Used since Roman times with a coldframe on top hot beds were often referred to in older gardening books but now are never seen in modern works. Yet they are an effective way to grow many tender crops. Moreover a hot bed is very ecological as instead of burning fuel or electricity a hot bed produces it's own heat by fermentation.
The traditional hot bed was a vast pile of steaming fresh horse manure, tanner's wastes (mostly oak bark and animal hide by-products), straw and leaves. This pile was then topped with a substantial layer of potting compost and covered with a cold frame if not made up in a greenhouse. These piles were made and turned several times a year and additional new manure worked in to keep the heat up. This was done carefully and mostly round the edges so as not to disturb crops growing on top except when, as with pineapples, they grew them in pots which could be moved from an old bed to a new with ease.
Hot beds were very effective at providing plentiful bottom heat and also gave off moist carbon dioxide and ammonia enriched fumes. At first these would be too strong and harm plants but after the first day or two the levels dropped and then they benefited the crops immensely. With boilers to raise the air temperature as well Victorian gardeners managed to raise and crop almost every sort of exotic crops you can imagine. And of course continued to use them to grow conventional crops well out of season.
We can now use soil warming cables to achieve much the same results but why go to that expense when we can still make hot beds for free ? Mostly to avoid the labour! All the bringing in of materials and their constant replenishment and renewal was a terrific amount of effort. Yes, but I could see if you had one already it would certainly get crops off to a flying start even if you did not turn and bolster it all the way through the season. And for so many greenhouse/ coldframe crops it is those early weeks of growth that are most crucial.
For many years whenever I made a compost heap I couldn't fail to be impressed by the amount of heat generated during the weeks after they were made up or turned. An iron bar plunged inside would soon heat too hot to touch let alone hold on to. I even fantasised about running pipes through a huge compost heap to heat my home. But inspired by an old book I stuck a thick layer of compost on top of a new Spring heap and placed a coldframe on that. I sowed melon seed -the plants grew like magic and had some of the best fruits I've ever grown.
Now if you have access to fresh horse or any other kind of animal manure then it makes sense to use them. But for most of us these are not so readily available. However you can cut out the 'middleman' and substitute grass clippings instead -after all, all muck was once grass! I find that even the makings of an ordinary garden compost heap will make an excellent hot bed -but only if they are well mixed together and carefully made up in one go with loads of clippings and stinging nettles.
As with any composting process the greater the number of different ingredients the better but for a hot bed you must ensure it will heat up. So there must be plentiful nitrogenous wastes and moisture; the fresh green wastes supply that. But also in order for the heat to continue for any length of time you need some fairly tough materials that are slower to degrade and also to allow air to gain ingress to the middle of the mass and fumes to leave. (I use a mixture of mostly grass, stinging nettles, last years leaves, soft and semi woody prunings, straw, urine and inoculations from a prior compost heap.)
It is essential to stop heat escaping where it is doing little good, or emerging too quickly or too fiercely under the crop. Thus insulating the sides of the bed is important but ideally some holes must be left to allow air to enter. Planks are too cold unless bolstered with an insulating cladding, as are bricks. Straw bales are excellent if you can get them and in a year or two go into the beds themselves. The top of the fermenting materials must be covered with a thick, i.e. more than a foot, of soil and or potting compost, this is to stop the heat cooking the crop's roots. The topping should also contain plenty of lime to absorb ammonia coming up from the composting beneath. (I re-use a previous hot bed mixed with extra lime, calcified seaweed and wood ashes as this potting compost!)
Hot beds certainly work best on a bigger rather than a smaller scale but by increasing the insulation smaller beds become possible. I have used a dead deep freezer unit very successfully to raise crops of melons outdoors. Even watermelons! The 'hotframe' unit is stood on bricks and equipped with a ventable inner lid of clear plastic or glass. I fill it up with the same goodly mixture leaving just space enough for a few inches of topping. As the mix heats and sinks the topping is added to and the seeds sown. The lid is opened and the plants ventilated during the day and closed (not totally) at night. As the seedlings grow the mass conveniently sinks leaving more room for the final plants. Aeration holes are sunk downwards and later filled with more topping, the plants are kept well watered but not fed. By the time the bed has cooled down the weather has warmed up anyway.
I've found melons, sweet potatoes and cucumbers respond so well to warmth at their roots I nearly always give them a mini hot bed at the start of the season even though it soon peters out. (Those I don't give this extra advantage to rarely do well at all except in very favourable years.) Plastic dustbins, a stack of car tyres , even insulated plastic bags. They all work and give better crops than plants grown the same in cold compost or in the soil. In every case do remember the mass will sink so you must let your plants drop with it. i.e. Do not tie them up unless you can easily arrange their supporting strings to continually lengthen or you will find them hung suspended.
Growing plants in pots or bags and standing them on top a hot bed means they can be lifted off and the bed remade to reheat which is really worthwhile early on in the season. But they will do best of all if they are grown in compost on the topping layer of a heap so I grow both and have some in the bed, and more in bags on and around.
I also make in-ground hot beds in my polytunnel border. I dig down a foot or two and the same wide, put in as much compostable material as will fill it and stand a bit proud and then put the soil back on top. This works pretty well but the plants never thrive quite as well as others do standing a little higher up on raised hot beds in bags or in tyre stacks and I think that is because the air is always rather cool and draughty down at ground level. Certainly those growing in bags have the advantage of also being movable initially so they can be kept much higher up standing on supports at the start of the season and dropped down as the year warms up.
Of course I find you can use a hot bed for more than just one crop, initially I get a summer crop of melons, cucumbers and watermelons. Sweet and chilli peppers, aubergines and Tomatillos in pots are stood in pots on and then around the hot beds. Where there is any spare space basil, Shungi-ku and stir fry vegetables are planted out from cells. Once autumn comes the beds have cooled and sunk and are very suitable for winter saladings such as chervil, rocket, Lamb's and Miner's lettuces, and especially for Pak choi which does well almost anytime of year.
From the shortest day on I insert sets of early potatoes such as Rocket in batches a fortnight apart squeezing them in amongst the salads. The following spring as I unearth the new potatoes the remains of the salads go in the next hot bed. The old hot bed material is so well decomposed that with the additions mentioned earlier it becomes potting compost itself -and it is conveniently inside the polytunnel where it's prewarmed ready for use. I use the topmost layer for all other crops but not for the ones it has been growing though I am happy to use the bottom of an old hot bed as part of the topping on a new one.
I'm sure you'll find like me that all of the more tender crops do wonderfully well on top a hot bed big or small. They thrive instead of languishing like their colder neighbours growing with cooler roots. Okra only grows well for me with a hot bed under it and ginger hardly ever does unless mixed in amongst other crops atop a gentle hot bed. Of course for convenience with some long lived crops such as pineapples I use soil warming cables during the depths of winter but give me a couple of fermenting hot beds for almost everything else.