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By December 31, 2015 0 Comments Read More →

How to Make Your Own Hotbed

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ventilated hotbed

Elizabeth McCorquodale explains an age old method of heating up the soil to give your plants a great start in life, instructions on how to make your own hotbed from scratch and how to get great results throughout the year.

See also:

Straw Bale Gardening

How to Protect Crops over Winter

HOTBEDS OF HISTORY

Manure-fuelled hotbeds have been around since the Egyptians (who used them to incubate eggs as well as to grow plants), and they 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, however, really began 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 six 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 1900s electricity took over, and greenhouse hotbeds and frames were heated by cables laid snake-like in the soil.

mixing straw and manure

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.

THE 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, and 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 can 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 minimising radiant heat loss, but the risk of waterlogging in the pit makes the above-ground model a better option in many cases. A sheltered spot against a south-facing 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 permits – is to site the 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.

Free Standing Hot Bed 1 2

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 a hotbed can be broken down into three parts: building the frame and the 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 make best use of the available 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, is polycarbonate roofing sheets fitted into a lightweight frame.

filling the 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 tending 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 60cm (2ft), but the optimum is 90cm (3ft), and the minimum length and width should be 60cm x 90cm (2ft x 3ft) in order to maintain an even temperature. Traditionally, the manure pile, whether free-standing or encased in a base of brick or timber, was 30–46cm (12–18in) 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 15cm (6in) 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 free-standing pile. To begin with, assemble an adequate quantity of old straw and fresh horse manure, the fresher the better, and using 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, 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 removed. Again, leave it in a pile to cook for three days, and on the sixth day, turn the heap again.

Perfectly cooked straw and manure.

Perfectly cooked straw and manure.

On the ninth day, repeat the process and then fork the fermenting mixture into its final position, adding it in 15cm (6in) 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 free-standing pile has been fashioned), leave it to heat up for three or four days. Insert a stick into the centre of the pile and leave it there to act as an improvised thermometer. 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 the 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 15cm (6in). 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.

hotbed in summer

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; if tacked to a piece of lath, the curtain can be rolled up easily each morning. During the day, regulate the temperature in the frame by opening and closing the lid, just as you would a cold frame.

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 revitalised several times over their long growing season.

planted hotbed

The result of the simple chemistry of straw and manure mixed and piled into a heap is a magical addition to the armoury of any 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.

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