There are a few primary elements necessary for organic materials to break down and become compost, the "magic ingredient" in so many successful gardens. Basically, you must provide the minimal conditions for the microbes which fuel the process to survive. They need air circulation, moisture, warmth, and a ready supply of organic materials, comprised of a balance of nitrogen-rich greens and carbon-rich browns. In the winter, the most difficult element to provide is warmth. Let's take a moment and consider some possibilities for shielding your compost from the frigid temperatures you may encounter, depending on your growing zone.
Two common schools of composting are often called "cold" composting, and "hot" composting. Cold composting requires a laissez-faire approach. It will work with any basic compost bin or pile, and it generally involves only adding your materials as you gather them. All organic material will, eventually, break down into compost, with very little interference from humans. If you are very patient, have limited time, or just no inclination to play around in rotting food-stuffs, this is the method for you. Cold composters are not much concerned with the effects of winter on their compost, as they are not trying to maintain high temperatures in the middle of their bins in the first place.
Hot composting is the more labor-intensive method, as it requires more attention to getting the balance of nitrogen to carbon ingredients right, with a ratio of 1:20 to 1:40 being the ideal, and usually involves some method of mixing the ingredients periodically. Compost tumblers would fall into this category, with their emphasis on turning the drum to re-distribute the ingredients every time you add more, as would any bin or method that required you to turn or mix the compost frequently. The advantage is that the compost progresses from "rough" to "finished" much more rapidly. It is the hot composters that dread the coming of sub-zero temperatures, as it basically brings the active process of composting to a halt. The materials will continue to deteriorate (as anyone who has left a carved pumpkin out in freezing temperatures can attest), but the bacterial process is seriously slowed by the cold temperatures.
As winter approaches, I would encourage either type of composting gardener to begin hoarding organic material. Leaves are available in great supply in the autumn, and you may find that your neighbors have very kindly collected and bagged them up for convenient removal. This presents you with two options: you can ask if you can have their bags of leaves, and take this opportunity to education them about the wonders of composting, or you can go into stealth mode, and snatch bags from the curb whenever the opportunity presents itself. I don't think there is much inherent risk in that, as I can't imagine anyone chasing you down to recover their bagged leaves.
Rather than adding all of your collected leaves at once, I advise storing them near your compost bin, so you can periodically add layers to your bin. During the summer months, I use a large plastic trash can with holes drilled 6 inches up the sides to grow potatoes. I repurpose that can during the winter months by filling it will mulched leaves and setting it beside my bin. I set a large brick on the lid to discourage it from blowing around my yard and redistributing the leaves over my lawn. My parents are blessed with a row of beautiful maple trees along their house, and they provide me with 8-10 large bags full of mulched leaves each year, as well. I store those in our shed until needed. After your first freeze, add a good 10-12 inch thick layer of leaves on top of your compost, to hold in the heat. It is easy to dig a little hole in the center and empty a countertop compost bucket into the leaves and recover them.
This is also a good time to reclaim straw that you may have spread elsewhere in your garden as mulch, perhaps in your strawberry patch or between your tomatoes. Of course, if you leave it in place, it will eventually break down and enrich the soil, but the more likely result will be that it will freeze and still be there in the spring, when you want to replant. Additionally, leaving leaves and decaying straw in your berry patch may encourage fungus to attack your plants. If you rake it up and add it to your compost, it will have a double effect: it will add to the carbon end of the balance, and it will provide lots of air pockets in the compost. I intentionally leave a few larger sticks and stems spread throughout my compost in the colder months, because I know that realistically, I will not head out there with a pitchfork and turn the stuff in subzero weather. Providing some air pockets throughout will help your beneficial microbes survive. Adding loosely wadded newspaper or cardboard paper towel tubes will have the same effect, creating air spaces within your compost.
You may also wish to add a second kitchen scrap collection bucket to your counter, or store them in a covered bucket under the sink. Again, I know myself, and my trips to the compost bin become less and less frequent as the temperatures drop. If I could, I would curl up and hibernate through the winter months. Reducing your trips to the compost bin can actually be a good thing, as you lose a little heat every time you open the lid. Despite your summertime commitment to regularly turning your compost, you want to leave it as undisturbed as possible during the winter, to avoid losing what heat you have managed to retain!
The true key to winter composting, however, lies in one word: insulation. If you are a dedicated composter, you may already own a compost thermometer, and have experienced the wonder of plunging the probe into the center of your pile to find the interior happily cooking away. As you most likely know, keeping it at a warm interior temperature speeds up the process, yielding compost more quickly. In the winter, however, it becomes much more difficult to maintain that inner temperature. If your pile or bin freezes solid throughout, your microbes are either dead, or hibernating until warmer temperatures return. Here are some ideas I've come across for shielding your bin from the cold.
First, consider your location. Ideally, your compost will be located in an area that is easily accessible from the house, so you don't have to shovel a long path to reach it. Mine is just outside my back door, around the corner. Second, if possible, put it in a shielded location, where the wind is not too intense. My location isn't strong on this point, as it is on the northeast corner of my house, but I didn't have much alternative. Thirdly, and probably most importantly, locate your bin where it will receive direct sunlight. Your compost is going to need all the solar help it can get! Covering it with a plastic tarp, preferably a dark colored one, will retain moisture and heat in your bin or pile.
