By Sam McCarthy, Do It With Worms
A few thoughts on winter composting with worms. I have been raising and selling red worms in the Santa Fe area for over 20 years and I have found that the winter survival of the worms is a non-issue if a few critical criteria are attended to:
1. Feeding and watering of the bin during the cold is a must. The internal warmth of the system depends on these inputs. An active pile is a warm pile. In my experience, those piles that lose their worms during the winter are almost exclusively in bins whose keepers withheld water fearing adding such would cause the bin to freeze. A dry bin will be less active and, thus, colder than a properly moistened one. Furthemore, worms will leave a dry bin as they require moisture to breathe. Additionally, mice will soon find, and nest in, dry bedding. Another mistake in terms of water management is to construct a bin with pallets or wire which allow for far too much airflow and drying of the bedding. Straw bales and solid wood walls minimize water loss while still “breathing”. The bin, absolutely, needs less water in the cold months, but maintenance of the moisture level is absolutely critical.
2. Bin volume is critical to retention of warmth. And by “bin volume” I am really referring to the volume of bedding, as the bedding is really doing the work, rather than the bin itself. I build wooden boxes that are approximately 3 feet square (volume approx. 200 gallons) and I have never had one freeze beyond a few inches from the surface. It should be noted that I experimented with a much smaller commercially-available plastic bin that, amazingly, survived a week or so of -15 F temps during the winter of 2011. The volume of that bin is on the order of 50 to 75 gallons and, due to the small volume, required much more attention as it dried out very rapidly as compared to a bin of larger volume. In my opinion, a bin of this type and size nearly guarantees failure for the beginning composter. Larger bins are easier to maintain. Also worth noting, high summer temperatures and solar exposure are serious challenges for the worms, and larger bins excel here, as well.
3. Regular additions of coarse bulking materials (bedding) is crucial. I find success in all seasons depends on an abundance of bedding. Red worms thrive in moistened straw and other materials like leaves and shredded paper. The more bedding the better. A very thick layer of bedding on top insulates, slows water-loss, and keeps flies and scavengers alike from finding the buried layer of food scraps. For all of these reasons, the bin should be kept full of bedding. For those who are experienced with building “conventional” bacteria-based compost piles, this method may seem odd at first, given that It seems strange to fill a bin with carbon, initially, with little to no nitrogen. However, once you see the worms thriving within the large volume of moistened bedding, and watch them quickly consume any additions of nitrogen-rich materials buried within the bedding, it will begin to make sense. The bedding provides both an energy-rich source of food -carbon- but, critically, also serves as the “house”. Nitrogen added over time suits the worms perfectly. As the bedding decomposes, the level of the pile falls, making room for additional layers of bedding and nitrogen. Finished castings -worm compost- builds from the bottom of the bin. In my experience, a 3’x3’x3′ bin will take about a year for an average household to fill. Bins made of bales can be any size, keeping in mind the fact that increasing the surface area relative to the overall volume, will increase water-loss. I find the cube shape to be ideal.
4. Additional considerations. During the cold months, a small bit of mixing or light turning of the bedding when adding food scraps and water will stimulate microbial activity and produce warmth without harming the worms. As well, the additional heat from a layer of manure can be helpful. Layers of nitrogen-rich materials may, temporarily, become too hot for the worms, but they successfully retreat to the cooler confines of the bin, returning to the soon-cooled layer.
Over the years at my booth at the Santa Fe Farmers Market, I have been amazed to meet many successful vermi-composters from climates with much colder winters than ours. I hope these recommendations will help keeping your worms happy all winter long!