How to Build a Hot Compost Heap

Finished hot box with lid

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Stick canes upright in gaps

An easy guide

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A compost heap added to as you go along will product compost eventually, but a “hot” heap, built all in one go, will do it faster, and with good luck and a following wind, will heat up sufficiently to kill weed seeds and roots and fungal spores.   Lawrence D. Hills, founder of HDRA (now Garden Organic), recommended several types of compost heap, but what they all had in common was the provision of trenches or plastic guttering underneath to let in air at the base of the heap, and holes through the middle to act as little chimneys, drawing the air up.  This, combined with solid sides to stop the air from escaping there, allows enough oxygen to flow through to enable the bacteria that make the compost to heat it to the 60-70°c necessary to, as he says “cook the weed seeds like grains of rice, simmer convolvulus roots like narrow potatoes and kill disease spores like a steam steriliser”.  

You need a fairly large bin to do this: a good way of making one is from old pallets that you should be able to get from warehouses round town by smiling sweetly and explaining what you want them for. I’ve been lucky enough to acquire some of the hinged wooden squares that are placed on pallets to stop heavy items moving round on them in transit. They just stack on top of one another and are absolutely ideal but very hard to come by. At any rate, the bin needs to be a metre or more square in order to generate enough heat.

The added ingredients are a length or two of plastic guttering you should be able to get off a skip (knock on the owner’s door and ask permission first) some long canes or similar (plastic electrical ducting is good) a piece of carpet for insulation, and something waterproof to make a lid. Cut the lengths of guttering into four or eight pieces (depending on whether you want one hole in the middle or several spaced round the heap) and lay them to form a cross, large enough for the ends to stick out of the bottom of the bin.

Save your compost materials so that you can put them in all at once. THIS IS VERY IMPORTANT. As well as your greens and browns (see below) save your pee for a week or so beforehand – this is your compost activator. Use it to moisten any dry material as you add it. If you can get hold of some fresh manure, it will also make a very good activator, sandwiched in layers with the other materials.   Then you can start filling the bin.

Stick the canes upright in the ground in the gaps between pieces of guttering. Put a twiggy layer about 4-6”  (10-15 cm) in the bottom of the bin, and then add alternate layers of green material (fresh weeds, the remains of vegetable plants, grass clippings, kitchen waste – not cooked, and no meat or dairy which attract rats) and brown (shredded or scrunched paper or cardboard, straw, shredded prunings, dried-out weeds). I also add layers of seaweed, picked up from the beach after onshore gales (it’s illegal to take it out of the sea) and left in the rain for a day or rinsed with the hose. It’s often recommended to add layers of soil, but there will easily be enough soil on the roots of the plants, and too much soil will stop the heap from heating.

When the bin is full to the top, cover with carpet, cutting holes in it for the canes, then take the canes out.   Finally, put a lid of some description on to keep out the rain. Don’t lay it directly on top of the bin – you want to allow the wind to blow through to help your “chimneys” to draw. Leave it for a few days, and you should notice that it has started to sink down and that there is warm air coming out of the holes in the carpet. Your heap is cooking.