Winter Vermicomposting: How to Keep a Worm Bin Warm When It’s Cold

With the approach of winter here in the Northern Hemisphere, the single most common question I get is “I live in a cold state. How do I keep my worms alive during the winter?”

My first answer is “Move them indoors or move to San Diego and we don’t have to worry about this!”

For some reason, people find this answer unacceptable, so I have to move to Plan B.

In this article, I’m going to discuss how you can keep a worm bin or worm composting setup warm over the winter using nature when possible, but get some manmade help when necessary.

How to Keep Worms Warm In the Winter

As you may have already learned in the Ultimate Guide to Vermicomposting (you did read that didn’t you?), we want to keep your vermicompost between 55°-95°F (13°-32°C).

Before we get into how you can keep your composting worms warm in the cold winter months, it’s important to remember that it’s the temperature inside your vermicompost, not the temperature outside your bin that matters.

If vermicomposting in cold weather, you should also manage your expectations and understand the worms simply likely won’t process your waste as consistently or reproduce as well if your bin is kept in an area subject to the low temperatures and low humidity that come with winter.

Location, Location, Location: Keep in a Heated Area

This piece of advice is a little glib, but it’s true; managing temperature is much easier if you’re able to move your worm bin indoors where the worms can enjoy similar temperatures to what you’re subject to.

While highly-committed vermicomposters might keep their bins in the kitchen, living room or other heavily-trafficked areas, most “normal” vermicomposters – if there is such a thing – may opt to keep the worm bin in a utility room or spare bedroom that will remain heated with the rest of the house.

But if you’re reading this, then chances are this isn’t possible. So let’s explore other options.

More Location: Keep In a Basement or an Unheated Garage

Urban Worm Thermometer

If a worm bin – even a beauty like the Urban Worm Bag! – doesn’t fit your interior decor tastes, you can still keep the bin in an unheated basement or a garage.

Basements are ideal in all seasons due to the relatively constant temperature and humidity. Even an unheated basement is fine because you’ll get some geothermal effect as the Earth’s temperature just a few feet below the surface is constant, making it warmer than the outside air temperature in the winter and cooler than the outside air temperature in the summer.

I keep my worm bins outside of my Philadelphia-area home in a barn built into the side of a slope in my back yard. Even that partial protection helps keep the temperatures in the bottom of the barn noticeably warmer than the cold outside air.

An attached garage will provide surprising benefit as well and the anecdotal evidence I’ve found suggests than an unheated garage will remain 20°F warmer than the outside air in the dead of winter. And if you drive your car regularly and actually park it in your garage (you weirdo), then you’ve got the space-heating benefit of the car radiating heat as the engine cools.

Whenever possible, try to place the bin near your car or a wall your garage shares with your heated home to help capture some of radiating heat.

Boost Microbial Activity to Produce Heat

In a normal hot compost pile, you are trying to get thermophilic or heat-loving microbes to proliferate, which creates heat, which produces more thermophilic microbes, and so on.

While hot composting is quite different than vermicomposting, you can stimulate microbial activity by boosting the quantity of organic waste and managing your carbon-to-nitrogen ratio to a lower C:N than might otherwise be recommended.

Vermicomposting should, in most cases, feature a C:N of 50 or higher, which would normally prevent the thermophilic reaction  – and 130°+ temperature! – that happens when the C:N is 25 or 30:1.

If all of this is confusing, feel free to mess around with the Urban Worm Compost Calculator and play with various recipes to see what works for you.

Now in order to create this reaction, a hot compost pile needs a decent amount of volume to trap the heat and get the inside of the pile cooking. Of course, we don’t want to achieve true hot composting in an enclosed worm bin, but we can increase the temperature through a careful regimen of increased feeding.

Image showing how to use feeding to heat a worm bin

To give you an idea of how this works, I placed a Bluetooth-enabled thermometer called a SensorPush into our commercial-scale Michigan SoilWorks CFT so we can monitor temperatures digitally and see the temperatures on our smartphones.

Notice the spike in temperature on Tuesday afternoon.

You can see where we fed and achieved about a 14°F increase over the next couple of days before the temps started trending downwards again.

Now I am normally very cautious about overfeeding due to the potential for anaerobic conditions and overheating. And I’ve screwed this up before!

But the heat generated by a worm bin can be blunted by cold outdoor temperatures. And just because temperatures can spike in a worm bin, it doesn’t mean that all of the material in the bin will be hot. Worms are great at finding the areas – typically the edges – of a worm bin where temperatures are more moderate.

You can help ensure worms have a cool area to escape to by placing these large feedings in smaller, localized areas in your worm bin. This “pocket feeding” technique means placing a heavy feeding in only a fraction of your bin, making sure the remainder is a hospitable environment for them.

