Far from being the ultimate new central heating fuel, what I’m suggesting is something last used by Victorian head gardeners: a hotbed. You can use it as an eco-friendly heated propagator for raising seedlings or as underfloor heating for “forcing” early veg and best of all it’s free.
First find your manure. Horse is best. What you want is the freshest you can get. It must not be well rotted.
Now you need a cold frame and a sheltered sunny spot with welldrained soil. Dig a hole 6in deep and about 1ft larger all round than the floor area of your cold frame. Spread a little gritty sand in the base for drainage then sort the straw from the pure manure and pile it into the hole so it makes a rectangular heap about 2ft high, with slightly sloping sides.
Press it well down all over with a spade, like a big mud pie, so the whole thing stays firm.
Pile the strawy stuff over the outside for insulation, give it a final firming and sit your cold frame squarely on top. Then spread a 6in layer of good topsoil or potting compost inside the frame and close the lid.
If you happen to have a greenhouse or a walk-in polytunnel, make a hotbed inside that on top of the soil border but there’s no need to use a cold frame since the structure itself acts as a cover.
Leave your bed of manure to sit and stew for two weeks. It should start to heat up. You’ll know by the steam. Now you need a soil thermometer. Stick it in to the hotbed in several places so you have a good idea of average temperatures inside and check it every few days.
To start with the heat builds up fast then it slowly drops and when it gets down to 75F (24C), a week or so later, you can start sowing seeds in the compost.
Victorian gardeners used their hotbeds to raise young vegetable plants and sow salads, leeks and early brassicas from late February to give them an early start.
They also sowed half-hardy veg such as French beans, courgettes and bedding plants in March, though today I’d play safe and raise those on warm…