glass or rigid plastic lids. 

 

Your hotbed can also be used to grow plants in pots. Rather than planting directly into the soil, it is possible to sink planted pots in up to their rims giving you the advantage of being able to lift plants without disturbing their roots. The exotic, highly esteemed pineapple was grown in this way in Victorian hothouses so that they could be lifted and the manure or tanners bark replaced and revitalized several times over their long growing season.

 

The result of the simple chemistry of straw and manure, mixed and piled into a heap, is a magical addition to the armoury of the home farmer.  It gives us some small control over the weather at seed planting time and extends the range of vegetables, fruit and even flowers that we are able to grow.  It is easy to see why hotbeds have stood the test of time.