But even if you overdo it and turn your bin into a crock pot, not all is lost. If you’ve got a roomy worm bin like the Urban Worm Bag or the Hungry Bin, chances are your worms will find an area to their liking. (Psst….the Urban Worm Bag is fabric and *must* be kept protected from precipitation and direct sunlight if kept outdoors.)

Increase Thermal Mass: Keep Your Bin Nice and Full!

There’s a quote – often attributed to Joseph Stalin – that I keep coming back to which asserts that “Quantity has a quality all its own.”

Stalin was rumored to have used this to describe an overwhelming tank attack against the German army. But if Stalin had a worm bin, he might have said the same thing to keep his worm comrades alive during the winter!

A worm bin with a large volume will offer better thermal mass which describes an ability to absorb, store, and release heat. What this means is that the contents of a full, large worm bin will offer a natural buffer against temperature extremes.

This concept – which works in the summer as well as the winter – means that the temperatures in your worm bin will lag well behind the ambient temperatures. In fact, your winter worm bin may actually be the coldest at the very time that it’s the warmest outside!

See the image to the right for a graph-based example of this concept.

Create a Hot Compost Pile Outdoors, Compost Guy Style!

This next option combines the two principles above: overfeeding and taking advantae of thermal mass. And it may have some interest to business-minded vermicomposters.

My friend Bentley “Compost Guy” Christie of Red Worm Composting has a very popular series of blog posts about how he keeps a thriving worm population going in the back yard of his suburban Ontario, Canada home. I highly recommend you A) check it out and B) if you have the space, try to replicate what he does, which is create a hybrid compost/vermicompost pile.

Bentley’s hybrid pile is 80 degrees. In February. In Canada.

He gathers gobs of fall leaves and mixes them with gobs of pumpkin, coffee grounds and other high-nitrogen materials. He layers straw on top of the material for insulation and covers with a tarp in a long pile called a “windrow.”

If this sounds like a recipe for full-on hot composting, it is!

But unlike in a smaller, enclosed worm bin, a nice, big winter windrow like this will allow worms to move in and out of the hot zones to their liking. Bentley, ever the creative guy that he is, found that he could create a “Walking Windrow.”

By adding fresh material to the end, creating heat and attracting the majority of the worms to the fresher, warmer decomposing matter, effectively “walked” the active zone along the ground, keeping worms thriving, reproducing, and creating copious amounts of worm castings in their wake over the winter.

He had so much success in creating such a densely-populated vermicompost that he sells this mix to Canadian customers in the springtime, and makes some good coin doing it.

Add Insulation Where You Can

One thing Bentley does with his winter windrows is to insulate with straw. Insulation serves to minimize heat loss through conduction, convection, and radiation. And by using dry, loosely-packed straw, Bentley was creating a large layer of millions of little air spaces that slow heat loss through convection – or heat transfer through the air.

By boosting microbial activity, you can create heat.

And by insulating it, you can keep it.

If you’re handy, you might consider a small frame around your bin with some form of insulating material, whether it’s a blanket or actual insulation. If you can’t insulate the entire bin itself, then you’re stuck insulating what you can, which is the surface. You can use carpet, bubble wrap, or the Urban Worm Blanket – possibly over a layer of leaves or straw – to slow heat loss.

Try Seed Starting Mats to Keep Worms Warm

If you can’t shelter your worm bin indoors or corral nature to serve your vermi-purposes, then there’s no shame in resorting to man-made technology.

I am really enthusiastic about seed starting mats and how they can deliver a nice, even heat across the surface of your vermicompost in ways that soil heating cables cannot.

The mats I am using are manufactured by Vivosun and the mat size I am most excited about is 20 x 20.75 inches, almost perfectly right-sized for the Urban Worm Bag and Hungry Bin.

The other is 48 x 20 inches and I’ll use this one to test out on part of the surface of my larger commercial vermicomposter. Once placed on top of the surface of your vermocompost, Vivosun mats are designed to deliver about a 20°F increase over ambient temperatures, but you can buy the optional thermostat to have a bit more control over the temperatures.

The consistency of warmth across the entire surface of the mat is what I find appealing. I have tried soil heating cables, but the heat is too localized to the cable itself and you have to make sure the cables don’t cross over one another.

I also tried a really hacky warming method involving incandescent rope lighting and rigid foam insulation, and almost burning my barn down as a result when the rope lighting got crossed over itself, melting its plastic sheathing.

Thankfully, there are better – and safer! – ways to do this and I think the Vivosun seed starting mats are a great option.

Summary: Use Nature Where You Can, Artificial Help When You Must

Let’s be honest.

