Without fail I get questions this time of year about keeping worms outside: Can I (How can I) keep my worm bin outside over winter? What about the garage/shed? etc.
I do not take my worm bins outside except to harvest, but here are some tips and tricks reported by others and my own observations about keeping red wigglers outside in cold weather.
First, the facts. You cannot vermicompost in freezing conditions. Red wigglers will die if the soil they are in freezes. I know they can become dormant and cocoons can survive, but in general most will die and your vermicomposting will cease when the pile freezes. In reality, when your soil temp drops down to 40F and below, your composting is so slow as to be stopped. Just like your fridge, things below 40F decompose very slowly.
In my 10 years of vermicomposting I have had worms survive outdoors only one year. It was in my outdoor composter (Earth Machine http://t1.gstatic.com/images?q=tbn:MI_dfU_Gk4cwsM:http://ecx.images-amazon.com/images/I/51HGE6GMJQL._SL500_AA280_.jpg ). It was 3-4 years ago. We had a relatively warm winter with no long cold spells and little snow cover. I went outside in early March and I took the lid off and discovered 3 red wigglers hanging out in the warm condensation on the lid. First and last time ever. I can only guess they were dormant and found a spot they could survive (not a deep freeze year and sun warmed the container just enough?). Typically, the Earth Machine is a compost popsicle. I wait until early April for the soil temp in the outdoor composters hit 50F, then I bring a bunch of red wigglers out to get them going. I am really bad about mixing the outdoor compost piles, so I sacrifice some red wigglers to do the work for me.
So if your bins are outside, you have 3 options: 1) bring your bin indoors, 2) start over with new worms in an indoor bin, 3) insulate and heat your bin to keep it from freezing.
I cannot use option 1 because of Bert's rules ("No bad smells and not bugs in the house"). Outdoor bins typically have insects, so that rules that out for me. If you (and your family) are OK with that possibility, then option 1 may be for you. These two options are pretty simple.
The more difficult proposition is option 3: keeping the bin outdoors and keeping it from freezing (and ideally between 60-70F).
You could with a big enough pile continue to add a hot manure to keep the soil warm enough. The key would be hot but not too hot (a pile rather than an impermeable enclosure like a bin or tub). I know my brother's horse manure windrow in Northeast PA works year round. They add to it frequently and it can be found steaming with active red wigglers year round. Reportedly, chicken or pig manure will also work.
Others have reported heating/seed starting mats under plastic wading pools in the shed with a styrofoam cover work. This would keep the soil temp relatively constant.
Another person proposed digging a pit below the frost line and insulating the sides and top with closed cell foam insulation and a trap door lid. Then piling hay bales over the door. I don’t know whether this was successful, but my frost line here is fairly deep and it wouldn’t be practical. Also, this would not make sense to me because one of the reasons I started vermicomposting was to NOT go outside to compost in the cold weather.
If you have experience with any of these, please leave a comment.
As for me, I will continue to compost indoors in my basement.