When spent plants, weeds and kitchen wastes end up together in a compost pile, they will eventually decompose into compost. Cold compost is left alone to do its thing, which requires no labour but does not give first-rate results. Hot compost is a managed process that produces crumbly black gold better than anything you can buy in bags, and making it can be great fun! Late summer is an ideal time to develop your hot composting skills, because you probably have plenty of material on hand, and warm temperatures will help the process along by limiting temperature loss from overnight cooling.
Hot composting produces greater volume than cold composting, hot compost contains far fewer weed seeds, and it is much richer in substances that promote plant growth. Besides being good for your garden, hot composting is good for you, too. Making a batch takes about three weeks, and every few days you will get a full body workout forking and turning your smouldering compost, which is way more fun than going to the gym.
Hot Composting Basics
When putting together a no-frills compost pile, most gardeners try to make layers of bulky materials (leaves, old mulches, dead plants) and high-energy green stuff like kitchen waste, or perhaps chicken manure or processed pellets. With hot composting, you mix the materials instead of layering them, and you try to work with smaller pieces, too. I have the most success combining half-rotted weeds and plants that have been piling up all summer with the contents of my stationary composter, that dungeon of kitchen waste, and then tweaking the mix with fresh green grass clippings or shredded comfrey leaves. You want a rich mix, so this is a great time to clear the freezer of things you will never eat, which often make great additions to hot compost.
Don’t worry about getting a complete mix the day you build your pile, because hot compost is forked and turned again and again. Go for building a sizeable heap instead, because large heaps heat up better than small ones. A hot composting project that starts out at least 3 feet (1 metre) tall and wide will usually give good results. A metal stake installed in the middle of a new hot composting project can serve as a rustic heat gauge you can feel with your hand, plus you can wiggle it to aerate the pile.
How and When to Turn a Hot Compost Pile
Once a hot heap is put together it is best to leave it alone for four days. This allows time for moisture to equalise while colonies of beneficial bacteria are becoming established. It can be hard to wait, but patience has its rewards. After four days I usually detect significant pockets of heat as I chop through the pile with a hoe, which is always exciting. Then I fork the material back into a pile, and cover it with a tarp (or the top section of my stationary composter) to keep out rain.
The protected heap is mixed and turned every two to three days for two weeks, with noticeable heat or even steam evident through the first three turnings. You can use a thermometer if you like, which is the most accurate way to see if your compost pile has hit the magic numbers, which range between 130-140°F (55-63°C). Or, simply use your hand. If you can’t hold your fist in the mix for more than a few seconds, the heap has hit the ideal temperature range for its transition to top-quality compost. White, ash-like remains on bits of plant material in the heap are further evidence that your hot composting project is going well.
Expect the temperature to drop in your hot compost after a week or so, but continue to aerate the mix every few days for another week. Active decomposition is nearly complete by this point, but the compost will improve even more when given time to cure. I cure mine in buckets and empty flowerpots kept in a spot sheltered from rain, or you could simply cover the heap for a few weeks. Allowing hot compost to mellow for a month after it is done allows time for the microbes in the mix to stabilise, and the texture of the mixture relaxes, too. It becomes dark and crumbly, agreeable to sniff and a pleasure to use. Better make as much as you can, because it runs out fast.