The Urban Worm Company survives by making a buck when somebody needs a manmade product like the Urban Worm Bag, coco coir, and other products. But the thing I love about the vermicomposting community is its minimalism and DIY spirit.

So I highly encourage you to exhaust all natural options before bringing in artificial help. And I especially encourage you to combine some the techniques above to “stack” the benefits on top of one another.

Bring that worm bin into the garage, give it some larger-than-normal feedings, and drop a layer of straw on top.

Everyone’s situation is different, but whatever you can do to at least keep your bins surviving – or maybe even thriving – over the winter, the more worm castings you’ll have in the spring.

And oh by the way, you won’t have to replenish your worms at $35 per pound.

If you’ve got any other winter vermicomposting tips and tricks, please drop a comment below!

And if you liked what you read here, I invite you to join my e-mail list below and read the rest of my Vermicomposting 101 Series.

17 thoughts on “Winter Vermicomposting: How to Keep a Worm Bin Warm When It’s Cold

  1. I live in high desert (about 4200′) Sun Valley near Reno, NV. My worm bag is situated under the patio cover in a protected area. Last winter was a bit colder than normal and the worms survived. I have already noticed they aren’t as active right now, so maybe they are aware of some impending weather anomaly.

  2. Great blog Steve! I’m curious about the SensorPush – Have you tried it yet in your commercial size bin? Could you bury a few of them in there and monitor the temperature at different locations? And what about its moisture sensing in different depths of the bin?

    1. Thanks Dan! 🙂 I’ve got two in the CFT but I fouled up one of them when I let it get too wet. We’ve put them in baggies, but obviously that messes up the moisture reading. For a beeifer solution, a commercial vermicomposter might look for a product called the Kestrel Drop at kestrelmeters.com.

  3. i have the common plastic bins. last year i used a thick layer abou 3 inches of shredded leaves an if you use a heat mat where do you put it on top or botto m thanksd then a piece of cardboard on top staid 55 degrees in my garage in Washington state

  4. Thanks for this info, Steve. I got started with vermicomposting this past spring with an Urban Worm Bag (it’s been going great!), so this will be my first winter. I want to use the Vivosun seed mat you recommended, since my bag is in an unheated basement and will get extended freezing temps.

    Can you give any further guidance on how to use the seed mat? Do you zip the bag up and lay the mat on top?

  5. I’m thinking of moving my bin to my unfinished basement for the winter (my unheated, detached shed gets too cold and I don’t have a garage). But my bin is currently full of what looks like fly larvae on the top. In the summer, I ignore this, but I really don’t want them hatching/developing inside my house. Is it a bad idea to move it indoors? Or should empty the entire contents into my regular composter and start over inside?

    1. Hi Alison,
      Yes, I would remove any larvae (like black soldier fly larvae) that you can find. Once they pupate and become flies, they only live for 1-2 days, but that will still not be a pleasant addition to your home!

      Where are you located?

      1. Central Virginia – so warmer than PA, but we’ll still hit teens and possibly lower for at least a few winter weeks.

  6. I once got a truck load of stale bread from the bread store. After grinding some with a food processor I put a 2″ layer on my bed which was 3’x5′ covered with insulation. It got to 122 degrees before I noticed. Next I tried about a gallon buried in one end, bed is 90 gallons, with similar heating results on that end. Don’t know how it would work in a bag but would be fun to try.

  7. In my garage in Reno I have 12 of the black containers that plants come in (obtained for free at local nursery recycle bin), they are about 15 gallons each. I have them stacked two high and form a circle around two lights that are 40 watts each. I have them covered with a assortment of old blankets and towels. At night time temperatures have been in the low 20’s. When I run the lights all night ( frequently only need one light) my temperature around the bins stays in the 58 to 62 range and the worms stay very active,consuming everything I put in there and are producing like crazy. Inside the containers the temp is around 7 degrees higher, so like today when the temp is 48 inside the garage the soil temperature inside of the bin is 65 degrees, outside the bin but under the coverings the temp is 58

  8. I live in a part of australia that is unusual in that I do not have temperatures conducive to keeping worms. In summer its lucky to get over 25 C and for 3 months of the year it’s under 10 deg C, and for the other 6 its usually under 17. I have my worms in a bath, which has a wooden shell around it, and is packed with insulation both under and between exterior bath walls and inner wood walls. In winter I have a scrunched up bit of black plastic on top, with glass on top of that.I’m not crazy about the gassing off VOCs from the plastic so I might try another dark material. The bath goes into winter full and it gives worms some benefit I build them up in spring and summer and mainly just take the worm tea. It stays on almost 1/2 of the year here.I also have a worm farm inside, adjacent to the kitchen, handily. It means I have buffer for my bulk worming operations, and some genetic diversity is a good thing too.

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