Mouse eviction! Or how to fix up an under-active compost bin

Probably the most common problem that can occur with backyard composting is attracting pests, particularly the rodent kind. The inside of a plastic compost bin is insulated from the elements and full of tasty treats so it’s easy to see why they are attracted to it. Another aspect of the average backyard composter is that although it may be a nice warm mouse house it’s not HOT, as in hot composting. Even if you regularly forget about those veggies in the fridge and end up wasting quite a few of them, or cook most of your meals from scratch it’s unlikely that you will generate enough nitrogen rich veggie scraps on a regular basis to start generating heat in your compost bin. This is true even for households of 5 or so people, including little ones who may generate more scraps in the form of half eaten snacks etc. Why does it matter if the compost bin gets hot or not? This determines how quickly the scraps break down. A handful of scraps thrown into a cold composting system may no longer be food safe for us humans but they can remain fresh enough to be attractive to mice and rats for weeks. Whereas a bulk amount of the same veggie scraps in a hot composting system will generate heat and in turn hasten the degradation even more by essentially cooking the veggie scraps. In a day or two they will no longer resemble veggies and the mice and rats will no longer be interested. They also may not find it so comfortable once the interior of the compost bin is up around 60-70 degrees Celsius!

cute! but preventing compost from being made

So besides wasting heaps of veggies (which would be a terrible idea for many reasons) what are you to do? Limit access and make it uncomfortable for the little thieves! The presence of mice or rats may also indicate a compost bin that is too dry. Nothing will break down to any great extent if there is not enough water. All of the bacteria and fungi that do the hard work need water to survive, after all. Don’t just flood the mice out though, as too much water in the compost will also slow things down, and produce bad smells along with methane and other greenhouse gases.

When a HCC contributor sent me the above photo it was easy to suggest a course of action. Peter does the right thing and sends the really tasty kitchen wastes like bread crusts and leftover rice to the HCC, but composts most of the vegetable waste in his own bin. If you’ve ever heard anyone say ‘you can’t compost bread’, what they actually mean is ‘bread is great in compost, it breaks down easily, but unless you have a pest proof system then you’ll end up attracting mice/rats as they love the stuff’. Peter’s heap must not be completely pest proof yet. It’s surprising how small of a hole mice can fit through. Someone once told me they only need about a square centimeter to squeeze through. The best way that I’ve found to keep them out of a plastic composter is to ensure that the base is impenetrable with a few inches of soil piled up against it. Just like making a sand castle, just pile it up around the base of the unit and pat it down to compact it slightly. Yes, the critters can technically still dig through, but they are more likely to run off and investigate other options before they bother.

You can also make the scraps inaccessible from the top (in case they’re running up the sides and squeezing into a lid that’s slightly ajar, for instance). If there are lots of tasty scraps in the bin, you can cover them all up with, what else? compost! The HCC ‘semi-compost’ is perfect for this. ‘Semi-compost’ is what I have started to refer to when there is a pile of material that has cycled through the compost bays yet still needs to break down quite a bit.

the pile in front is ‘semi-compost’, the pile in back is just leaves!

And while you’re adding semi-compost why not heat things up a bit? For Peter’s heap I supplied a nice big bag of coffee grounds, as well as a pile of semi-compost. The idea is to add a nitrogen source that will speed up the decomposition of the whole. Watering this in is a good idea, especially if the pile is dry, but also to distribute the coffee grounds down through the existing mass. Then the whole lot can be topped off with the semi-compost, which the mice won’t be inclined to dig through to get to the tasty bits and will also inoculate the heap with good bacteria and fungi. You can also use grass clippings or a thin layer of manure for a nitrogen rich boost. Finally, (and perhaps waiting a day for the mice to vacate), build your ‘soil moat’ around the base, packing it up against the composter walls to secure the perimeter. Easy peasy! I would then let this whole lot sit for a few weeks, and then turn the whole lot over.

coffee grinds (as shown with portafilter), followed by water and a nice big pile of semi-compost and the mice will vacate! Pack soil around the base for extra reinforcement

If you have a compost bin in a similar situation I’d be happy to supply the semi-compost and even do all the hard work for you. All with the aim of diverting more kitchen scraps from landfill! I would ask for a donation to keep the HCC chugging away. If you think it’s worth it to keep the scraps out of landfill (and to not expand the rodent population) then you can enquire at



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