Many gardeners recognise the benefits of composting. “Black gold!” they exclaim, and yet still buy in mulch in great volume for their veg beds or garden borders, or bag upon bag of potting compost. I have been in that boat myself. I have had compost bays for a good number of years, enthusiastically loading in veg peelings, mown grass, a bit of shredded paper and the odd bit of prunings. This generates a certain amount of finished compost, eventually, but not enough. For the last two or three years I have either bought in copious quantities of spent mushroom compost, or barrowed what felt like tonnes of rotted horse muck across boggy fields then carted it across town to my plot.

Well, no more. This year I have resolved to be self sufficient in compost. This means massively increasing the volume of compost I generate. This is all very well, but I have a fairly modest sized garden that generates only a certain amount of raw materials, usually accumulated slowly. Here are the lessons I have learned in my attempts to be self sufficient.

  1. A slowly accumulated heap stays cool.
  2. A cool heap generates low volumes of finished compost – ie less of the raw material is converted to finished compost.
  3. A cool heap takes 6-8 months to generate this low volume of finished compost.
  4. Finished compost generated this way is likely to include a lot of weed seeds (the heap has not got hot enough to kill the weed seeds)

The finished product here is still a good and worthy addition to any garden. BUT it’s not enough for me.

In the past, I have looked on with envy at those who casually refer to their heaps generating routinely high temperatures of 60 plus degrees Celsius, wondering how the hell they did it. My slowly accumulated heap never used to get anywhere close to that hot. Hot is good because a hot heap works a lot faster, and generates a higher volume of finished compost – ie more of the raw material is converted into finished compost. But how do you get a hot heap? There are a few critical factors:

  1. Roughly the right mix of ingredients
  2. The right volume of ingredients
  3. Maintaining aerobic conditions in the heap

Let’s take those one by one. For an efficient compost heap, the conditions must be right for efficient bacterial decomposition – it is this that generates the heat. In practice this means a good balance between nitrogen and carbon in the heap. It is possible to get quite geeky about this, with carefully balanced ratios in individual ingredients. I have found that it is more than sufficient to ensure that “green”, or high nitrogen and “brown”, or high carbon items are added in roughly a ratio of 1:2 green:brown. That’s one part green to two parts brown, one bucket of green for every two of brown. Loosely speaking, anything that is or was recently alive is “green”, anything that is now or has been for a while dead is “brown”. Confusingly, some “green” items are actually brown in colour!

Here are some examples from each category:

Green:

  • Freshly mown grass
  • Fresh manure
  • Chicken coop sweepings
  • Prunings or cleared plants, preferably shredded
  • Coffee grounds
  • Vegetable peelings

Brown:

  • fallen leaves
  • cardboard
  • newspaper
  • straw
  • sawdust or wood shavings
  • twigs, chipped bark/branches
  • other examples

To get the necessary high temperatures, it is essential to have enough green and brown materials handy to build a heap at least 1 cubic metre – 1 metre high, 1 metre deep, 1 metre wide. Bigger is even better. Green and brown materials should be mixed in roughly the 1:2 proportions described above. I have been successful with the following mix, built up in thin layers just two or three cm deep:

  • Fresh horse manure, collected from a local stables. Fresher the better.
  • Cardboard, collected from garden centre or supermarket recycle bins
  • Spent hops, collected from local brewery
  • More cardboard
  • Repeat until full, watering down every few layers

4 bags of manure, 4 bags of hops, and a substantial pile of cardboard is necessary to fill a single compost bay about 3’x3’x3′. Top tip, thick cardboard is difficult to tear into pieces, it’s hard work on the hands – it’s much easier when wet, so if it hasn’t rained recently, give it a good soaking with the hose. If I have other ingredients to hand, I add in as a bonus layer from time to time. Once the heap is full, I have covered with either cardboard or plastic sheet. This prevents too much additional moisture getting in if it rains, and also helps keep the heat in.

If one was not in a hurry, it would be perfectly OK to leave a heap built like this alone for a few months. Good compost would result, and in reasonable volume. To maximise the return, however, it is necessary to put in some physical effort….

After 4 or 5 days, a heap built in this way will have got quite hot, somewhere between 53 and 65 degrees Celsius. I have a compost thermometer, but a rough and ready way to test the temperature is to thrust a fist into the heap. If it is too hot to leave in for more than a few seconds, it is at optimal temperature. All that heat is generated by bacterial decomposition. It doesn’t matter much what the ambient temperature is. I have had this process work in the dog days of summer and in the chill of approaching winter. On day 5, I turn the heap. The goal of this process is to add oxygen into the heap, giving the bacteria a boost, and to turn the cooler parts on the outside of the heap into the hot-zone in the middle. I have 5 compost bays, so in practice this means I turn the new heap from a full bay into an empty one. If I just had one bay I would have to fork it all out and back in again. The act of forking it across like this adds the necessary air and also gives the ingredients a good old mix along the way.

how to make a hot compost heap

I aim to turn the heap every 2 or 3 days, by torchlight in the evening if necessary! If I keep up this pace, the heap turns into a dark, crumbly compost within 3 to 4 weeks. I have found it fascinating to observe the change from a stratified heap of different ingredients into something recognisable as finished compost. That’s worth repeating – 3 to 4 weeks! This is lightning fast compared to 6-8 months for a cold, slowly accumulated heap. A heap managed this way also loses very little in volume. To a close approximation, it is true to say you get out more or less what you put in. The compost geeks that I have referred to in my research say that you should leave this finished compost to “cure” or mature for a few weeks before using. In practice this means I let it sit in the finished bay, or transfer to bags or pots till I’m ready to use it.

how to make a hot compost heap

Following this method I have already been able to mulch all my no-dig veg beds. I have also mulched two new borders, and I expect to be able to give a nice thick mulch to the borders in the rest of the garden over the next month or two as the next batches become ready. I am ridiculously excited by the whole process.

how to make a hot compost heap

I’m going to do a separate post soon following the evolution of a recently made hot heap from build to finished product.

Every garden should have a compost heap, but to generate truly impressive volumes of weed-free compost, every garden should have a hot heap!

Some useful links:

I’ll be back soon with more composting fun. Happy to answer questions, I am eager to share the compost love.

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