How can I keep deer and mice out of my compost?

Answer: Compost piles can attract a number of pests, including deer, bears, voles, mice, raccoons, and skunk. If your compost area is close to your production area, consider composting only leaves and vegetable scraps. Leftover vegetables that have oil or a lot of seasoning can attract animals easily. Fruits may also be enticing but rather than leaving them out of the compost, I would just suggest burying them a little deeper and breaking up large pieces such as melon rinds. You don’t necessarily want to turn the compost every time you add something new, but maybe add some ‘browns’ to cover it up. I think a tarp would help keep the smell down but not necessarily the animals out. We conducted a study in which we covered our compost with a tarp and pumped air through a PVC pipe with air holes that ran underneath. Effectively what we were trying to do was increase the oxygen while holding in the water to see if we could rush the compost process. We ended up burning out our compost very quickly but it was likely our source of materials.

The most important factor is preventing it from going anaerobic. Once this happens, you will begin to get that putrid smell and methane will be produced. The best way to prevent anaerobic composting is to be careful about compaction of layers that may become impermeable to water and air. We use used bedding from the chicken coop, which is composed of straw and chicken manure. This layer often compacts from too much material on top, or heavy precipitation, sealing out the oxygen. We just have to turn it to mix it up again to loosen the material. Just make sure these layers of leaves are breaking down properly.

Another reason we end up going anaerobic is because we “burn out.” The proper composting temperature is between 130 and 150 degrees F. This zone allows the biodiversity of microbes to flourish, making for a probiotic amendment for your soil while cooking weed seeds. When temperatures begin to push 180 degrees F, all the good aerobic bacteria are killed, leaving you with an anaerobic bacteria dominated compost. These are not great amendments for your soil. You will be able to identify this by a silver-white layer a few inches down on the compost. This means the C:N ratio is imbalanced. Too much nitrogen and your temperature will sky rocket out of control. You are shooting for a 30:1 C:N ratio:

60:1 = C:N for leaves
25:1 = C:N for vegetable scraps (varies)
Therefore: 6 parts vegetable scraps + 1 part leaves [((25*6)+60C)/7N]=30:1

Note: I have seen different C:N ratios posted for the same materials and it impossible to be precise. This does help if you start to introduce more ingredients, such as manure which has a 15:1 ratio. Again, all manure is different and it depends on what they are eating so don’t worry too much about exact amounts. Just know you will need more vegetable scraps than leaves unless you increase the nitrogen with another material.

Instead, focus on the temperature. The first thing, if you don’t have one already, would be to purchase a compost thermometer. You would want one that has a long stem to reach deeper into the compost to see how far the active zone is. (I have used meat thermometers as well but they don’t have very long stems). If you see the temperature going above 160 degrees F, turn it. It is likely going to overheat if you don’t stop the rapid enzymatic processes. I also turn the compost every time it goes below 130 degrees F, assuming it is running out of oxygen. After a few cycles (turning each time and sufficiently watering), the compost will finally quit rising in temperature. That’s when you know It is done.

Typically the smells occur during the first cycle. If you can make that first cycle happen quickly by making sure everything has enough H2O, O, C, N, you will reduce the risk of luring in animals.

Deer are pesky and seem to find their way over any fence and deterrents rarely, if ever, work. You would need nearly a 10-foot perimeter fence to keep deer out. You could also consider a 3D fence, which works by confusing the deer into thinking the fence is bigger by running a series of wires on a slant.

I’ve mostly dealt with voles, which are a bit smarter than mice. The main difference is that mice are easier to bait, where voles run along paths that they build. Mice can be lured into traps. One easy trap to build is a rolling log mouse trap. They best part is that you can catch multiple mice without having to reset a trap. If you end up with voles, which have longer snouts, I have found a few ways to get rid of them. Spring traps can simply be laid in their little runways that they build. They will scurry along their same paths right into it without using any peanut butter or some sort of bait. Secondly, I try to disrupt their habitat. Turn the compost often or even dig around the edges of your compost area where they are burrowing. You can even flood their tunnels or around the compost. If all fails, I use castor oil. You can purchase it at any garden store or nursery. It is OMRI certified so you can use it in organic operations. It essentially deters voles from the area and has worked great for me. Though it won’t eliminate a problem, it will move them from a specific area.

Learn more about composting in the ATTRA publication Composting – The Basics. This publication provides a foundation of information for those interested in composting. It addresses a wide range of topics, from the materials that are needed to begin a compost pile to techniques for successfully managing the composting process. A troubleshooting list describing common problems and how to address them also is included. In addition, the basics of vermicomposting—using worms to generate compost—are described. Finally, the publication contains an introduction to composting for small agricultural operations such as market gardens.