One of the simplest methods of insulating your compost bins from the cold wind is to stack straw bales around the sides of your bin. Straw bales are not difficult to find in the autumn, and are often sold for $3-5 a bale in my area. Straw is a great insulator, but allows air and moisture to circulate. I even read of someone abandoning their tumbler during the winter months, and instead building a square enclosure of straw to house their composting ingredients. If you would like to read about an interesting use for those bales of straw in the spring, visit our Strawbale Gardening forum here on Dave's Garden. If you are already a strawbale gardener, put those bales to work in the winter months, when they would otherwise sit idle!
Alternately, if you have an abundance of leaves and a mulching lawn mower, you can collect the mulched leaves in bags, and strap or tie them around the outside of your bin. You can use the brown paper lawn waste bags, which are themselves compostable, if you have dry winters. If you tend to get lots of snow or rain in the winter months, you may have a mess on your hands if the paper bags dissolve. Instead, consider putting the leaves in black plastic trash bags and packing them around the bin, securing them with ties or weights to prevent them from blowing around in a stiff wind. The black plastic has the added bonus of collecting solar energy. Just be sure to recycle or reuse them in the spring, as you use up their contents for mulch or compost.
If you have a loose pile for composting, you may need to concentrate your pile into a more compact form to help retain the heat. This would be another case in which a frame of straw bales might be advisable. Another option would be to cover your pile with a tarp or piece of heavy plastic. If you can find a dark-colored tarp, to absorb any solar energy on those rare sunny winter days, all the better! Just don't seal the pile off too tightly, as your microbes need air to live.
If you have bins made of wood, pallets, or chicken wire stretched around corner posts, you have more options. Line the inside walls of your empty bin with cardboard, then add a second wall of cardboard inside the first. Leave a space between them, and fill it with insulating material, such as straw, mulched leaves, or even crumpled plastic shopping bags. You may then fill the space in the center with your organic materials. The cardboard is compostable the following spring, counting toward your carbon ratio, assuming it is not laminated. This does reduce the size of your usable space, but if it provides the necessary conditions for your compost to "cook," it is a sacrifice worth making. Better to have a smaller bin that is actively breaking down your waste than a large, sprawling one that freezes solid and has to start over in the spring!
If you live in a colder growing zone, Mother Nature herself may provide you with a great insulator: snow! Survival classes will teach you to dig a cave down into the snow if trapped in a blizzard, allowing your body to retain its heat. Snow can have the same effect on your compost bin, if you pile it thickly around the sides and even over the top. Wouldn't it be fun to dig out the lid of your compost bin, lift the hatch, and see steam rising from your compost when you dig past the top insulating layer? If you have a two-bin method, one for nearly finished compost, and one for adding new materials, this would be an ideal method for the almost-finished bin that doesn't have to be accessible. It also provides needed moisture when warmer temperatures return and the snow melts around your bin.
If you are partial to digging holes, as some gardeners are, you can even use the ground itself to insulate your compost. If you have access to a large plastic garbage can, you can partially bury it 6-12" into the ground. Drill large air holes 6-12" inches from the top, and stack straw bales around the exposed portion of the sides, being careful not to block the air holes. You can also fill any spaces between the bales and the can with soil, leaves, straw, or other insulating material. Keep the top on, to discourage roving critters from feasting on your kitchen scraps, and you have a winter compost bin!
Regardless of the preparations you make, barring the ability to provide some sort of heat source, your compost bin will progress more slowly in the winter. If you have an extended period of extreme cold, your materials may freeze through in spite of the efforts you made to prevent it. If that happens, don't despair. In the spring, stir up your compost, add moisture if needed, and add a new batch of organic materials, along with a couple of shovelfuls of soil from your garden. The combination of air, moisture, food, and organisms will get to work and restart the process.
For more information about composting, visit the Soil and Composting forum here on Dave's Garden. The "sticky" at the top of the forum is a great place to begin learning about the basics of composting!
Also, browse through these previous articles written on the subject.
Back to Basics Composting, by Kathy LaLiberte
Composting, On the Not-So-Grand Scale, by Sally G. Miller
Countertop Compostables Containers, by Sally G. Miller
Household Composting, by Tamara Galbraith
Low Cost Compost for All, by Caleb Garvin
There are many more articles on composting available here on Dave's Garden. To find them, click on the tag "soil and compost" in the Helpful Links box at the bottom of this page. Alternately, click on the Guides and Information tab at the top of your homepage, scroll down to Articles (the seventh category down), and enter "compost" in the search box. There is a wealth of information available on Dave's Garden, if you only know how to find it!
Special thanks go out to Daves' Garden member Bev Walker, for her picture of her compost tumbler and bin. The first image, at the top of the article, is from Flickr Commons, and is attributed to net_efekt. All other images are from Flickr Commons; the photographer may be found by hovering your cursor over the picture.