Review by: Margaret Rose Johnston
Introduction
My husband Stuart (an engineer) and I live in Cumbria, and I have always loved gardening, but our space is limited. We have a small 5.5 x 2.3-metre garden that is completely paved, with all our plants in pots and containers. This made conventional composting a challenge, until we found the HOTBIN Mini Composter.
Composting was new to me. Especially hot composting. When we received the HOTBIN this spring, it was a real revelation. What impressed me the most was how little space it took up, how neat it looked, and most importantly: it didn't smell. Turning our household and garden waste into perfect compost in just a matter of weeks was something I had never experienced before. The HOTBIN processes all our household and garden waste in weeks and has been producing perfect compost since March. I can only highly recommend it.
I found monitoring the temperature particularly interesting, and I enjoyed seeing how the added waste directly influenced the temperature of the heap, which helped me better understand the composting process.
But because we wanted to go a step further, Stuart built a special temperature sensor using a Raspberry Pi Pico microcontroller. This device, which we placed directly on the boards against a north-facing fence in partial shade, sends temperature readings to our computer every 30 minutes. It is fascinating to see live data that helps me learn which materials compost the best.
What is the best way to make compost in a small garden?
Wood chips and shredded paper: initially I used the wood chips that came with the HOTBIN Mini, but I soon started making my own bulking agent. I discovered that a 2:1 mix of paper waste (mainly shredded paper and cardboard) works best, adjusting the amount of paper based on the moisture content of the waste.
Kitchen waste: because most of our kitchen waste goes to the worm farm, we don't produce much kitchen waste for the HOTBIN. What does go into the HOTBIN includes things that are not suitable for worms, such as onion skins, orange peels, cat food, and coffee grounds. Cutting everything into small pieces of about 2 cm speeds up the process.
Garden waste: this makes up the bulk of what goes into the HOTBIN. It mainly consists of deadheaded flowers and prunings, at least two bowls per week, sometimes more. I keep excess waste in an old compost bag and add it when needed, cutting all waste into 5 cm pieces.
My top 7 tips - Lessons learned for composting in a small garden
First use of the HOTBIN: starting in March of this year, when the air temperature was still very cold, it took a few weeks to get the HOTBIN up to the right temperature. I used the kick-start bottle a few times, but this didn't seem to have much effect. Adding a large bag of fresh grass clippings (mixing well with the rest of the waste) made the difference, causing the temperature to rise quickly and stay high.
Composting grass: since I discovered that adding fresh grass clippings is the most reliable way to increase and maintain the temperature quickly, I try to add half a bin bag of grass every few weeks. Note that it is important to mix the grass well with the rest of the added waste so that it doesn't mat. Since I don't have a lawn myself, I collect grass clippings from the local council.
Waste digestion: waste that I was able to compost quickly and easily included: fleshy plants, such as red valerian and cineraria, lilies (Arum lilies, Asiatic and Oriental lilies, bulbs, stems, and flowers), and partially fallen rotten apples. Tough, strap-like cordyline leaves take much longer.
Feeding the HOTBIN: so far, I have found that adding 1 to 2 washing-up bowls of waste every 3 days works well. First, I add shredded paper and mix it with the partially composted material at the top. Then I add the kitchen and garden waste. I mix all the new material and pull some of the deeper, hot material up to mix with the new waste, hoping this will plant bacteria into the new waste and thus accelerate the composting process. When the temperature has peaked (well above 40 °C) and is just starting to drop again, I add new waste. It can take about 12 to 24 hours to return to the temperature it was when I opened the HOTBIN to add the material.
Temperature: the temperature seems to be quite self-regulating. It usually stays between 40 °C and 60 °C, although it climbed almost to 70 °C a few times at the height of summer. Usually, the temperature rises quickly a few hours after new material is added. Then it stabilizes and begins to fall.
Compost: the first batch of compost was surprisingly successful. It was warm, moist (not wet), odorless (only an earthy smell), and the material was well composted and crumbly. The material that had been put in was unrecognizable and the compost was full of worms. There were only a few dry twigs and stems that I put back, see photos. It actually looked better in reality than in the photos.
Removing the compost: I find this quite difficult to do without emptying the entire contents. The last time, I just emptied everything out. I used the well-composted material as mulch in the garden, and I put the rest back in the HOTBIN to continue composting. I mixed the old material thoroughly with new material and added more shredded grass and paper. This seemed to work very well.
I plan to continue using the HOTBIN Mini and the temperature sensor to improve my knowledge of the composting process. It has been a very interesting adventure, and I can only highly recommend the HOTBIN Mini to anyone with limited garden space. Meanwhile, Stuart continues to tinker with his device and is working on a lower power consumption variant that will help optimize battery life.
