There are a number of things that I do to help the worms survive the winter:
Temperature measurement method and results
See the main temperature control page for suggestions for making temperature measurements.
Winter - January 19, 2011
As with previous winters, I no longer use my worm bin once outdoor temperatures go below freezing. Instead I do what in Ontario we call green bin composting, basically composting through a citywide program. The following measurements were made on January 19, 2011, in the middle of winter since previous ones weren't made until March. The warmer worm bin temperature, 11.9C (53F) when compared to the March 2008 measurement of 7.1C (45F) despite the colder ground and outdoor temperatures is probably due to my having put more food in the bin prior to leaving it for the season.
These measurements are promising for anyone who would like to do worm composting in a winter greenhouse to help keep the greenhouse warm. Keep in mind, this worm bin is partially buried in ground that is at -0.6C (31F) and so there is a lot of heat loss to the ground. In a greenhouse, the surrounding temperature may be warmer.
Winter - March 17, 2008
Having learned from the winter of 2007 (see Winter 2007 subsection below), I decided not to open the bin in 2008 unless the outdoor temperature was above freezing (0C, 32F). 2007 was amazingly warm, we had snow only in February! 2008 was more like the old days with snow from December to March. So in December, I put lots of food and dry newpaper for insulation and left it. As of this date, March 17, I still haven't opened it, and as you can see by the temperature measurements below, the worms may very well be active!
Actually, for the first five minutes after having dug the hole, the ground temperature was 3.4C (38F) and the worm area temperature was 9C (48F). But it took me about 10 minutes for the mercury thermometer to reach outdoor temperature and so I had to wait to take the photo. By then, the temperatures had cooled to those in the above photo. Needless to say, I quickly filled in the hole afterwards.
Spring - April 7, 2008
Finally, on April 7th I opened the bin. It was filled with wet newspaper, understandable since I'd put a lot in the fall for insulation. And as you can see from the photo, the worms were plentiful and big and there was plenty of food left. I removed about 3 inches of wet newspaper and put on a fresh layer of dry. Ready for the summer.
Winter - 2007
The following measurements show that even though it was much colder outdoors, the temperature in the area where some of the worms were was fairly constant, even if a bit below freezing. It turns out that the material in the bin did freeze starting from the top down. By the time of the measurements, the remote temperature sensor was in an area that did freeze. There were still about two or three inches of depth below that that did not freeze.
I did have to stop putting banana peels in the bin as it was too cold for microbes to break them down. When I realized this, I switched from putting raw food in the bin to blending the food in a blender first to make it bite sized for the worms.
Also, at one point I realized that each time I opened the bin to put food in, the next time I looked at that area it would be frozen. The act of exposing that area to freezing temperatures caused it to loose what little heat it had. I then simply put a lot of blended food into the bin and left it closed until warmer weather.
When spring came, whichever worms and cocoons survived flourished immediately, filling the bin.
Bury the worm composting bin
The temperature under the ground is fairly constant day to day as the charts on this page show. It's warmer than the surface in winter. Burying the composting bin means that the bottom and sides of the bin will be at the temperature of the ground.
In my case I live in an apartment building and given people's squirmishness about compost smell (non-existant) and worms, I really have only one location at the end of our parking lot farthermost from the building. Unfortunately, when digging, about 70mm (3 inches) down, I ran into some very thick and healthy roots that I didn't want to cut. Heck, even if cutting them wouldn't normally bother you, the trees they're attached to are providing my shade! To make up for the lack of depth I piled up soil around the bin but it would still be best if it were buried deeper.
Note that before burying the bin I first wrapped plastic, I had a roll of polyethylene handy, around the bottom and most of the sides of the bin for waterproofing.
Worm compost bins need ventilation - but who wants cold winter air?! So for my air intake, which are the lower holes in the back of my bin, I use air that first circulates through buried tubes to be heated in winter by the surrounding ground as the following illustration shows.
The longer the tubes are the better. However, the tubes should be long enough only to heat the air to the desired temperature. If the tubes are too long then there could be too much friction for the air to be pulled in at all. Instead it will be stagnant and very little air will enter from them. Mine are probably shorter than I'd like but I don't own my property and can do only so much.
Also, the winter frost line around here (Ottawa, Ontario, Canada) is roughly around 1.2 metres (4 feet) so my bin and my tubes are not deep enough for this to really work in the winter. It should be below the frost line. You'd probably be better off insulating the bin very well instead and keeping it above ground. That way the heat generated by the composting will stay inside the bin more and less will leak to the frosty ground. But as my worms did survive the winter, it either worked or did no harm. In case it fails, it's a good idea to make a mini-bin as backup.
Insulate the bin to converve heat generated by composting
When the temperatures started to drop below 0C (32F) I insulated the bin to retain any heat generated by the composting. To do this I did two things.
First, I kept the bin filled to the top with loose bedding (shredded newspaper.) The newpaper closer down to the worms is kept moist to the worms liking but the newpaper higher up is kept dry for better insulation.
Second, I packed dry leaves all around the parts of the bin that aren't buried. I used leaves because they were free, plentiful, and environmentally friendly. The leaves have to be dry to act as good insulation so I covered them with plastic. The plastic is stapled to the bin at the top and tucked in under the leaves further from the bin. Though the cover also has leaves I made sure it can still open. The air instake remains in the back, with fly mesh and and plastic shield to keep out rain, and the fly mesh covered air out holes remain in the front.