Everyone who has ever had a decent compost heap cooking away in the winter frost knows how hot fermenting vegetable matter - with or without added animal manure - can get. Many gardeners plant cucumbers or other early spring vegetables into the top of cooling compost heaps to take advantage of both the heat and the fertility of the heap.

What they are doing is using a basic hotbed. All vegetable matter ferments naturally when piled up, but heaped manure works even better, and if a heap is large enough, and carefully enough prepared, then the heat can last for months.

Hotbeds have been in use from antiquity and were, and continue to be, used almost exclusively for growing (or forcing) early spring vegetable and melon crops during the cold weather of mid-to-late winter when normally such crops would not germinate. By forcing crops in hotbeds they could then be ready months earlier than otherwise - a real bonus in Europe when there was a dearth of green vegetables from February to May.

According to Aristotle, the ancient Egyptians used dung heaps to hasten the hatching of eggs, while the Roman emperor Tiberius so loved his cucumbers that his gardeners grew them for him year round on wheeled hotbeds (wheeled so they could be moved into shelter if the weather grew too inclement).

But it was the Arabs who made the first true hotbeds for early forcing of vegetables. The Dark Age Arabs who lived on the Iberian Peninsula (Spain and Portugal), or Moors as they were known, used the dung of donkeys, horses and occasionally pigeons, to construct their hotbeds. Their raised hotbeds were fairly small, about two feet in height and about three square, and were used primarily to raise seedlings. Once seeds were planted in December, the Moors covered the top of the hotbed with large vegetable leaves, such as cabbage or cauliflower leaves, to retain heat and moisture and watering occasionally as needed. Once the seedlings were strong, then they would be transplanted out into prepared garden beds, usually about April.

Evidence of hotbed use can be found in Europe from the late Dark Ages, but they appear to have been relatively common from the thirteenth century. It is impossible to know if hotbeds had been in use for centuries, but the first recorded use of them occurs about this time, or if the northern Europeans had only recently imported the technique (possibly from the Crusades which, while a military and social disaster, nonetheless resulted in the Europeans adopting many Arabic techniques, ranging from hotbed to Arabic numerals). But, from the thirteenth century, soon hotbeds were steaming ahead across the gardens of Europe.

By the end of the sixteenth century most gardening treatises mentioned hot beds and, over the following two centuries, they were put to good use growing many of the exotic fruits, vegetables and plants brought home to England and Europe from their increasing exploration of the new worlds. Universities across Europe developed botanical gardens, and were often at the forefront of growing new and exotic species using hotbeds.

It wasn't just the universities using hotbeds, of course. Many kitchen gardens used them, particularly within the larger estates, but anyone who had access to a steaming compost heap had access to a hotbed.

All they needed do was to add a foot or more of soil, ensure that the developing seedlings had some protection from the frosts and other inclement weathers (matting, glass domes, clay pots, blankets, straw etc.), and they, too, could grow early vegetables. Hotbeds continued in use for centuries, right through to the nineteenth century, and although they have gone somewhat out of fashion now (particularly since the prevalence of greenhouses or hothouses, they are still a cheap and effective means of growing early seedlings.

To build a hotbed all you need is a well dug heap of composting manure, perhaps mushroom compost, and build it up to at least a metre (about a yard) square by a metre deep. Add to this at least 50 centimeters of good soil, and also a frame for glass or some such to protect the seedling from frost and cold, and you're set to go.

You do not need to use hotbeds exclusively for vegetables or melons. Anything can grow in them, flowers included, and the top of the compost heap is a wonderful place to grow nasturtiums.

See also some eighteenth-century directions for constructing a hotbed.