The traditional hot bed technique is not so widely used in these days of heaters. But it’s a marvellous way to turn a cold frame into a heated greenhouse in spring. And if you’ve got access to the raw materials, it won’t cost you a penny.
To make the bed, thoroughly mix together equal parts of fresh horse manure – the fresher, the better – and deciduous leaves, and turn the heap three or four times during the course of a fortnight. The leaves will help to moderate the heat and release it over a longer period. Dig a hole 3ft deep and about 12in longer and wider than the frame. At the end of the two weeks, fill the hole with the fermenting mixture, firming it in layers, and position the frame on top.
Level the surface, cover it with 6-8in of fine soil and close the frame. Allow the heat to build up for a day or so before sowing or planting. Don’t sow or plant directly into the soil because the young roots could get damaged by the powerful mix. Instead sow or plant as usual in pots or seed trays, then bury the containers up to their rims in the warm soil. Open the frame whenever the air temperature inside tops 21°C (70°F).
A hot bed made in February or March -when vegetable sowing and planting gets under way – will release its heat for up to eight weeks. Afterwards, you can plant ridge cucumbers, marrows or melons directly into the soil – they will enjoy the rich root run.